LTC3528/LTC3528B
1
3528f
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
1A, 1MHz Synchronous
Step-Up DC/DC Converters
in 3mm × 2mm DFN
The LTC
®
3528/LTC3528B are synchronous, fi xed frequency
step-up DC/DC converters with output disconnect. High
effi ciency synchronous rectifi cation, in addition to a 700mV
start-up voltage and operation down to 500mV once
started, provides longer run-time for single or multiple
cell battery-powered products.
A switching frequency of 1MHz minimizes solution foot-
print by allowing the use of tiny, low profi le inductors and
ceramic capacitors. The current mode PWM is internally
compensated, simplifying the design process. The LTC3528
enters Burst Mode operation at light loads, while the
LTC3528B features continuous switching at light loads.
Anti-ringing circuitry reduces EMI by damping the inductor
in discontinuous mode. Additional features include a low
shutdown current, open-drain power good output, short-
circuit protection and thermal overload protection.
The LTC3528/LTC3528B are offered in an 8-lead 3mm ×
2mm × 0.75mm DFN package.
■ Delivers 3.3V at 200mA from a Single Alkaline/
NiMH Cell or 3.3V at 400mA from Two Cells
■ VIN Start-Up Voltage: 700mV
■ 1.6V to 5.25V VOUT Range
■ Up to 94% Effi ciency
■ Output Disconnect
■ 1MHz Fixed Frequency Operation
■ VIN > VOUT Operation
■ Integrated Soft-Start
■ Current Mode Control with Internal Compensation
■ Burst Mode
®
Operation with 12μA Quiescent Current
(LTC3528)
■ Low Noise PWM Operation (LTC3528B)
■ Internal Synchronous Rectifi er
■ Logic Controlled Shutdown: <1μA
■ Anti-Ringing Control
■ Low Profi le (3mm × 2mm × 0.75mm) DFN Package
■ Medical Instruments
■ Flash-Based MP3 Players
■ Noise Canceling Headphones
■ Wireless Mice
■ Bluetooth Headsets
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
SW
VIN
499k
4.7μH
287k
4.7μF
10μF
33pF
3528 TA01a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
1.8V TO 3.2V
VOUT
3.3V
400mA
OFF ON
GND
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA01b
70
60
0.1 1 10 1000
50
40
30
100
1000
10
1
0.1
0.01
EFFICIENCY (%)
POWER LOSS (mW)
EFFICIENCY
POWER LOSS
VOUT = 3.3V
VIN = 2.4V
Effi ciency and Power Loss
LTC3528/LTC3528B
2
3528f
ELECTRICAL CHARACTERISTICS
ABSOLUTE MAXIMUM RATINGS
VIN Voltage ................................................... –0.3V to 6V
SW Voltage
DC ............................................................ –0.3V to 6V
Pulsed < 100ns ........................................ –0.3V to 7V
⎯
S
⎯
H
⎯
D
⎯
N, FB Voltage ........................................ –0.3V to 6V
VOUT ............................................................. –0.3V to 6V
PGOOD ......................................................... –0.3V to 6V
Operating Temperature Range
(Notes 2, 5) .............................................. –40°C to 85°C
Storage Temperature Range ................... –65°C to 125°C
(Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Start-Up Voltage ILOAD = 1mA ●0.70 0.88 V
Output Voltage Adjust Range
TA = 0°C to 85°C
●1.7
1.6
5.25
5.25
V
V
Feedback Voltage (Note 7) ●1.170 1.200 1.230 V
Feedback Input Current VFB = 1.3V 1 50 nA
Quiescent Current—Shutdown V
⎯
S
⎯
H
⎯
D
⎯
N = 0V, Not Including Switch Leakage, VOUT = 0V 0.01 1 μA
Quiescent Current—Active Measured on VOUT, Nonswitching (Note 4) 300 500 μA
Quiescent Current—Burst Measured on VOUT, FB > 1.230V 12 20 μA
N-Channel MOSFET Switch Leakage Current VSW = 5V 0.1 10 μA
P-Channel MOSFET Switch Leakage Current VSW = 5V, VOUT = 0V 0.1 10 μA
N-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.175 Ω
P-Channel MOSFET Switch On Resistance VOUT = 3.3V 0.250 Ω
N-Channel MOSFET Current Limit ●1.0 1.5 A
Current Limit Delay Time to Output (Note 3) 60 ns
Maximum Duty Cycle VFB = 1.15V ●88 93 %
Minimum Duty Cycle VFB = 1.3V ●0%
Frequency ●0.7 1.0 1.3 MHz
The ● denotes the specifi cations which apply over the specifi ed operating
temperature range of –40°C to 85°C, otherwise specifi cations are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V, unless otherwise noted.
TOP VIEW
9
DDB PACKAGE
8-LEAD (3mm × 2mm) PLASTIC DFN
5
6
7
8
4
3
2
1SHDN
FB
PGOOD
VOUT
VIN
SGND
PGND
SW
TJMAX = 125°C, θJA = 76°C/W (NOTE 6)
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC3528EDDB#PBF
LTC3528BEDDB#PBF
LTC3528EDDB#TRPBF
LTC3528BEDDB#TRPBF
LCYD
LDDG
8-Lead (3mm × 2mm) Plastic DFN
8-Lead (3mm × 2mm) Plastic DFN
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based fi nish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/
PIN CONFIGURATION
ORDER INFORMATION
LTC3528/LTC3528B
3
3528f
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3528E is guaranteed to meet performance specifi cations
from 0°C to 85°C. Specifi cations over –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: Specifi cation is guaranteed by design and not 100% tested in
production.
PARAMETER CONDITIONS MIN TYP MAX UNITS
⎯
S
⎯
H
⎯
D
⎯
N Input High Voltage 0.88 V
⎯
S
⎯
H
⎯
D
⎯
N Input Low Voltage 0.25 V
⎯
S
⎯
H
⎯
D
⎯
N Input Current V
⎯
S
⎯
H
⎯
D
⎯
N = 1.2V 0.3 1 μA
PGOOD Threshold Percentage Referenced to Feedback Voltage Rising –7 –10 –13 %
PGOOD Low Voltage IPGOOD = 1mA
VOUT = 1.6V, IPGOOD = 1mA
0.05
0.05
0.1
0.2
V
V
PGOOD Leakage Current VPGOOD = 5.5V 0.01 1 μA
ELECTRICAL CHARACTERISTICS
The ● denotes the specifi cations which apply over the specifi ed operating
temperature range of –40°C to 85°C, otherwise specifi cations are at TA = 25°C. VIN = 1.2V, VOUT = 3.3V, unless otherwise noted.
Note 4: Current measurements are made when the output is not switching.
Note 5: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specifi ed maximum operating junction
temperature may result in device degradation or failure.
Note 6: Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
76°C/W.
Note 7: The IC is tested in a feedback loop to make the measurement.
TYPICAL PERFORMANCE CHARACTERISTICS
Effi ciency vs Load Current and VIN
for VOUT = 1.8V (LTC3528)
Effi ciency vs Load Current and VIN
for VOUT = 3V (LTC3528)
(TA = 25°C unless otherwise noted)
LOAD CURRENT (mA)
0.01
EFFICIENCY (%)
POWER LOSS (mW)
60
80
100
100
3528 G01
40
20
50
70
90
30
10
0
10
100
1000
1
0.1
0.01
0.1 110 1000
VIN = 1V
VIN = 1.2V
VIN = 1.5V
EFFICIENCY
POWER
LOSS
LOAD CURRENT (mA)
0.01
EFFICIENCY (%)
POWER LOSS (mW)
60
80
100
100
3528 G26
40
20
50
70
90
30
10
0
10
100
1000
1
0.1
0.01
0.1 110 1000
VIN = 1V
VIN = 1.5V
VIN = 2.4V
EFFICIENCY
POWER
LOSS
LTC3528/LTC3528B
4
3528f
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 G03
70
60
0.1 1 10 1000
50
40
30
100
1000
10
1
0.1
EFFICIENCY (%)
POWER LOSS (mW)
VIN = 1.2V
VIN = 2.4V
VIN = 3.6V
VIN = 4.2V
EFFICIENCY
POWER
LOSS
No-Load Input Current vs VIN
(LTC3528)
Maximum Output Current vs VIN
Minimum Load Resistance During
Start-Up vs VIN Start-Up Delay Time vs VIN
Burst Mode Threshold Current
vs VIN
Effi ciency vs Load Current and VIN
for VOUT = 5V (LTC3528)
Effi ciency vs Load Current and VIN
for VOUT = 3.3V (LTC3528)
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 G02
70
60
0.1 1 10 1000
50
40
30
100
1000
10
1
0.1
0.01
EFFICIENCY (%)
POWER LOSS (mW)
VIN = 1.2V
VIN = 1.8V
VIN = 2.4V
VIN = 3V
EFFICIENCY
POWER
LOSS
VIN (V)
1
10
IIN (μA)
30
50
70
90
110
130
23 45
3528 G04
VOUT = 1.8V
VOUT = 3V
VOUT = 3.3V
VOUT = 5V
VIN (V)
1
IOUT (mA)
700
2.5
3528 G05
400
200
1.5 2 3
100
0
800
600
500
300
3.5 4 4.5
VOUT = 1.8V
VOUT = 3.3V
VOUT = 5V
VIN (V)
0.7
10
RLOAD (Ω)
1000
100
10000
0.8 0.9 1
3528 G06
VIN (V)
1
DELAY (μs)
120
2.5
3528 G07
90
70
1.5 2 3
60
50
130
110
100
80
3.5 4 4.5
VIN (V)
1
IOUT (mA)
20
30
1.4
3528 G08
10
01.1 1.2 1.3
RISING
FALLING
1.5
40 VOUT = 1.8V
VIN (V)
1
IOUT (mA)
20
30
3528 G09
10
01.5
RISING
VOUT = 3V
FALLING
22.5
50
40
VIN (V)
1
0
IOUT (mA)
20
40
RISING
FALLING
60
1.5 2
3528 G10
2.5 3
VOUT = 3.3V
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C unless otherwise noted)
Burst Mode Threshold Current
vs VIN
Burst Mode Threshold Current
vs VIN
LTC3528/LTC3528B
5
3528f
TEMPERATURE (°C)
–50 –30
600
START-UP VOLTAGE (mV)
700
850
–10 30 50
3528 G17
650
800
750
10 70 90
VOUT (V)
1
10.5
CURRENT (μA)
11.5
12.5
13.5
23
3528 G18
45
VIN = 1.2V
Oscillator Frequency Change
vs VOUT RDS(ON) vs VOUT
Oscillator Frequency Change
vs Temperature RDS(ON) Change vs Temperature VFB vs Temperature
Start-Up Voltage vs Temperature
Fixed Frequency VOUT Ripple and
Inductor Current Waveforms
Burst Mode Quiescent Current vs
VOUT (LTC3528)
VIN (V)
1
0
IOUT (mA)
20
40
RISING
FALLING
60
23
3528 G11
4
VOUT = 5V
VOUT (V)
1.5
FREQUENCY CHANGE (%)
–1.50
0
0.25
0.50
2.5 3.5 4
3528 G12
–2.00
–0.50
–1.00
–1.75
–0.25
–2.25
–0.75
–1.25
234.5 5
NORMALIZED TO VOUT = 3V
VOUT (V)
1.5
300
350
450
34
3528 G13
250
200
2 2.5 3.5 4.5 5
150
100
400
RDS(ON) (mΩ)
PMOS
NMOS
TEMPERATURE (°C)
–50
–5
FREQUENCY CHANGE (%)
–4
–2
–1
0
5
2
–10 30 50
3528 G14
–3
3
4
1
–30 10 70 90
NORMALIZED TO 25°C
TEMPERATURE (°C)
–50 –30
–20
CHANGE (%)
0
30
–10 30 50
3528 G15
–10
20
10
10 70 90
TEMPERATURE (°C)
–50
1.180
VFB (V)
1.185
1.190
1.195
1.200
–30 –10 10 30
3528 G16
50 70 90
VOUT
20mV/DIV
2μs/DIV 3528 G19
VIN = 1.2V
VOUT = 3.3V
COUT = 22μF
CFF = 33pF
IOUT = 100mA
IL
200mA/DIV
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C unless otherwise noted)
Burst Mode Threshold Current
vs VIN
LTC3528/LTC3528B
6
3528f
PIN FUNCTIONS
Load Step Response (Fixed
Frequency, 1.2V to 3.3V)
Load Step Response (Burst Mode
Operation, 1.2V to 3.3V, LTC3528)
⎯
S
⎯
H
⎯
D
⎯
N (Pin 1): Logic Controlled Shutdown Input. There is
an internal 4MΩ pull-down resistor on this pin.
•
⎯
S
⎯
H
⎯
D
⎯
N = High: Normal operation
•
⎯
S
⎯
H
⎯
D
⎯
N = Low: Shutdown, quiescent current < 1μA
FB (Pin 2): Feedback Input. Connect resistor divider tap
to this pin. The output voltage can be adjusted from 1.6V
to 5.25V by:
VV
R
R
OUT =+
⎛
⎝
⎜⎞
⎠
⎟
120 1 2
1
.•
PGOOD (Pin 3): Power Good Comparator Output. This
open-drain output is low when VFB < 10% from its regu-
lation voltage.
VOUT (Pin 4): Output Voltage Sense and Drain Connection
of the Internal Synchronous Rectifi er. PCB trace length
from VOUT to the output fi lter capacitor (4.7μF minimum)
should be as short and wide as possible.
SW (Pin 5): Switch Pin. Connect inductor between SW
and VIN. Keep PCB trace lengths as short and wide as
possible to reduce EMI. If the inductor current falls to
zero, or
⎯
S
⎯
H
⎯
D
⎯
N is low, an internal anti-ringing switch is
connected from SW to VIN to minimize EMI.
PGND (Pin 6): Power Ground. Provide a short direct PCB
path between PGND and the (–) side of the input and
output capacitors.
SGND (Pin 7): Signal Ground. Provide a short direct PCB
path between SGND and the (–) side of the input and
output capacitors.
Load Step Response (Fixed
Frequency, 3.6V to 5V)
Load Step Response (Burst Mode
Operation, 3.6V to 5V, LTC3528)
VOUT and IIN During Soft-Start
Burst Mode Waveforms
5μs/DIV 3528 G20
VIN = 3.6V
VOUT = 5V
COUT = 22μF
CFF = 33pF
ILOAD = 30mA
VOUT
20mV/DIV
INDUCTOR
CURRENT
100mA/DIV
VOUT
1V/DIV
SHDN PIN
200μs/DIV 3528 G21
VIN = 1.2V
VOUT = 3.3V
COUT = 10μF
L = 4.7μH
IIN
200mA/DIV
VOUT
100mV/DIV
20μs/DIV 3528 G22
VIN = 3.6V
VOUT = 5V
COUT = 10μF
L = 4.7μH
LOAD
CURRENT
200mA/DIV
VOUT
100mV/DIV
50μs/DIV 3528 G23
VIN = 3.6V
VOUT = 5V
COUT = 10μF
L = 4.7μH
LOAD
CURRENT
200mA/DIV
VOUT
100mV/DIV
50μs/DIV 3528 G24
VIN = 1.2V
VOUT = 3.3V
COUT = 10μF
L = 4.7μH
LOAD
CURRENT
100mA/DIV
VOUT
100mV/DIV
50μs/DIV 3528 G25
VIN = 1.2V
VOUT = 3.3V
COUT = 10μF
L = 4.7μH
LOAD
CURRENT
100mA/DIV
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C unless otherwise noted)
LTC3528/LTC3528B
7
3528f
BLOCK DIAGRAM
8
Σ
+
–
+
–
GATE DRIVERS
AND
ANTI-CROSS
CONDUCTION
LOGIC
CLK
UVLO PK
PK
COMP
SLOPE
COMP
IZERO
COMP
ERROR AMP
SLEEP COMP
IZERO
WAKE
EXPOSED
PAD
+
–
WELL
SWITCH
MODE
CONTROL
(LTC3528)
UVLO
VREF VREF
4M
SHDN
VBEST
START-UP
1MHz
OSC
TSD
THERMAL
SHUTDOWN
SHUTDOWN
ANTI-RING
VSEL
VIN
5
4
SW
VOUT
L1
4.7μH
VB
SHUTDOWN
CLAMP
BURST
SOFT-START
VREF
+
–
VREF – 10%
FB
FB
VOUT
2
9
SGND
3528 BD
7
PGND
6
FB
R2
COUT
10μF
VOUT
1.6V
TO 5.25V
R1
1
PGOOD
3
CIN
4.7μF
VIN
0.7V
TO 5V
VIN (Pin 8): Battery Input Voltage. Connect a minimum
of 1μF ceramic decoupling capacitor from this pin to
ground.
Exposed Pad (Pin 9): The Exposed Pad must be soldered
to the PCB ground plane. It serves as another ground
connection and as a means of conducting heat away
from the die.
PIN FUNCTIONS
LTC3528/LTC3528B
8
3528f
OPERATION
The LTC3528/LTC3528B are 1MHz synchronous boost
converters housed in an 8-lead 3mm × 2mm DFN package.
With the ability to start-up and operate from inputs less
than 0.88V, the devices feature fi xed frequency, current
mode PWM control for exceptional line and load regula-
tion. The current mode architecture with adaptive slope
compensation provides excellent transient load response
and requires minimal output fi ltering. Internal soft-start and
internal loop compensation simplifi es the design process
while minimizing the number of external components.
With its low RDS(ON) and low gate charge internal N-chan-
nel MOSFET switch and P-channel MOSFET synchronous
rectifi er, the LTC3528 achieves high effi ciency over a wide
range of load current. Burst Mode operation maintains
high effi ciency at very light loads, reducing the quiescent
current to 12μA. Operation can be best understood by
referring to the Block Diagram.
LOW VOLTAGE START-UP
The LTC3528/LTC3528B includes an independent start-up
oscillator designed to operate at an input voltage of 0.70V
(typical). Soft-start and inrush current limiting are provided
during start-up, as well as normal operating mode.
When either VIN or VOUT exceeds 1.6V typical, the IC enters
normal operating mode. Once the output voltage exceeds
the input by 0.24V, the IC powers itself from VOUT instead
of VIN. At this point the internal circuitry has no dependency
on the VIN input voltage, eliminating the requirement for
a large input capacitor. The input voltage can drop as low
as 0.5V. The limiting factor for the application becomes
the availability of the power source to supply suffi cient
power to the output at the low voltages, and the maximum
duty cycle, which is clamped at 90% typical. Note that
at low input voltages, small voltage drops due to series
resistance become critical, and greatly limit the power
delivery capability of the converter.
LOW NOISE FIXED FREQUENCY OPERATION
Soft-Start
The LTC3528/LTC3528B contains internal circuitry to pro-
vide soft-start operation. The internal soft-start circuitry
slowly ramps the peak inductor current from zero to its
peak value of 1.5A (typical), allowing start-up into heavy
loads. The soft-start time is approximately 0.5ms. The
soft-start circuitry is reset in the event of a commanded
shutdown or a thermal shutdown.
Oscillator
An internal oscillator sets the frequency of operation to
1MHz.
Shutdown
The converter is shut down by pulling the
⎯
S
⎯
H
⎯
D
⎯
N pin below
0.25V, and activated by pulling
⎯
S
⎯
H
⎯
D
⎯
N above 0.88V. Note
that
⎯
S
⎯
H
⎯
D
⎯
N can be driven above VIN or VOUT, as long as it
is limited to less than the absolute maximum rating.
Error Amplifi er
The error amplifi er is a transconductance type. The nonin-
verting input is internally connected to the 1.20V reference
and the inverting input is connected to FB. Clamps limit
the minimum and maximum error amp output voltage for
improved large-signal transient response. Power converter
control loop compensation is provided internally. A voltage
divider from VOUT to ground programs the output voltage
via FB from 1.6V to 5.25V.
VV
R
R
OUT =+
⎛
⎝
⎜⎞
⎠
⎟
120 1 2
1
.•
Current Sensing
Lossless current sensing converts the peak current signal
of the N-channel MOSFET switch into a voltage which
is summed with the internal slope compensation. The
summed signal is compared to the error amplifi er output to
provide a peak current control command for the PWM.
Current Limit
The current limit comparator shuts off the N-channel
MOSFET switch once its threshold is reached. The current
limit comparator delay to output is typically 60ns. Peak
switch current is limited to approximately 1.5A, indepen-
dent of input or output voltage, unless VOUT falls below
0.7V, in which case the current limit is cut in half.
(Refer to Block Diagram)
LTC3528/LTC3528B
9
3528f
Zero Current Comparator
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifi er
when this current reduces to approximately 20mA. This
prevents the inductor current from reversing in polarity,
improving effi ciency at light loads.
Synchronous Rectifi er
To control inrush current and to prevent the inductor
current from running away when VOUT is close to VIN, the
P- channel MOSFET synchronous rectifi er is only enabled
when VOUT > (VIN + 0.24V).
Anti-Ringing Control
The anti-ringing control connects a resistor across the
inductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. The ringing
of the resonant circuit formed by L and CSW (capacitance
on SW pin) is low energy, but can cause EMI radiation.
Output Disconnect
The LTC3528/LTC3528B is designed to allow true output
disconnect by eliminating body diode conduction of the
internal P-channel MOSFET rectifi er. This allows for VOUT
to go to zero volts during shutdown, drawing no current
from the input source. It also enables inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output discon-
nect, a Schottky diode cannot be connected between SW
and VOUT. The output disconnect feature also allows VOUT
to be forced above the programmed regulation voltage,
without any reverse current into a battery on VIN.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3528/
LTC3528B will enter thermal shutdown. All switches will be
turned off and the soft-start capacitor will be discharged.
The device will be enabled again when the die temperature
drops by approximately 15°C.
Burst Mode OPERATION
The LTC3528 will automatically enter Burst Mode operation
at light load current and return to fi xed frequency PWM
mode when the load increases. Refer to the Typical Perfor-
mance Characteristics to see the output load Burst Mode
threshold vs VIN. The load at which Burst Mode operation
is entered can be changed by adjusting the inductor value.
Raising the inductor value will lower the load current at
which Burst Mode operation is entered.
In Burst Mode operation, the LTC3528 continues switch-
ing at a fi xed frequency of 1MHz, using the same error
amplifi er and loop compensation for peak current mode
control. This control method minimizes output transients
when switching between modes. In Burst Mode opera-
tion, energy is delivered to the output until it reaches the
nominal regulated value, then the LTC3528 transitions to
sleep mode where the outputs are off and the LTC3528
consumes only 12μA of quiescent current from VOUT. Once
the output voltage has drooped slightly, switching resumes
again. This maximizes effi ciency at very light loads by
minimizing switching and quiescent current losses. Burst
Mode output ripple, which is typically 1% peak-to-peak,
can be reduced by using more output capacitance (10μF
or greater).
As the load current increases, the LTC3528 automatically
leaves Burst Mode operation. Note that larger output ca-
pacitor values may cause this transition to occur at lighter
loads. The regulator will also leave Burst Mode operation if
a load transient occurs which causes the inductor current
to repeatedly reach current limit. Once the LTC3528 has
left Burst Mode operation and returned to normal opera-
tion, it will remain there until the output load is reduced
below the Burst threshold.
Burst Mode operation is inhibited during start-up and
until soft-start is done and VOUT is at least 0.24V greater
than VIN.
The LTC3528B features continuous PWM operation at
1MHz. At very light loads, the LTC3528B will exhibit
pulse-skip operation.
OPERATION
(Refer to Block Diagram)
LTC3528/LTC3528B
10
3528f
APPLICATIONS INFORMATION
VIN > VOUT OPERATION
The LTC3528/LTC3528B will maintain voltage regulation
even when the input voltage is above the desired output
voltage. Note that the effi ciency is much lower in this mode,
and the maximum output current capability will be less.
Refer to the Typical Performance Characteristics.
SHORT-CIRCUIT PROTECTION
The LTC3528/LTC3528B output disconnect feature allows
an output short circuit while maintaining a maximum
internally set current limit. To reduce power dissipation
under short-circuit conditions, the peak switch current
limit is reduced to 750mA (typical).
SCHOTTKY DIODE
Although not required, adding a Schottky diode from
SW to VOUT will improve effi ciency by about 2%. Note
that this defeats the output disconnect and short-circuit
protection features.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3528/LTC3528B de-
mands careful attention to board layout. A careless layout
will not produce the advertised performance. Figure 1
shows the recommended component placement. A large
ground copper area with the package backside metal pad
properly soldered will help to lower the chip temperature.
A multilayer board with a separate ground plane is ideal,
but not absolutely necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3528/LTC3528B can utilize small surface mount
chip inductors due to their fast 1MHz switching frequency.
Inductor values between 2.2μH and 4.7μH are suitable for
most applications. Larger values of inductance will allow
slightly greater output current capability (and lower the
Burst Mode threshold) by reducing the inductor ripple cur-
rent. Increasing the inductance above 10μH will increase
size while providing little improvement in output current
capability.
The minimum inductance value is given by:
LVV V
Ripple V
IN MIN OUT MAX IN MIN
>
()
() ( ) ()
•–
.• •12 OOUT MAX
μH
()
where:
Ripple = Allowable inductor current ripple (amps peak-
peak)
VIN(MIN) = Minimum input voltage
VOUT(MAX) = Maximum output voltage
SHDN
FB
PGOOD
LTC3528
VOUT
3528 F01
COUT
VIN
VIN CIN
SGND
PGND
SW
MULTIPLE VIAS
TO GROUND PLANE
8
5
6
7
1
4
3
2
+
Figure 1. Recommended Component Placement for Single Layer Board
LTC3528/LTC3528B
11
3528f
APPLICATIONS INFORMATION
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
types, improving effi ciency. The inductor should have low
ESR (series resistance of the windings) to reduce the I2R
power losses, and must be able to handle the peak induc-
tor current without saturating. Molded chokes and some
chip inductors usually do not have enough core area to
support the peak inductor currents of 1.5A seen on the
LTC3528/LTC3528B. To minimize radiated noise, use a
shielded inductor. See Table 1 for suggested components
and suppliers.
Table 1. Recommended Inductors
VENDOR PART/STYLE
Coilcraft
(847) 639-6400
www.coilcraft.com
LPO2506, MSS5131
MSS6122, MOS6020
ME3220, DO1608C
1812PS
Coiltronics SD14, SD18, SD20
SD25, SD52
Sumida
(847) 956-0666
www.sumida.com
CD43
CDC5D23B
CDRH5D18
CR43
TDK VLP, VLF
VLCF, SLF
Toko
(408) 432-8282
www.tokoam.com
D53, D63
D73, D75
Wurth
(201) 785-8800
www.we-online.com
WE-TPC type M, MH
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints.
A 10μF to 22μF output capacitor is suffi cient for most ap-
plications. Values larger than 22μF may be used to obtain
extremely low output voltage ripple and improve transient
response. X5R and X7R dielectric materials are preferred
for their ability to maintain capacitance over wide voltage
and temperature ranges. Y5V types should not be used.
The internal loop compensation of the LTC3528/LTC3528B
is designed to be stable with output capacitor values of 10μF
or greater. Although ceramic capacitors are recommended,
low ESR tantalum capacitors may be used as well.
A small ceramic capacitor in parallel with a larger tantalum
capacitor may be used in demanding applications which
have large load transients. Another method of improving
the transient response is to add a small feed-forward
capacitor across the top resistor of the feedback divider
(from VOUT to FB). A typical value of 33pF will generally
suffi ce.
Low ESR input capacitors reduce input switching noise
and reduce the peak current drawn from the battery. It
follows that ceramic capacitors are also a good choice
for input decoupling and should be located as close as
possible to the device. A 10μF input capacitor is suffi cient
for most applications. Larger values may be used without
limitations. Table 2 shows a list of several ceramic capaci-
tor manufacturers. Consult the manufacturers directly for
detailed information on their selection of ceramic parts.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE WEBSITE
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
Taiyo-Yuden (408) 573-4150 www.t-yuden.com
TDK (847) 803-6100 www.component.tdk.com
LTC3528/LTC3528B
12
3528f
TYPICAL APPLICATIONS
1 Cell to 1.8V
SW
VIN
499k
4.7μH
1M
4.7μF
10μF
33pF
3528 TA02a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
0.88V TO 1.6V
VOUT
1.8V
250mA
OFF ON
GND
Dual 1 Cell to 1.8V, 3V Sequenced Supply
SW
VIN
499k
475k
4.7μH
1M
4.7μF
10μF
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
0.88V TO 1.6V
VOUT1
1.8V
250mA
OFF ON
GND
SW
VIN
499k
4.7μH
324k
4.7μF
10μF
33pF
3528 TA03a
LTC3528
SHDN
PGOOD
VOUT
FB
VOUT2
3V
200mA
GND
33pF
Effi ciency
Output Voltage Sequencing
VOUT2
VOUT1
VIN
PGOOD1
200μs/DIV
0.5V/DIV
3528 TA03b
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA02b
70
60
0.1 1 10 1000
50
40
30
EFFICIENCY (%)
VIN = 0.9V
VIN = 1.2V
VIN = 1.5V
LTC3528/LTC3528B
13
3528f
TYPICAL APPLICATIONS
1 Cell to 3.3V Effi ciency
Effi ciency2 Cell to 3.3V
SW
VIN
499k
4.7
m
H
287k
4.7
m
F
10
m
F
33pF
3528 TA04a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
0.88V TO 1.6V
VOUT
3.3V
200mA
OFF ON
GND
SW
VIN
499k
4.7μH
287k
4.7μF
10μF
33pF
3528 TA05a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
1.8V TO 3.2V
VOUT
3.3V
400mA
OFF ON
GND
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA04b
70
60
0.1 1 10 1000
50
40
30
EFFICIENCY (%)
VIN = 0.9V
VIN = 1.2V
VIN = 1.5V
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA05b
70
60
0.1 1 10 1000
50
40
30
EFFICIENCY (%)
VIN = 1.8V
VIN = 2.4V
VIN = 3V
LTC3528/LTC3528B
14
3528f
TYPICAL APPLICATIONS
2 Cell to 5V Effi ciency
Effi ciencyLi-Ion to 5V
SW
VIN
1M
4.7μH
316k
4.7μF
22μF
3528 TA06a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
1.8V TO 3.2V
VOUT
5V
300mA
OFF ON
GND
SW
VIN
1M
4.7μH
316k
4.7μF
22μF
3528 TA07a
LTC3528
SHDN
PGOOD
VOUT
FB
VIN
2.7V TO 4.2V
VOUT
5V
400mA
OFF ON
GND
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA06b
70
60
0.1 1 10 1000
50
40
30
EFFICIENCY (%)
VIN = 1.8V
VIN = 2.4V
VIN = 3V
LOAD CURRENT (mA)
0.01
80
90
100
100
3528 TA07b
70
60
0.1 1 10 1000
50
40
30
EFFICIENCY (%)
VIN = 2.8V
VIN = 3.6V
VIN = 4.2V
LTC3528/LTC3528B
15
3528f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE DESCRIPTION
DDB Package
8-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1702 Rev B)
2.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
0.56 ± 0.05
(2 SIDES)
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
2.15 ±0.05
(2 SIDES)
3.00 ±0.10
(2 SIDES)
14
85
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0 – 0.05
(DDB8) DFN 0905 REV B
0.25 ± 0.05
0.50 BSC
PIN 1
R = 0.20 OR
0.25 × 45°
CHAMFER
0.25 ± 0.05
2.20 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.61 ±0.05
(2 SIDES)
1.15 ±0.05
0.70 ±0.05
2.55 ±0.05
PACKAGE
OUTLINE
0.50 BSC
LTC3528/LTC3528B
16
3528f
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
LT 0807 • PRINTED IN USA
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ThinSOT is a trademark of Linear Technology Corporation.
