1
®
FN9106.3
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143 |Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2004 All Rights Reserved. Dynamic VID™ is a trademark of Intersil Americas Inc.
All other trademarks mentioned are the property of their respective owners.
ISL6244
Multi-Phase PWM Controller
The ISL6244 provides core-voltage regulation by driving 2 to
4 interleaved synchronous-rectified buck-converter channels
in parallel. Interleaving the channel timing results in
increased ripple frequency which reduces input and output
ripple currents. The reduction in ripple results in lower
component cost, reduced dissipation, and a smaller
implementation area.
The ISL6244 uses cost and space-saving rDS(ON) sensing
for channel current balance, active voltage positioning, and
over-current protection. Output voltage is monitored by an
internal differential remote sense amplifier. A high-bandwidth
error amplifier drives the output voltage to match the
programmed 5-bit DAC reference voltage. The resulting
compensation signal guides the creation of pulse width
modulated (PWM) signals to control companion Intersil
MOSFET drivers. The OFS pin allows direct offset of the
DAC voltage from 0V to 100mV using a single external
resistor. The entire system is trimmed to ensure a system
accuracy of ±1%.
Outstanding features of this controller IC include
Dynamic VIDTM technology allowing seamless on-the-fly VID
changing without the need of any external components.
Battery “feed-forward” is provided to allow for traditional
control schemes over total input voltage variation. Output
voltage “droop” or active voltage positioning is optional.
When employed, it allows the reduction in size and cost of
the output capacitors required to support load transients. A
threshold-sensitive enable input allows the use of an
external resistor divider for start-up coordination with Intersil
MOSFET drivers or any other devices powered from a
separate supply.
Superior over-voltage protection is achieved by gating on the
lower MOSFET of all phases to reduce the output voltage.
Under-voltage conditions are detected, but PWM operation
is not disrupted. Over-current conditions cause a hiccup-
mode response as the controller repeatedly tries to restart.
After a set number of failed startup attempts, the controller
latches off. A power good logic signal indicates when the
converter output is between the UV and OV thresholds.
Features
• Multi-Phase Power Conversion
- 2, 3 or 4 Phase Operation
• Active Channel Current Balancing
• Precision rDS(ON) Current Sharing
- Lossless
-Low Cost
• Precision CORE Voltage Regulation
- Differential Remote Output Voltage Sensing
- Programmable Reference Offset
- ±1% System Accuracy
• Microprocessor Voltage Identification Input
- 5-Bit VID Input
- 0.800V to 1.550V in 25mV Steps
- Dynamic VID Technology
• Programmable Droop Voltage
• Excellent Dynamic Response
- Combined Input Voltage Feed-Forward and Pulse-by-
Pulse Average Current Mode
• Over Current Protection
• Digital Soft Start
• Threshold Sensitive Enable Input
• High Ripple Frequency (160kHz to 4MHz)
• QFN Package:
- Compliant to JEDEC PUB95 MO-220 QFN - Quad Flat
No Leads - Package Outline
- Near Chip Scale Package footprint, which improves PCB
efficiency and has a thinner profile
• Pb-Free Available (RoHS Compliant)
Applications
• AMD Hammer Family Processor Voltage Regulator
• Low Output Voltage, High Current DC-DC Converters
• Voltage Regulator Modules
Data Sheet December 28, 2004
2FN9106.3
December 28, 2004
Pinout
ISL6244CR
(32 LEAD QFN 5x5)
TOP VIEW
Ordering Information
PART NUMBER TEMP. (°C) PACKAGE PKG. DWG. #
ISL6244CR 0 to 70 32 Ld 5x5 QFN L32.5x5
ISL6244CRZ
(Note 1)
0 to 70 32 Ld 5x5 QFN
(Pb-Free)
L32.5x5
ISL6244HR -10 to 100 32 Ld 5x5 QFN L32.5x5
ISL6244HRZ
(Note 1)
-10 to 100 32 Ld 5x5 QFN
(Pb-Free)
L32.5x5
NOTES:
1. Intersil Pb-free products employ special Pb-free material sets;
molding compounds/die attach materials and 100% matte tin
plate termination finish, which are RoHS compliant and
compatible with both SnPb and Pb-free soldering operations.
Intersil Pb-free products are MSL classified at Pb-free peak
reflow temperatures that meet or exceed the Pb-free
requirements of IPC/JEDEC J STD-020.
2. Add “-T” suffix for 32 QFN 5x5 Tape and Reel packages.
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
32 31 30 29 28 27 26 25
9 10111213141516
VID2
VID1
VID0
NC
OFS
COMP
FB
NC ISEN3
ISEN2
GND
PWM2
PWM1
ISEN1
ISEN4
PWM4
PGOOD
FS
EN
GND
VFF
VID4
NC
VID3
IOUT
VDIFF
VSEN
RGND
GND
GND
VCC
PWM3
NC = NO CONNECT
ISL6244
3FN9106.3
December 28, 2004
Absolute Maximum Ratings
Supply Voltage, VCC (Note 3) . . . . . . . . . . . . . . . . . . . . . . . . . . .+7V
Input, Output, or I/O Voltage . . . . . . . . . . GND -0.3V to VCC + 0.3V
ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class II
Operating Conditions
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5V ±5%
Ambient Temperature. . . . . . . . . . . . . . . . . . . . . . . . .-10°C to 100°C
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . 125°C
Thermal Information
Thermal Resistance θJA (°C/W) θJC (°C/W)
QFN Package (Notes 4, 6). . . . . . . . . . 32 4
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . 150°C
Maximum Storage Temperature Range. . . . . . . . . . . -65°C to 150°C
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300°C
CAUTION: Stress above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational section of this specification is not implied.
NOTES:
3. For VCC > 5.5V, current must be limited to 25mA.
4. θJA is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See Tech
Brief TB379.
5. Tolerance does not include the VID offset error or any external component tolerances.
6. For θJC, the “case temp” location is the center of the exposed metal pad on the package underside.
Electrical Specifications Operating Conditions: VCC = 5V, TA = -10°C to 100°C. Unless Otherwise Specified.
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
VCC SUPPLY CURRENT
Nominal Supply VCC = 5VDC; EN = 5VDC; RT = 100kΩ ±1% 8.0 10.8 14.0 mA
Shutdown Supply VCC = 5VDC; EN = 0VDC; RT = 100kΩ ±1% 8.0 10.3 13.0 mA
POWER-ON RESET AND ENABLE
POR Threshold VCC Rising 4.25 4.35 4.60 V
VCC Falling 3.75 3.85 4.00 V
ENABLE Threshold EN Rising 1.215 1.240 1.265 V
Hysteresis 82 92 102 mV
REFERENCE VOLTAGE AND DAC
System Accuracy (Note 5) -1.2 - 1.2 %VID
0 to 70°C -1 - 1 %VID
VID on Fly Step Size RT = 100kΩ-25-mV
VID Pull Up -30 -20 -10 µA
VID Input Low Level --0.8V
VID Input High Level 2.0 - - V
PIN-ADJUSTABLE OFFSET
OFS Current - 100 - µA
Offset Accuracy ROFS = 5kΩ ±1% 92.0 100.0 108.0 mV
ROFS = 5kΩ ±1% , 0 to 70°C 94.0 100.0 106.0
Maximum Offset - - 100.0 mV
OSCILLATOR
Accuracy -12.5 - 12.5 %
RT = 100K 245 280 315 kHz
Adjustment Range 0.08 - 1.0 MHz
VFF Range 0.5 - 2.5 V
Max Duty Cycle -75- %
ISL6244
4FN9106.3
December 28, 2004
ERROR AMPLIFIER
Open-Loop Gain RL = 10kΩ to ground -72-dB
Open-Loop Bandwidth CL = 100pF, RL = 10kΩ to ground -18-MHz
Slew Rate CL = 100pF, Load = ±400mA 3 7.1 11 V/µs
Maximum Output Voltage RL = 10kΩ to ground 3.6 4.5 - V
Source Current 3.0 7.0 11.5 mA
Sink Current 1.6 3.0 5.4 mA
REMOTE-SENSE AMPLIFIER
Input Impedance -80-kΩ
Bandwidth -20-MHz
Slew Rate -6-V/µs
SENSE CURRENT
IOUT Accuracy ISEN1 = ISEN2 = ISEN3 = ISEN4 = 50µA-5 - 5 %
ISEN Offset Voltage -6-mV
Over-Current Trip Level 68 85 102 µA
POWER GOOD AND PROTECTION MONITORS
PGOOD Low Voltage IPGOOD = 4mA - - 0.4 V
Under-Voltage Offset From VID VSEN Falling 320 370 420 mV
Over-Voltage Threshold VSEN Rising 2.08 2.13 2.20 V
Electrical Specifications Operating Conditions: VCC = 5V, TA = -10°C to 100°C. Unless Otherwise Specified. (Continued)
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
ISL6244
5FN9106.3
December 28, 2004
Typical Operating Performance
FIGURE 1. SOFT-START WAVEFORM FIGURE 2. INRUSH CURRENT AT VIN 19V @ 52A
FIGURE 3. INRUSH CURRENT AT VIN 10.8V @ 52A FIGURE 4. TRANSIENT WAVEFORM FROM 0A TO 52A
FIGURE 5. INDUCTOR CURRENT TRANSIENT FIGURE 6. VID CHANGES FROM 1.60V TO 1.20V
ISL6244
6FN9106.3
December 28, 2004
FIGURE 7. ISL6244 DROOP: VBAT = 8.4V FIGURE 8. FOUR PHASE CURRENT BALANCE @ 52A
FIGURE 9. ISL6244 EFFICIENCY vs LOAD FIGURE 10. ISL6244 DROOP: VBAT = 21V
Typical Operating Performance
1.400
1.450
1.500
1.550
1.600
1.650
0204060
OUTPUT CURRENT (A)
VCORE (V)
VBAT 8.4V
Vo- (AMD SPEC)
Vo+ (AMD SPEC)
TARGET
50
60
70
80
90
100
0204060
OUTPUT CURRENT (A)
VCORE (V)
10 30 50
Vbat = 8.4 Vbat = 10.8 Vbat = 14.4
Vbat = 19 Vbat = 21
1.400
1.450
1.500
1.550
1.600
1.650
0204060
OUTPUT CURRENT (A)
VCORE (V)
VBAT 21V
Vo- (AMD SPEC)
Vo+ (AMD SPEC)
TARGET
ISL6244
7FN9106.3
December 28, 2004
Functional Pin Description
ISL6244CR
(32 LEAD QFN 5x5)
TOP VIEW
GND
Bias and reference ground for the IC.
VFF
This pin is connected to VIN through a 10:1 voltage divider to
allow for battery “feed-forward,” which improves stability over
varying input line.
VID4, VID3, VID2, VID1, VID0
The state of these five inputs program the internal DAC,
which provides the reference voltage for output regulation.
Connect these pins to either open-drain or active pull-up
type outputs. Pulling these pins above 2.9V can cause a
reference offset inaccuracy.
OFS
Connecting a resistor between this pin and ground creates a
positive offset voltage which is added to the DAC voltage,
allowing easy implementation of load-line regulation. For no
offset, simply tie this pin to ground.
FB and COMP
The internal error amplifier inverting input and output
respectively. Connect the external R-C feedback
compensation network of the regulator to these pins.
IOUT
The current carried out of this pin is proportional to output
current and can be used to incorporate output voltage droop
and/or load sharing. The scale factor is set by the ratio of the
ISEN resistors and the lower MOSFET rDS(ON). If droop is
desired, connect this pin to FB. When not used for droop or
load sharing, simply leave this pin open.
VSEN, RGND, VDIFF
VSEN and RGND are the inputs to the differential remote-
sense amplifier. Connect these pins to the sense points of
the remote load. Connect an appropriately sized feedback
resistor, RFB, between VDIFF and FB.
VCC
Supplies all the power necessary to operate the chip. The IC
starts to operate when the voltage on this pin exceeds the
rising POR threshold and shuts down when the voltage on
this pin drops below the falling POR threshold. Connect this
pin directly to a +5V supply.
ISEN1, ISEN2, ISEN3, ISEN4
Current sense inputs. A resistor connected between these
pins and their respective phase node sets a current
proportional to the current in the lower MOSFET during it’s
conduction interval. This current is used as a reference for
channel balancing, load sharing, protection, and load-line
regulation. Inactive channels should have their respective
sense inputs left open.
PWM1, PWM2, PWM3, PWM4
Pulse-width modulating outputs. Connect these pins to the
individual ISL620X driver PWM input pins. These logic
outputs command the driver IC(s) in switching the half-
bridge configuration of MOSFETs. The number of active
channels is determined by the state of PWM3 and PWM4. If
PWM3 is tied to VCC, this indicates to the controller that two
channel operation is desired. In this case, PWM 4 should be
left open or tied to VCC. Shorting PWM4 to VCC indicates
that three channel operation is desired.
PGOOD
Power good is an open-drain logic output that changes to a
logic low when the voltage at VDIFF is 350mV below the VID
setting (under-voltage) or above 2.2V (over-voltage).
FS
A pin for setting the switching frequency of the regulator.
Place a resistor from this pin to ground to set the switching
frequency between 80kHz and 1MHz.
EN
This pin enables the ISL6244 regulator.
1
2
3
4
5
6
7
8
24
23
22
21
20
19
18
17
32 31 30 29 28 27 26 25
9 10111213141516
VID2
VID1
VID0
NC
OFS
COMP
FB
NC ISEN3
ISEN2
GND
PWM2
PWM1
ISEN1
ISEN4
PWM4
PGOOD
FS
EN
GND
VFF
VID4
NC
VID3
IOUT
VDIFF
VSEN
RGND
GND
GND
VCC
PWM3
NC = NO CONNECT
ISL6244
8FN9106.3
December 28, 2004
Typical Application: 3-Phase Buck Converter with rDS(ON) Current Sensing
+5V
PWM1
ISEN1
PWM2
ISEN3
PWM3
ISEN2
VSEN VOUT
DRIVER
ISL6207
PWM
EN
+5V BOOT
UGATE
PHASE
LGATE
VIN
VCC
FS
PWM4
ISEN4 NC
GND
GND
VCC
RFB
RC
CC
RISEN1
RT
DRIVER
ISL6207
PWM
EN
+5V BOOT
UGATE
PHASE
LGATE
VIN
VCC
GND
RISEN2
DRIVER
ISL6207
PWM
EN
+5V BOOT
UGATE
PHASE
LGATE
VIN
VCC
GND
RISEN3
VDIFF
RGND
IOUT
FB
ISL6244
COMP
OFS
ROFS
PGOOD
VID0
VID1
VID2
VID3
VID4
VFF
FIGURE 11. TYPICAL APPLICATION
VIN
EN
3x10µF
IRF7811W 0.56µH
820PTC
6x330µF
SI4362
0.56µH
820PTC
SI4362
3x10µF
IRF7811W
0.56µH
820PTC
SI4362
3x10µF
IRF7811W
10K
90K
107K
2.5K
2.43K
12nF
715
603
6800nF 12x22µF
CERAMIC
low ESR
RADJ2
RADJ1
0.1µF
0.1µF
0.1µF
ISL6244
9FN9106.3
December 28, 2004
Theory of Operation
Multi-Phase Power Conversion
Microprocessor load current profiles have changed to the
point where the multi-phase power conversion advantage is
pronounced. The technical challenges associated with
producing a single-phase converter which is both cost-
effective and thermally viable have forced a change to the
cost-saving approach of multi-phase. The ISL6244 controller
helps reduce the complexity of implementation by integrating
vital functions and requiring minimal output components.
The block diagram in Figure 12 provides a top level view of
multi-phase power conversion using the ISL6244 controller.
diff
VSEN
IOUT 1/N
CURRENT
SENSE
&
PHASE
DETECT
I3
I2
I1
+
2.2V
OV
85µA
+
+
-
N PHASES
+
I4
+
+
-
+
+
x 0.2
100µA
OFS
VID4
VID3
VID2
VID1
VID0
FB
DYNAMIC
VID
DAC
e/a
VDIFF
RGND
COMP
ISEN1
ISEN2
ISEN3
PWM1
PWM2
PWM3
VFF
EN
1.23V
FS
PGOOD VCC
POR
AND
SOFT START
OSCILLATOR
AND
SAWTOOTH
+
+ -
+
+ -
6V
PWM4
ISEN4
UV
+
-
GND
OC
AVERAGE
FIGURE 12. BLOCK DIAGRAM
350mV
ISL6244
10 FN9106.3
December 28, 2004
Interleaving
The switching of each channel in a multi-phase converter is
timed to be symmetrically out of phase with each of the other
channels. In a 3-phase converter, each channel switches 1/3
cycle after the previous channel and 1/3 cycle before the
following channel. As a result, the three-phase converter has
a combined ripple frequency three times greater than the
ripple frequency of any one phase. In addition, the peak-to-
peak amplitude of the combined inductor currents is reduced
in proportion to the number of phases (Equations 1 and 2).
Increased ripple frequency and lower ripple amplitude mean
that the designer can use less per-channel inductance and
lower total output capacitance for any performance
specification.
Figure 13 illustrates the multiplicative effect on output ripple
frequency. The three channel currents (IL1, IL2, and IL3),
combine to form the AC ripple current and the DC load
current. The ripple component has three times the ripple
frequency of each individual channel current. Each PWM
pulse is terminated 1/3 of a cycle, or 1.33µs for fS = 250kHz,
after the PWM pulse of the previous phase. The peak-to-peak
current waveforms for each phase is about 7A, and the dc
components of the inductor currents combine to feed the load.
To understand the reduction of ripple current amplitude in the
multi-phase circuit, examine the equation representing an
individual channel’s peak-to-peak inductor current.
In Equation 1, VIN and VOUT are the input and output
voltages respectively, L is the single-channel inductor value,
and fS is the switching frequency.
The output capacitors conduct the ripple component of the
inductor current. In the case of multi-phase converters, the
capacitor current is the sum of the ripple currents from each
of the individual channels. Compare Equation 1 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 2. Peak-to-peak ripple current decreases by an
amount proportional to the number of channels. Output-
voltage ripple is a function of capacitance, capacitor
equivalent series resistance (ESR), and inductor ripple
current. Reducing the inductor ripple current allows the
designer to use fewer or less costly output capacitors.
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multi-phase topologies can
improve overall system cost and size by lowering input ripple
current and allowing the designer to reduce the cost of input
capacitance. The example in Figure 14 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 14 delivers 52A to a 1.20V
load from a 19V input. The RMS input capacitor current is
6.5A. Compare this to a single-phase converter also
stepping down 19V to 1.20V at 52A. The single-phase
converter has 11.96A RMS input capacitor current. The
single-phase converter must use an input capacitor bank
with twice the RMS current capacity as the equivalent three-
phase converter.
Figures 28, 29 and 30 in the section entitled Input Capacitor
Selection can be used to determine the input-capacitor RMS
current based on load current, duty cycle, and the number of
channels. They are provided as aids in determining the
optimal input capacitor solution. Figure 31 shows the single
phase input-capacitor RMS current for comparison.
FIGURE 13. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
1µs/DIV
PWM2, 5V/D IV
PWM1, 5V/D IV
IL2, 7A/DIV
IL1, 7A/DIV
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IPP VIN VOUT
–()VOUT
LfSVIN
------------------------------------------------------= (EQ. 1)
ICPP,VIN NV
OUT
–()VOUT
LfSVIN
------------------------------------------------------------= (EQ. 2)
FIGURE 14. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
INPUT-CAPACITOR CURRENT, 15A/DIV
1µs/DIV
CHANNEL 1
INPUT CURRENT
15A/DIV
CHANNEL 2
INPUT CURRENT
15A/DIV
CHANNEL 3
INPUT CURRENT
15A/DIV
ISL6244
11 FN9106.3
December 28, 2004
PWM Operation
The timing of each converter leg is set by the number of
active channels. The default channel setting for the ISL6244
is four. One switching cycle is defined as the time between
PWM1 pulse termination signals. The pulse termination
signal is an internally generated clock signal which triggers
the falling edge of PWM1. The cycle time of the pulse
termination signal is the inverse of the switching frequency
set by the resistor between the FS pin and ground. Each
cycle begins when the clock signal commands the channel-1
PWM output to go low. The PWM1 transition signals the
channel-1 MOSFET driver to turn off the channel-1 upper
MOSFET and turn on the channel-1 synchronous MOSFET.
In the default channel configuration, the PWM2 pulse
terminates 1/4 of a cycle after PWM1. The PWM 3 output
follows another 1/4 of a cycle after PWM2. PWM4 terminates
another 1/4 of a cycle after PWM3.
If PWM3 is connected to VCC, then two channel operation is
selected and the PWM2 pulse terminates 1/2 of a cycle later.
Connecting PWM4 to VCC selects three channel operation and
the pulse-termination times are spaced in 1/3 cycle increments.
Once a PWM signal transitions low, it is held low for a
minimum of 1/4 cycle. This forced off time is required to
ensure an accurate current sample. Current sensing is
described in the next section. After the forced off time
expires, the PWM output is enabled. The PWM output state
is driven by the position of the error amplifier output signal,
VCOMP
, minus the current correction signal relative to the
sawtooth ramp as illustrated in Figure 1. When the modified
VCOMP voltage crosses the sawtooth ramp, the PWM output
transitions high. The MOSFET driver detects the change in
state of the PWM signal and turns off the synchronous
MOSFET and turns on the upper MOSFET. The PWM signal
will remain high until the pulse termination signal marks the
beginning of the next cycle by triggering the PWM signal low.
Current Sensing
During the forced off time following a PWM transition low, the
controller senses channel load current by sampling the
voltage across the lower MOSFET rDS(ON), see Figure 15. A
ground-referenced amplifier, internal to the ISL6244,
connects to the PHASE node through a resistor, RISEN. The
voltage across RISEN is equivalent to the voltage drop
across the RDS(ON) of the lower MOSFET while it is
conducting. The resulting current into the ISEN pin is
proportional to the channel current, IL. The ISEN current is
then sampled and held after sufficient settling time every
switching cycle. The sampled current, In, is used for
channel-current balance, load-line regulation and
overcurrent protection. From Figure 15, the following
equation for In is derived
where IL is the channel current.
If RDS(ON) sensing is not desired, an independent current-
sense resistor in series with the lower MOSFET source can
serve as a sense element. The circuitry shown in Figure 15
represents channel n of an N-channel converter. This
circuitry is repeated for each channel in the converter, but
may not be active depending upon the status of the PWM3
and PWM4 pins as described in the previous section.
Channel-Current Balance
The sampled current, In, from each active channel is used to
gauge both overall load current and the relative channel
current carried in each leg of the converter. The individual
sample currents are summed and divided by the number of
active channels. The resulting average current, IAVG,
provides a measure of the total load current demand on the
converter and the appropriate level of channel current. Using
Figures 15 and 16, the average current is defined as:
where N is the number of active channels and IOUT is the
total load current.
The average current is then subtracted from the individual
channel sample currents. The resulting error current, IER, is
then filtered before it adjusts VCOMP
. The modified VCOMP
signal is compared to a sawtooth ramp signal and produces
a pulse width which corrects for any imbalance and drives
the error current toward zero. Figure 16 illustrates Intersil’s
patented current-balance method as implemented on
channel-1 of a multi-phase converter.
InILrDS ON()
RISEN
----------------------=(EQ. 3
)
FIGURE 15. INTERNAL AND EXTERNAL CURRENT-SENSING
CIRCUITRY
In
ISEN ILrDS ON()
RISEN
--------------------------=
-
+
ISEN(n)
RISEN
SAMPLE
&
HOLD
ISL6244 INTERNAL CIRCUIT EXTERNAL CIRCUIT
VIN
CHANNEL N
UPPER MOSFET
CHANNEL N
LOWER MOSFET
-
+
ILrDS ON()
IL
(EQ. 4)
IAVG I1I2…IN
++
N
----------------------------------=
IAVG IOUT
N
------------- rDS ON()
RISEN
----------------------=
ISL6244
12 FN9106.3
December 28, 2004
Two considerations designers face are MOSFET selection
and inductor design. Both are significantly improved when
channel currents track at any load level. The need for
complex drive schemes for multiple MOSFETs, exotic
magnetic materials, and expensive heat sinks is avoided,
resulting in a cost-effective and easy to implement solution
relative to single-phase conversion. Channel-current
balance insures the thermal advantage of multi-phase
conversion is realized. Heat dissipation is spread over
multiple channels and a greater area than single phase
approaches.
Voltage Regulation
The output of the error amplifier, VCOMP
, is compared to the
sawtooth waveform to modulate the pulse width of the PWM
signals. The PWM signals control the timing of the Intersil
MOSFET drivers and regulate the converter output to the
specified reference voltage. Three distinct inputs to the error
amplifier determine the voltage level of VCOMP
. The internal
and external circuitry which control voltage regulation is
illustrated in Figure 17.
Most multi-phase controllers simply have the output voltage
fed back to the inverting input of the error amplifier through a
resistor. The ISL6244 features an internal differential
remote-sense amplifier in the feedback path. The amplifier
removes the voltage error encountered when measuring the
output voltage relative to the local controller ground
reference point, resulting in a more accurate means of
sensing output voltage. Connect the microprocessor sense
pins to the non-inverting input, VSEN, and inverting input,
RGND, of the remote-sense amplifier. The remote-sense
amplifier output, VDIFF
, is then tied through an external
resistor to the inverting input of the error amplifier.
A digital to analog converter (DAC) generates a reference
voltage based on the state of logic signals at pins VID4
through VID0. The DAC decodes a 5-bit logic signal (VID)
into one of the discrete voltages shown in Table 1. Each VID
input offers a 20µA pull-up to an internal 2.5V source for use
with open-drain outputs. External pull-up resistors or active-
high output stages can augment the pull-up current sources,
but a slight accuracy error can occur if they are pulled above
2.9V. The DAC-selected reference voltage is connected to
the non-inverting input of the error amplifier.
The ISL6244 features a second non-inverting input to the
error amplifier which allows the user to directly offset the
DAC reference voltage in the positive direction only. The
offset voltage is created by an internal current source which
feeds out the OFS pin into a user selected external resistor
to ground.
The resulting voltage across the resistor, VOFS, is internally
divided down by five to create the offset voltage. This
method of offsetting the DAC voltage is more accurate than
external methods of level-shifting the FB pin.
FIGURE 16. CHANNEL-1 PWM FUNCTION AND CURRENT-
BALANCE ADJUSTMENT
÷ N
IAVG
I4 *
I3 *
I2
Σ
-
+
+-
+
-
f(jω)
PWM1
I1
VCOMP
SAWTOOTH SIGNAL
IER
NOTE: *Channels 3 and 4 are optional.
FIGURE 17. OUTPUT-VOLTAGE AND LOAD-LINE
REGULATION
-
+
IAVG
REFERENCE
EXTERNAL CIRCUIT ISL6244 INTERNAL CIRCUIT
COMP
CC
RC
RFB
FB
IOUT
VDIFF
VSEN
RGND
-
+
VDROOP
ERROR AMPLIFIER
-
+
VOUT
REMOTE
SENSE
DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
VOLTAGE
VCOMP
+
OFS 1/5
100µA
ROFS
OFFSET
VOLTAGE
-
+
VOFS
GND
POINTS
ISL6244
13 FN9106.3
December 28, 2004
The integrating compensation network shown in Figure 17
assures that the steady-state error in the output voltage is
limited to the error in the reference voltage (output of the
DAC) plus offset errors in the OFS current source, remote-
sense and error amplifiers. Intersil specifies the guaranteed
tolerance of the ISL6244 to include all variations in current
sources, amplifiers and the reference so that the output
voltage remains within the specified system tolerance of
±1%. The 1% does not include the VID offset tolerance or
any external component tolerances.
FEED-FORWARD RAMP COMPENSATION
The ISL6244 features a VFF pin for setting the pulse width
modulator gain. The VFF voltage is set by a resistor divider
network from the battery voltage, as illustrated in Figure 18.
The VFF voltage sets the peak-to-peak voltage of the ramp
oscillator relative to the battery voltage. By feeding the
battery voltage forward, the pulse width modulation gain,
Gmod, is independent of battery voltage, see Equation 5.
The ramp modulator gain is then set by the ratio of the
maximum duty cycle, dMAX, to the amount of attenuation
programmed by the resistor network on the VFF pin. For
typical applications, select RADJ1 to be 9 times the value of
RADJ2 for a 1/10 attenuation of the battery voltage, resulting
in a constant pulse width modulator gain of 7.5 over the
entire range of battery voltage (see note below).
NOTE: the VFF voltage must be bounded between 0.5V and 2.5V.
.
LOAD-LINE REGULATION
Microprocessor load current demands change from near no-
load to full load often during operation. The resulting sizable
transient current slew rate causes an output voltage spike
since the converter is not able to respond fast enough to the
rapidly changing current demands. The magnitude of the
spike is dictated by the ESR and ESL of the output
capacitors selected. In order to drive the cost of the output
capacitor solution down, one commonly accepted approach
is active voltage positioning. By adding a well controlled
output impedance, the output voltage can effectively be level
shifted in a direction which works against the voltage spike.
The average current of all the active channels, IAVG, flows
out IOUT, see Figure 17. IOUT is connected to FB through a
load-line regulation resistor, RFB. The resulting voltage drop
across RFB is proportional to the output current, effectively
creating an output voltage droop with a steady-state value
defined as
TABLE 1. VOLTAGE IDENTIFICATION CODES
VID4 VID3 VID2 VID1 VID0 DAC
0 00001.550
0 00011.525
0 00101.500
0 00111.475
0 01001.450
0 01011.425
0 01101.400
0 01111.375
0 10001.350
0 10011.325
0 10101.300
0 10111.275
0 11001.250
0 11011.225
0 11101.200
0 11111.175
1 00001.150
1 00011.125
1 00101.100
1 00111.075
1 01001.050
1 01011.025
1 01101.000
1 01110.975
1 10000.950
1 10010.925
1 10100.900
1 10110.875
1 11000.850
1 11010.825
1 11100.800
1 1111Shutdown
Gmod
dMAX VBATTERY
⋅
RADJ2
RADJ1 RADJ2
+
------------------------------------------------- VBATTERY
⋅
-----------------------------------------------------------------------------------------0.75x10 7.5===
(EQ. 5)
FIGURE 18. BATTERY VOLTAGE FEED-FORWARD
COMPENSATION
RADJ1
SAWTOOTH
VBATTERY
VFF
RADJ2
SIGNAL
-
+
PWM1
PWM
SAWTOOTH
GENERATOR
VCOMP
ISL6244 INTERNAL CIRCUITRY
VDROOP IAVG RFB
=(EQ. 6)
ISL6244
14 FN9106.3
December 28, 2004
In most cases, each channel uses the same RISEN value to
sense current. A more complete expression for VDROOP is
derived by combining equations 15 and 16.
Droop is an optional feature of the ISL6244. If active voltage
positioning is not required, simply leave the IOUT pin open.
REFERENCE OFFSET
Typical microprocessor tolerance windows are centered
around a nominal DAC set point. Implementing a load-line
would require offsetting the output voltage above this
nominal DAC set point, centering the load-line within the
static specification window. The ISL6244 features an internal
100µA current source which feeds out the OFS pin. Placing
a resistor from OFS and ground allows the user to set the
amount of positive offset desired directly to the reference
voltage. The voltage developed across the OFS resistor,
ROFS, is divided down internally by a factor of 5 and directly
counters the DAC voltage at the error amplifier non-inverting
input. Select the resistor value based on the voltage offset
desired, VOFS, using Equation 19.
DYNAMIC VID
Next generation microprocessors can change VID inputs at
any time while the regulator is in operation. The power
management solution is required to monitor the DAC inputs
and respond to VID voltage transitions or ‘on-the-fly’ VID
changes, in a controlled manner, supervising the safe output
voltage transition within the DAC range of the processor
without discontinuity or disruption.
The ISL6244 will register a VID change within 1 to 2 clock
cycles. If the VID change is stable for an additional 1 to 2
clock cycles, the controller will begin executing the output
voltage change. The controller begins incrementing the
reference voltage by making 25mV steps every two
switching cycles until it reaches the new VID code.
The total time required for a VID change, tDV, is dependent
on the switching frequency (fS), the size of the change
(∆VID), and the time before the next switching cycle begins.
Since the ISL6244 recognizes VID-code changes only at the
beginning of switching cycles, up to one full cycle may pass
before a VID change registers. This is followed by a one-
cycle wait before the output voltage begins to change. The
uncertainty in Equation 9 is due to the possibility that the VID
code change may occur between two and four full cycles
before being recognized.
The time required for a converter running with fS = 250kHz
to make a 1.2V to 1.4V reference-voltage change is between
72µs and 80µs as calculated using Equation 9. This example
is also illustrated in Figure 7.
Operation Initialization
Before converter operation is initialized, proper conditions
must exist on the enable and disable inputs. Once these
conditions are met, the controller begins a soft-start interval.
Once the output voltage is within the proper window of
operation, the PGOOD output changes state to update an
external system monitor.
Enable and Disable
The PWM outputs are held in a high-impedance state to
assure the drivers remain off while in shutdown mode. Three
separate input conditions must be met before the ISL6244 is
released from shutdown mode.
First, the bias voltage applied at VCC must reach the internal
power-on reset (POR) circuit rising threshold. Once this
threshold is met, proper operation of all aspects of the
ISL6244 is guaranteed. Hysteresis between the rising and
falling thresholds insures that once enabled, the ISL6244 will
not inadvertently turn off unless the bias voltage drops
substantially. See the Electrical Specifications for specifics
on POR rising and falling thresholds.
VDROOP IOUT
N
------------- rDS ON()
RISEN
---------------------- R FB
=(EQ. 7)
ROFS
5V⋅OFS
100µA
-----------------------=(EQ. 8)
2
fS
-----VID∆
0.025
---------------1+
tDV 2
fS
-----VID∆
0.025
---------------2+
≤<(EQ. 9)
FIGURE 19. DYNAMIC-VID WAVEFORMS FOR 250kHz ISL6244
BASED MULTI-PHASE BUCK CONVERTER
15µs/DIV
VREF
, 100mV/div
VOUT
, 100mV/div
1.2V
VID, 5V/div
01110 00110
1.2V
VID CHANGE OCCURS
ANYWHERE HERE
ISL6244
15 FN9106.3
December 28, 2004
Second, the ISL6244 features an enable input (EN) for
power sequencing between the controller bias voltage and
another voltage rail. The enable comparator holds the
ISL6244 in shutdown until the voltage at EN rises above
1.23V. The enable comparator has about 90mV of hysteresis
to prevent bounce. It is important that the driver ICs reach
their POR level before the ISL6244 becomes enabled. The
schematic in Figure 20 demonstrates sequencing the
ISL6244 with the ISL620X family of Intersil MOSFET drivers
which require 5V bias.
The 11111 VID code is reserved as a signal to the controller
that no load is present. The controller will enter shutdown
mode after receiving this code and will start up upon
receiving any other code. This code is not intended as a
means of enabling the controller when a load is present.
To enable the controller, VCC must be greater than the POR
threshold; the voltage on EN must be greater than 1.23V;
and VID cannot be equal to 11111. Once these conditions
are true, the controller immediately initiates a soft-start
sequence.
Soft-Start
The soft-start time, tSS, is determined by an 11-bit counter
that increments with every pulse of the phase clock. For
example, a converter switching at 250kHz per phase has a
soft-start time of
During the soft-start interval, the soft-start voltage, VRAMP,
increases linearly from zero to 140% of the programmed
DAC voltage. At the same time a current source, IRAMP, is
decreasing from 160µA down to zero. These signals are
connected as shown in Figure 21 (IOUT may or may not be
connected to FB depending on the particular application).
The ideal diodes in Figure 21 assure that the controller tries
to regulate its output to the lower of either the reference
voltage or VRAMP. Since IRAMP creates an initial offset
across RFB of (RFB x 160µA), the first PWM pulse will not be
seen until VRAMP is greater than the RFB IRAMP offset. This
produces a delay after the ISL6244 enables before the
output voltage starts moving. For example, if VID = 1.5V,
RFB = 1kΩ and TSS = 8.3ms, the delay time can be
expressed using Equation 11.
Following the delay, the soft start ramps linearly until VRAMP
reaches VID. For the system described above, this first
linear ramp will continue for approximately
The final portion of the soft-start sequence is the time
remaining after VRAMP reaches VID and before IRAMP gets to
zero. This is also characterized by a slight change in the slope
of the output voltage ramp which, for the current example,
exists for a time of
This behavior is seen in the example in Figure 22 of a
converter switching at 500kHz. For this converter, RFB is
set to 2.67kΩ leading to TSS = 4.0ms, tDELAY = 700ns,
tRAMP1 = 2.23ms, and tRAMP2 = 1.17ms.
FIGURE 20. POWER SEQUENCING USING THRESHOLD-
SENSITIVE ENABLE (EN) FUNCTION
-
+
1.23V (±2%)
EXTERNAL CIRCUITISL6244 INTERNAL CIRCUIT
EN
+5V
VCC
+5V
POR
CIRCUIT
OV LATCH
SIGNAL
3.64kΩ
1.40kΩ
ENABLE
COMPARATOR
TSS 2048
fSW
-------------8.3ms== (EQ. 10)
FIGURE 21. RAMP CURRENT AND VOLTAGE FOR
REGULATING SOFT-START SLOPE
AND DURATION
-
+
IAVG
REFERENCE
EXTERNAL CIRCUIT ISL6244 INTERNAL CIRCUIT
COMP
CC
RC
RFB
FB
IOUT
VDIFF
ERROR AMPLIFIER
VOLTAGE
VCOMP
VRAMP
IRAMP
IDEAL DIODES
tDELAY TSS
11.4 VID
()
RFB160 10 6
–
×
-----------------------------------------+
--------------------------------------------------- 560µs== (EQ. 11)
tRAMP1 TSS
1.4
-----------tDELAY
–
=
5.27ms=
(EQ. 12)
tRAMP2 TSS tRAMP1
–tDELAY
–=
2.34ms=
(EQ. 13)
ISL6244
16 FN9106.3
December 28, 2004
NOTE: Switching frequency 500kHz and RFB = 2.67kΩ.
Fault Monitoring and Protection
The ISL6244 actively monitors voltage and current feedback
to detect fault conditions. Fault monitors trigger protective
measures to prevent damage to a microprocessor load. One
common power good indication signal is provided for linking
to external system monitors. The schematic in Figure 23
outlines the interaction between the fault monitors and the
power good signal.
Power Good Signal
The power good pin (PGOOD) is an open-drain logic output
which indicates that the converter is operating properly and
the output voltage is within a set window. The under-voltage
(UV) and over-voltage (OV) comparators create the output
voltage window. The controller also takes advantage of
current feedback to detect output over-current (OC)
conditions. PGOOD pulls low during shutdown and releases
high during soft-start once the output voltage reaches the
DAC level. Once high, PGOOD will only transition low when
the controller is disabled or a fault condition is detected. It
will return high under certain circumstances once a fault
clears.
Under-Voltage Protection
The voltage on VDIFF is internally offset by 350mV before it
is compared with the DAC reference voltage. By positively
offsetting the output voltage, an UV threshold is created
which moves relative to the VID code. During soft-start, the
slow rising output voltage eventually exceeds the UV
threshold. If a fault condition arises during operation and the
output voltage drops below the UV threshold, PGOOD will
immediately pull low, but converter operation will continue.
PGOOD will return high once the output voltage again
reaches regulation.
If the ISL6244 is disabled during operation, the PGOOD
signal will not pull low until the output voltage decays below
the UV threshold.
Over-Voltage Protection
When the output of the differential amplifier (VDIFF) reaches
2.2V, PGOOD immediately goes low indicating a fault. Two
protective actions are taken by the ISL6244 to protect the
microprocessor load.
All PWM outputs are commanded low, directing the Intersil
drivers to turn on the lower MOSFETs. This shunts the
output to ground preventing any further increase in output
voltage. The PWM outputs remain low until VDIFF falls to the
programmed DAC level at which time they go into a high-
impedance state. The Intersil drivers respond by turning off
both upper and lower MOSFETs. If the over-voltage
condition recurs, the ISL6244 will again command the lower
MOSFETs to turn on. The ISL6244 will continue to protect
the load in this fashion as long as the over-voltage repeats.
Once an over-voltage condition is detected, normal PWM
operation ceases and PGOOD remains low until the
ISL6244 is reset. Cycling the voltage on EN below 1.23V or
the bias to VCC below the POR-falling threshold will reset
the controller.
Over-Current Protection
The ISL6244 monitors individual channel current to detect an
over-current condition. Each channel current is continually
compared with a constant 85µA reference current. Once any
of the currents exceeds the reference current, the comparator
triggers the converter to shutdown. The POR circuit places all
PWM signals in a high-impedance state which commands the
drivers to turn off both upper and lower MOSFETs. PGOOD
pulls low and the system remains in this state while the
controller counts 2048 phase clock cycles. This is followed by
a soft-start attempt (see Soft-Start).
FIGURE 22. SOFT-START WAVEFORMS FOR ISL6244 BASED
MULTI-PHASE BUCK CONVERTER
1ms/DIV
EN, 5V/DIV
VOUT, 500mV/DIV
tDELAY tRAMP1 tRAMP2
FIGURE 23. POWER GOOD AND PROTECTION CIRCUITRY
POR
CIRCUIT
-
+
2.2V
VDIFF
-
+
85µA
ICHx
-
+
DAC
REFERENCE
OV
OC
UV
350mV
PGOOD
-
+
ISL6244
17 FN9106.3
December 28, 2004
During the soft-start interval, the over-current protection
circuitry remains active. As the output voltage ramps up, if
an over-current condition is detected, the ISL6244
immediately places all PWM signals in a high-impedance
state. The ISL6244 repeats the 2048-cycle wait period and
follows with another soft-start attempt, as shown in
Figure 24. This hiccup mode of operation repeats up to
seven times. On the eighth soft-start attempt, the part
latches off. Once latched off, the ISL6244 can only be reset
when the voltage on EN is brought below 1.23V or VCC is
brought below the POR falling threshold. Upon completion of
a successful soft-start attempt, operation will continue as
normal, PGOOD will return high, and the OC latch counter is
reset.
During VID-on-the-fly transitions, the OC comparator output
is blanked. The quality and mix of output capacitors used in
different applications leads to a wide output capacitance
range. Depending upon the magnitude and direction of the
VID change, the change in voltage across the output
capacitors could result in significant current flow. Summing
this instantaneous current with the load current already
present could drive the average current above the reference
current level and cause an OC trip during the transition. By
blanking the OC comparator during the VID-on-the-fly
transition, nuisance tripping is avoided.
Application Information
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multi-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced below. In
addition to this guide, Intersil provides complete reference
designs that include schematics, bills of materials, and
example board layouts for all common microprocessor
applications.
Power Stages
The first step in designing a multi-phase converter is to
determine the number of phases. This determination
depends heavily on the cost analysis which in turn depends
on system constraints that differ from one design to the next.
Principally, the designer will be concerned with whether
components can be mounted on both sides of the circuit
board and the total board space available for power-supply
circuitry. Generally speaking, the most economical solutions
are those where each phase handles between 15 and 20A.
In cases where board space is the limiting constraint, current
can be pushed as high as 30A per phase, but these designs
require heat sinks and forced air to cool the MOSFETs.
MOSFETs
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct; the switching frequency;
the capability of the MOSFETs to dissipate heat; and the
availability and nature of heat sinking and air flow.
LOWER MOSFET POWER CALCULATION
The calculation for heat dissipated in the lower MOSFET is
simple, since virtually all of the heat loss in the lower
MOSFET is due to current conducted through the channel
resistance (rDS(ON)). In Equation 14, IM is the maximum
continuous output current; IPP is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (VOUT/VIN); and
L is the per-channel inductance.
An additional term can be added to the lower-MOSFET loss
equation to account for additional loss accrued during the
dead time when inductor current is flowing through the
lower-MOSFET body diode. This term is dependent on the
diode forward voltage at IM, VD(ON); the switching
frequency, fS; and the length of dead times, td1 and td2, at
the beginning and the end of the lower-MOSFET conduction
interval respectively.
Thus the total maximum power dissipated in each lower
MOSFET is approximated by the summation of PL and PD.
0A
0V 5ms/DIV
OUTPUT VOLTAGE,
OUTPUT CURRENT, 20A/DIV
FIGURE 24. OVERCURRENT BEHAVIOR IN HICCUP MODE
500mV/DIV
PLrDS ON()
IM
N
------
21d–()
ILPP,21d–()
12
--------------------------------+= (EQ. 14)
PDVDON()
fSIM
N
------IPP
2
---------+
td1 IM
N
------IPP
2
---------
–
td2
+
=(EQ. 15)
ISL6244
18 FN9106.3
December 28, 2004
UPPER MOSFET POWER CALCULATION
In addition to rDS(ON) losses, a large portion of the upper-
MOSFET losses are due to currents conducted across the
input voltage (VIN) during switching. Since a substantially
higher portion of the upper-MOSFET losses are dependent
on switching frequency, the power calculation is more
complex. Upper MOSFET losses can be divided into
separate components involving the upper-MOSFET
switching times; the lower-MOSFET body-diode reverse-
recovery charge, Qrr; and the upper MOSFET rDS(ON)
conduction loss.
When the upper MOSFET turns off, the lower MOSFET does
not conduct any portion of the inductor current until the
voltage at the phase node falls below ground. Once the
lower MOSFET begins conducting, the current in the upper
MOSFET falls to zero as the current in the lower MOSFET
ramps up to assume the full inductor current. In Equation 16,
the required time for this commutation is t1 and the
approximated associated power loss is PUP,1.
The upper MOSFET begins to conduct and this transition
occurs over a time t2. In Equation 17, the approximate power
loss is PUP,2.
A third component involves the lower MOSFET’s reverse-
recovery charge, Qrr. Since the inductor current has fully
commutated to the upper MOSFET before the lower-
MOSFET’s body diode can draw all of Qrr, it is conducted
through the upper MOSFET across VIN. The power
dissipated as a result is PUP,3 and is approximately
Finally, the resistive part of the upper MOSFET’s is given in
Equation 19 as PUP,4.
In this case, of course, rDS(ON) is the on resistance of the
upper MOSFET.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 16, 17, 18 and 19. Since the power
equations depend on MOSFET parameters, choosing the
correct MOSFETs can be an iterative process that involves
repetitively solving the loss equations for different MOSFETs
and different switching frequencies until converging upon the
best solution.
Current Sensing
The ISEN pins are denoted ISEN1, ISEN2, ISEN3 and
ISEN4. The resistors connected between these pins and
their respective phase nodes determine the gains in the
load-line regulation loop and the channel-current balance
loop. Select the values for these resistors based on the room
temperature rDS(ON) of the lower MOSFETs; the full-load
operating current, IFL; and the number of phases, N
according to Equation 20 (see also Figure 15).
In certain circumstances, it may be necessary to adjust the
value of one or more of the ISEN resistors. This can arise
when the components of one or more channels are inhibited
from dissipating their heat so that the affected channels run
hotter than desired (see the section entitled Channel-Current
Balance). In these cases, chose new, smaller values of RISEN
for the affected phases. Choose RISEN,2 in proportion to the
desired decrease in temperature rise in order to cause
proportionally less current to flow in the hotter phase.
In Equation 21, make sure that ∆T2 is the desired temperature
rise above the ambient temperature, and ∆T1 is the measured
temperature rise above the ambient temperature. While a
single adjustment according to Equation 21 is usually
sufficient, it may occasionally be necessary to adjust RISEN
two or more times to achieve perfect thermal balance
between all channels.
Load-Line Regulation Resistor
The load-line regulation resistor is labeled RFB in Figure
17. Its value depends on the desired full-load droop voltage
(VDROOP in Figure 17). If Equation 20 is used to select
each ISEN resistor, the load-line regulation resistor is as
shown in Equation 22.
If one or more of the ISEN resistors was adjusted for thermal
balance, as in Equation 21, the load-line regulation resistor
should be selected according to Equation 23. Where IFL is
the full-load operating current and RISEN(n) is the ISEN
resistor connected to the nth ISEN pin.
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed. Depending on whether the
regulator employs the optional load-line regulation as
described in Load-Line Regulation, there are two distinct
methods for achieving these goals.
PUP1,VIN IM
N
------IPP
2
---------+
t1
2
----
fS
≈(EQ. 16)
PUP 2,VIN IM
N
------IPP
2
---------
–
t2
2
----
fS
≈(EQ. 17)
PUP3,VINQrrfS
=(EQ. 18)
PUP4,rDS ON()
IM
N
------
2dIPP
2
12
----------
+≈(EQ. 19)
RISEN rDS ON()
50 10 6
–
×
----------------------- IFL
N
--------= (EQ. 20)
RISEN 2,RISEN
∆T2
∆T1
----------=(EQ. 21)
RFB VDROOP
50 10 6–
×
-------------------------= (EQ. 22)
RFB VDROOP
IFL rDS ON()
-------------------------------- R ISEN n()
n
∑
=(EQ. 23)
ISL6244
19 FN9106.3
December 28, 2004
COMPENSATING LOAD-LINE REGULATED
CONVERTER
The load-line regulated converter behaves in a similar
manner to a peak-current mode controller because the two
poles at the output-filter L-C resonant frequency split with
the introduction of current information into the control loop.
The final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, RC and CC.
Since the system poles and zero are effected by the values
of the components that are meant to compensate them, the
solution to the system equation becomes fairly complicated.
Fortunately there is a simple approximation that comes very
close to an optimal solution. Treating the system as though it
were a voltage-mode regulator by compensating the L-C
poles and the ESR zero of the voltage-mode approximation
yields a solution that is always stable with very close to ideal
transient performance.
The feedback resistor, RFB, has already been chosen as
outlined in Load-Line Regulation Resistor. Select a target
bandwidth for the compensated system, f0. The target
bandwidth must be large enough to assure adequate
transient performance, but smaller than 1/3 of the per-
channel switching frequency. The values of the
compensation components depend on the relationships of f0
to the L-C pole frequency and the ESR zero frequency. For
each of the three cases which follow, there is a separate set
of equations for the compensation components.
.
In Equations 24, L is the per-channel filter inductance
divided by the number of active channels; C is the sum total
of all output capacitors; ESR is the equivalent-series
resistance of the bulk output-filter capacitance; and VPP is
the peak-to-peak sawtooth signal amplitude as described in
Figure 16 and Electrical Specifications.
Once selected, the compensation values in Equations 24
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to RC. Slowly increase the
value of RC while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
CC will not need adjustment. Keep the value of CC from
Equations 24 unless some performance issue is noted.
The optional capacitor C2, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 25). Keep
a position available for C2, and be prepared to install a high-
frequency capacitor of between 22pF and 150pF in case any
leading edge jitter problem is noted.
FIGURE 25. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6244 CIRCUIT
ISL6244
COMP
CC
RC
RFB
FB
IOUT
VDIFF
-
+
VDROOP
C2 (OPTIONAL)
1
2πLC
------------------- f 0
>
RCRFB2πf0Vpp LC
0.75VIN
------------------------------------=
CC0.75VIN
2πVPPRFBf0
------------------------------------=
Case 1:
1
2πLC
------------------- f01
2πC ESR()
------------------------------<≤
RCRFBVPP 2π()
2f02LC
0.75 VIN
--------------------------------------------=
CC0.75VIN
2π()
2f02VPPRFB LC
-------------------------------------------------------------=
Case 2:
(EQ. 24)
f01
2πC ESR()
------------------------------>
RCRFB 2πf0VppL
0.75 VIN ESR()
------------------------------------------=
CC0.75VIN ESR()C
2πVPPRFBf0L
-------------------------------------------------=
Case 3:
FIGURE 26. COMPENSATION CIRCUIT FOR ISL6244 BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION
ISL6244
COMP
CC
RC
RFB
FB
IOUT
VDIFF
C2
C1
R1
ISL6244
20 FN9106.3
December 28, 2004
COMPENSATION WITHOUT LOAD-LINE REGULATION
The non load-line regulated converter is accurately modeled
as a voltage-mode regulator with two poles at the L-C
resonant frequency and a zero at the ESR frequency. A type
III controller, as shown in Figure 26, provides the necessary
compensation.
The first step is to choose the desired bandwidth, f0, of the
compensated system. Choose a frequency high enough to
assure adequate transient performance but not higher than 1/3
of the switching frequency. The type-III compensator has an
extra high-frequency pole, fHF. This pole can be used for added
noise rejection or to assure adequate attenuation at the error-
amplifier high-order pole and zero frequencies. A good general
rule is to chose fHF = 10f0, but it can be higher if desired.
Choosing fHF to be lower than 10f0 can cause problems with
too much phase shift below the system bandwidth.
In the solutions to the compensation equations, there is a
single degree of freedom. For the solutions presented in
Equations 25, RFB is selected arbitrarily. The remaining
compensation components are then selected according to
Equations 25.
In Equations 25, L is the per-channel filter inductance
divided by the number of active channels; C is the sum total
of all output capacitors; ESR is the equivalent-series
resistance of the bulk output-filter capacitance; and VPP is
the peak-to-peak sawtooth signal amplitude as described in
Figure 16 and Electrical Specifications.
Output Filter Design
The output inductors and the output capacitor bank together
form a low-pass filter responsible for smoothing the pulsating
voltage at the phase nodes. The output filter also must
provide the transient energy during the interval of time after
the beginning of the transient until the regulator can
respond. Because it has a low bandwidth compared to the
switching frequency, the output filter necessarily limits the
system transient response leaving the output capacitor bank
to supply or sink load current while the current in the output
inductors increases or decreases to meet the demand.
In high-speed converters, the output capacitor bank is
usually the most costly (and often the largest) part of the
circuit. Output filter design begins with minimizing the cost of
this part of the circuit. The critical load parameters in
choosing the output capacitors are the maximum size of the
load step, ∆I; the load-current slew rate, di/dt; and the
maximum allowable output-voltage deviation under transient
loading, ∆VMAX. Capacitors are characterized according to
their capacitance, ESR, and ESL (equivalent series
inductance).
At the beginning of the load transient, the output capacitors
supply all of the transient current. The output voltage will
initially deviate by an amount approximated by the voltage
drop across the ESL. As the load current increases, the
voltage drop across the ESR increases linearly until the
load current reaches its final value. The capacitors selected
must have sufficiently low ESL and ESR so that the total
output-voltage deviation is less than the allowable
maximum. Neglecting the contribution of inductor current
and regulator response, the output voltage initially deviates
by an amount
The filter capacitor must have sufficiently low ESL and ESR
so that ∆V < ∆VMAX.
Most capacitor solutions rely on a mixture of high-frequency
capacitors with relatively low capacitance in combination
with bulk capacitors having high capacitance but limited
high-frequency performance. Minimizing the ESL of the
high-frequency capacitors allows them to support the output
voltage as the current increases. Minimizing the ESR of the
bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors also creates the majority of
the output-voltage ripple. As the bulk capacitors sink and
source the inductor ac ripple current (see Interleaving and
Equation 2), a voltage develops across the bulk-capacitor
ESR equal to IC,PP (ESR). Thus, once the output capacitors
are selected, the maximum allowable ripple voltage,
VPP(MAX), determines the lower limit on the inductance.
CC0.75VIN 2πfHF LC 1–
2π()
2f0fHF LCRFBVPP
-------------------------------------------------------------------=
RCVPP 2π
2f0fHFLCRFB
0.75VIN 2πfHF LC 1–
---------------------------------------------------------------------=
R1RFB CESR()
LC C ESR()–
-----------------------------------------=
C1LC C ESR()–
RFB
-----------------------------------------=
C20.75VIN
2π()
2f0fHF LCRFBVPP
-------------------------------------------------------------------=
(EQ. 25)
∆V ESL()
di
dt
----- ESR()∆I+≈(EQ. 26)
L ESR()
VIN NVOUT
–
VOUT
fSVINVPP MAX()
------------------------------------------------------------≥(EQ. 27)
ISL6244
21 FN9106.3
December 28, 2004
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
∆VMAX. This places an upper limits on inductance.
Equation 29 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output-voltage deviation than the leading edge. Equation 28
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
Input Supply Voltage Selection
The VCC input of the ISL6244 can be connected to either a
+5V supply directly or through a current limiting resistor to a
+12V supply. An integrated 5.8V shunt regulator maintains
the voltage on the VCC pin when a +12V supply is used. A
300Ω resistor is suggested for limiting the current into the
VCC pin to approximately 20mA.
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper-MOSFET loss calculation. These effects are
outlined in MOSFETs, and they establish the upper limit for
the switching frequency. The lower limit is established by the
requirement for fast transient response and small output-
voltage ripple as outlined in Output Filter Design. Choose the
lowest switching frequency that allows the regulator to meet
the transient-response requirements.
Switching frequency is determined by the selection of the
frequency-setting resistor, RT (see Figure 11). Figure 27 and
Equation 30 are provided to assist in the selecting the
correct value for RT
.
Input Capacitor Selection
The input capacitors are responsible for sourcing the ac
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the ac component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
For a two phase design, use Figure 28 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (IO), and the ratio
of the combined peak-to-peak inductor current (IC,PP) to IO.
Select a bulk capacitor with a ripple current rating which will
minimize the total number of input capacitors required to
support the RMS current calculated. The voltage rating of
the capacitors should also be at least 1.25 times greater
than the maximum input voltage.
(EQ. 28)
L2NCVO
∆I
()
2
--------------------- ∆VMAX ∆I ESR()–≤
(EQ. 29)
LN∆VMAX C⋅⋅()1.5VIN 2VO
–()
∆I
()
2
-------------------------------------------------------------------------------------
≤
FIGURE 27. RT vs SWITCHING FREQUENCY
100 1000 1000010 SWITCHING FREQUENCY (kHz)
10
100
1000
RT (kΩ)
RT10 11.09 1.13 fS
()log–
[]
=(EQ. 30)
0.3
0.1
0
0.2
INPUT-CAPACITOR CURRENT (IRMS/IO)
FIGURE 28. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR 2-PHASE
CONVERTER
00.4 1.00.2 0.6 0.8
DUTY CYCLE (VIN/VO)
IC,PP = 0
IC,PP = 0.5 IO
IC,PP = 0.75 IO
ISL6244
22 FN9106.3
December 28, 2004
Figures 29 and 30 provide the same input RMS current
information for three and four phase designs respectively.
Use the same approach to selecting the bulk capacitor type
and number as described above.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes. The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
close as possible to each upper MOSFET drain to minimize
board parasitics and maximize suppression.
MULTIPHASE RMS IMPROVEMENT
Figure 31 is provided as a reference to demonstrate the
dramatic reductions in input-capacitor RMS current upon the
implementation of the multiphase topology.
For example, compare the input rms current requirements of
a two-phase converter versus that of a single phase.
Assume both converters have a duty cycle of 0.25,
maximum sustained output current of 40A, and a ratio of
IC,PP to IO of 0.5. The single phase converter would require
17.3 Arms current capacity while the two-phase converter
would only require 10.9 Arms. The advantages become
even more pronounced when output current is increased
and additional phases are added to keep the component
cost down relative to the single phase approach.
Layout Considerations
Printed circuit board (PCB) layout is very important in high
frequency switching converter design. With components
switching at greater than 200kHz, the resulting current
transitions from one device to another cause voltage spikes
across the interconnecting impedances and parasitic circuit
elements. These voltage spikes can degrade efficiency, lead
to device over-voltage stress, radiate noise into sensitive
nodes, and increase thermal stress on critical components.
Careful component placement and PCB layout minimizes
the voltage spikes in the converter.
The following multi-layer printed circuit board layout
strategies minimize the impact of board parasitics on
converter performance and optimize the heat-dissipating
capabilities of the printed-circuit board. This section
highlights some important practices which should not be
overlooked during the layout process.
Component Placement
Determine the total implementation area and orient the
critical switching components first. Symmetry is very
important in multiphase converter placement and the
switching components dictate how the available space is
filled. The switching components carry large amounts of
energy and tend to generate high levels of noise. A tight
layout of the output inductors and MOSFETs with short, wide
DUTY CYCLE (VIN/VO)
FIGURE 29. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR 3-PHASE
CONVERTER
00.4 1.00.2 0.6 0.8
INPUT-CAPACITOR CURRENT (IRMS/IO)
0.3
0.1
0
0.2
IC,PP = 0
IC,PP = 0.25 IO
IC,PP = 0.5 IO
IC,PP = 0.75 IO
INPUT-CAPACITOR CURRENT (IRMS/IO)
FIGURE 30. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR 4-PHASE
CONVERTER
00.4 1.00.2 0.6 0.8
DUTY CYCLE (VIN/VO)
0.3
0.1
0
0.2
IC,PP = 0
IC,PP = 0.25 IO
IC,PP = 0.5 IO
IC,PP = 0.75 IO
FIGURE 31. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
00.4 1.00.2 0.6 0.8
DUTY CYCLE (VIN/VO)
INPUT-CAPACITOR CURRENT (IRMS/IO)
0.6
0.2
0
0.4
IC,PP = 0
IC,PP = 0.5 IO
IC,PP = 0.75 IO
ISL6244
23 FN9106.3
December 28, 2004
traces, such that space between the components is reduced
while creating the PHASE plane, is recommended. Stray
Inductance in the switch path adds to the voltage spikes
generated during the switching interval. By keeping the
phase plane small, the magnitude of the potential spikes is
minimized. If possible, duplicate the same placement and
layout of these components for each phase. Figure 32
illustrates the connection of critical components for one
output channel of a converter.
Place the ISL6207 drivers as close as possible to the
MOSFETs they control to reduce the parasitics due to trace
length between critical driver input and output signals.
Position one high-frequency ceramic input capacitor next to
each upper MOSFET drain. Place the bulk input capacitors
as close to the upper MOSFET drains as dictated by the
component size and dimensions. Long distances between
input capacitors and MOSFETs drains results in too much
trace inductance and a reduction in capacitor performance.
In Figure 32, CIN and COUT represent numerous physical
capacitors.
Locate the output capacitors between the inductors and the
load, while keeping them in close proximity around the
microprocessor socket. Care should be taken not to add
inductance in the circuit board traces that could cancel the
usefulness of the low inductance components.
The ISL6244 can be placed off to one side or centered
relative to the individual phase switching components.
Routing of sense lines and PWM signals will guide final
placement. Critical small signal components to place close
to the controller include the feedback resistor RFB,
frequency select resistor RFS, offset resistor ROFS,
feedforward ramp adjustment resistors RADJ1 and RADJ2,
and compensation components RC and CC. Because the
remote sense traces for VSEN and VRTN may be long and
routed close to switching nodes, a 1.0µF ceramic
decoupling capacitor can be placed between VSEN and
RTN of the package. This value may vary depending on the
impact of the converter response.
Bypass capacitors, CBP
, supply critical bypassing current for
the ISL6244 and ISL6207 drivers bias supplies and must be
placed next to their respective pins. Stray trace parasitics will
reduce their effectiveness, so keep the traces to these
components as short and wide as possible.
Plane Allocation and Routing
Dedicate at least one solid layer, usually a middle layer of
the PCB, for a ground plane and make all critical component
ground connections with vias to this layer. If two ground
layers can be used, it is beneficial to run all signal lines in
between these to shield them from radiative coupling. These
include the current sense lines from PHASE, PWM, VID and
Enable lines. Dedicate one additional solid layer as a power
plane and break this plane into smaller islands of common
voltage. Keep the metal runs from the PHASE terminal to the
output inductor short. The power planes should support the
input power and output power nodes. Use copper filled
areas on the top and bottom circuit layers for the phase
nodes. Use the remaining PCB layers for small signal
routing. Size the traces from the ISL6207 driver to the power
MOSFET gates and sources to carry at least 1A of
continuous current. When routing components in the
switching path, use short wide traces to reduce the
associated parasitics.
ISL6244
24 FN9106.3
December 28, 2004
FIGURE 32. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
V
OUT
+5V
VIA CONNECTION TO GROUND PLANE
ISLAND ON POWER PLANE LAYER ISLAND ON CIRCUIT PLANE LAYER
L
O1
C
OUT
C
IN
V
battery
KEY
VCC
USE INDIVIDUAL METAL RUNS
COMP
ISL6244
PWM
R
FB
R
C
C
BP
FB
VDIFF
ISEN R
ISEN(n)
(close to controller)
ISL6207
C
BOOT
C
BP
C
C
VCC
BOOT
ISOLATE OUTPUT STAGES
FOR EACH CHANNEL TO HELP
L
parasitic
PHASE
UGATE
LGATE
PWM
GND
R
ADJ1
R
ADJ2
GND
OFS
R
OFS
RGND
VSEN
FS
VFF
+5V
PHASE
R
FS
IOUT
(
close to LGATE if thermistor is used)
ISL6244
25
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
FN9106.3
December 28, 2004
ISL6244
Quad Flat No-Lead Plastic Package (QFN)
Micro Lead Frame Plastic Package (MLFP) L32.5x5
32 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
(COMPLIANT TO JEDEC MO-220VHHD-2 ISSUE C
SYMBOL
MILLIMETERS
NOTESMIN NOMINAL MAX
A 0.80 0.90 1.00 -
A1 - - 0.05 -
A2 - - 1.00 9
A3 0.20 REF 9
b 0.18 0.23 0.30 5,8
D 5.00 BSC -
D1 4.75 BSC 9
D2 2.95 3.10 3.25 7,8
E 5.00 BSC -
E1 4.75 BSC 9
E2 2.95 3.10 3.25 7,8
e 0.50 BSC -
k0.25 - - -
L 0.30 0.40 0.50 8
L1 - - 0.15 10
N322
Nd 8 3
Ne 8 8 3
P- -0.609
θ--129
Rev. 1 10/02
NOTES:
1. Dimensioning and tolerancing conform to ASME Y14.5-1994.
2. N is the number of terminals.
3. Nd and Ne refer to the number of terminals on each D and E.
4. All dimensions are in millimeters. Angles are in degrees.
5. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
7. Dimensions D2 and E2 are for the exposed pads which provide
improved electrical and thermal performance.
8. Nominal dimensions are provided to assist with PCB Land Pattern
Design efforts, see Intersil Technical Brief TB389.
9. Features and dimensions A2, A3, D1, E1, P & θ are present when
Anvil singulation method is used and not present for saw
singulation.
10. Depending on the method of lead termination at the edge of the
package, a maximum 0.15mm pull back (L1) maybe present. L
minus L1 to be equal to or greater than 0.3mm.
