LM25119PSQE - Texas Instruments High-Performance Analog

LM25119

September 28, 2010

Wide Input Range Dual Synchronous Buck Controller

General Description

The LM25119 is a dual synchronous buck controller intended

for step-down regulator applications from a high voltage or

widely varying input supply. The control method is based up-

on current mode control utilizing an emulated current ramp.

Current mode control provides inherent line feed-forward, cy-

cle-by-cycle current limiting and ease of loop compensation.

The use of an emulated control ramp reduces noise sensitivity

of the pulse-width modulation circuit, allowing reliable control

of very small duty cycles necessary in high input voltage ap-

plications. The switching frequency is programmable from

50kHz to 750kHz. The LM25119 drives external high-side and

low-side NMOS power switches with adaptive dead-time con-

trol. A user-selectable diode emulation mode enables discon-

tinuous mode operation for improved efficiency at light load

conditions. A high voltage bias regulator with automatic

switch-over to external bias further improves efficiency. Ad-

ditional features include thermal shutdown, frequency syn-

chronization, cycle-by-cycle and hiccup mode current limit

and adjustable line under-voltage lockout. The device is avail-

able in a power enhanced leadless LLP-32 package featuring

an exposed die attach pad to aid thermal dissipation.

Features

■Emulated peak current mode control

■Wide operating range from 4.5V to 42V

■Easily configurable for dual outputs or interleaved single

output

■Robust 3.3A peak gate drive

■Switching frequency programmable to 750kHz

■Optional diode emulation mode

■Programmable output from 0.8V

■Precision 1.5% voltage reference

■Programmable current limit

■Hiccup mode overload protection

■Programmable soft-start

■Programmable line under-voltage lockout

■Automatic switch-over to external bias supply

■Channel2 enable logic input

■Thermal Shutdown

■Leadless LLP32 (5mm x 5mm) package

Typical Application

30126201

LM25119 Wide Input Range Dual Synchronous Buck Controller

Connection Diagram

30126202

Top View

32–Lead LLP

Order Number Package Type NSC Package

Drawing

Supplied As

LM25119PSQ LLP-32 SQA32A 1000 Units on

Tape and Reel

LM25119PSQX LLP-32 SQA32A 4500 Units on

Tape and Reel

LM25119PSQE LLP-32 SQA32A 250 Units on Tape

and Reel

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LM25119

Pin Descriptions

Pin Name Description

1 VCC1 Bias supply pin. Locally decouple to PGND1 using a low ESR/ESL capacitor located as close to

controller as possible.

2 LO1 Low side MOSFET gate drive output. Connect to the gate of the channel1 low-side synchronous

MOSFET through a short, low inductance path.

3 PGND1 Power ground return pin for low side MOSFET gate driver. Connect directly to the low side of the

channel1 current sense resistor.

4 CSG1 Kelvin ground connection to the external current sense resistor. Connect directly to the low side of

the channel1 current sense resistor.

5 CS1 Current sense amplifier input. Connect to the high side of the channel1 current sense resistor.

6 RAMP1 PWM ramp signal. An external resistor and capacitor connected between the SW1 pin, the RAMP1

pin and the AGND pin sets the channel1 PWM ramp slope. Proper selection of component values

produces a RAMP1 signal that emulates the current in the buck inductor.

7 SS1 An external capacitor and an internal 10µA current source set the ramp rate of the channel1 error

amp reference. The SS1 pin is held low when VCC1 or VCC2 < 4V, UVLO < 1.25V or during thermal

shutdown.

8 VCCDIS Optional input that disables the internal VCC regulators when external biasing is supplied. If VCCDIS

>1.25V, the internal VCC regulators are disabled. The externally supplied bias should be coupled to

the VCC pins through a diode. VCCDIS has a 500kΩ pull-down resistor to ground to enable the VCC

regulators when the pin is left floating. The pull-down resistor can be overridden by pulling VCCDIS

above 1.25V with a resistor divider connected to the external bias supply.

9 FB1 Feedback input and inverting input of the channel1 internal error amplifier. A resistor divider from the

channel1 output to this pin sets the output voltage level. The regulation threshold at the FB1 pin is

0.8V.

10 COMP1 Output of the channel1 internal error amplifier. The loop compensation network should be connected

between this pin and the FB1 pin.

11 EN2 If the EN2 pin is low, channel2 will be disabled. Channel1 and all other functions remain active. The

EN2 has a 50kΩ pull-up resistor to enable channel2 when the pin is left floating.

12 AGND Analog ground. Return for the internal 0.8V voltage reference and analog circuits.

13 RT The internal oscillator is set with a single resistor between RT and AGND. The recommended

maximum oscillator frequency is 1.5MHz which corresponds to a maximum switching frequency of

750kHz for either channel. The internal oscillator can be synchronized to an external clock by

coupling a positive pulse into RT through a small coupling capacitor.

14 RES The restart timer pin for an external capacitor that configures the hiccup mode current limiting. A

capacitor on the RES pin determines the time the controller will remain off before automatically

restarting in hiccup mode. The two regulator channels operate independently. One channel may

operate in normal mode while the other is in hiccup mode overload protection. The hiccup mode

commences when either channel experiences 256 consecutive PWM cycles with cycle-by-cycle

current limiting. After this occurs, a 10µA current source charges the RES pin capacitor to the 1.25V

threshold which restarts the overloaded channel.

15 COMP2 Output of the channel2 internal error amplifier. The loop compensation network should be connected

between this pin and the FB2 pin.

16 FB2 Feedback input and inverting input of the channel2 internal error amplifier. A resistor divider from the

channel2 output to this pin sets the output voltage level. The regulation threshold at the FB2 pin is

0.8V.

17 DEMB Logic input that enables diode emulation when in the low state. In diode emulation mode, the low

side MOSFET is latched off for the remainder of the PWM cycle when the buck inductor current

reverses direction (current flow from output to ground). When DEMB is high, diode emulation is

disabled allowing current to flow in either direction through the low side MOSFET. A 50kΩ pull-down

resistor internal to the LM25119 holds DEMB pin low and enables diode emulation if the pin is left

floating.

18 SS2 An external capacitor and an internal 10µA current source set the ramp rate of the channel2 error

amp reference. The SS2 pin is held low when VCC1 or VCC2 < 4V, UVLO < 1.25V or during thermal

shutdown.

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LM25119

Pin Name Description

19 RAMP2 PWM ramp signal. An external resistor and capacitor connected between the SW2 pin, the RAMP2

pin and the AGND pin sets the channel2 PWM ramp slope. Proper selection of component values

produces a RAMP2 signal that emulates the current in the buck inductor.

20 CS2 Current sense amplifier input. Connect to the high side of the channel2 current sense resistor.

21 CSG2 Kelvin ground connection to the external current sense resistor. Connect directly to the low side of

the channel2 current sense resistor.

22 PGND2 Power ground return pin for low side MOSFET gate driver. Connect directly to the low side of the

channel2 current sense resistor.

23 LO2 Low side MOSFET gate drive output. Connect to the gate of the channel2 low-side synchronous

MOSFET through a short, low inductance path.

24 VCC2 Bias supply pin. Locally decouple to PGND2 using a low ESR/ESL capacitor located as close to

controller as possible.

25 SW2 Switching node of the buck regulator. Connect to channel2 bootstrap capacitor, the source terminal

of the high-side MOSFET and the drain terminal of the low-side MOSFET.

26 HO2 High side MOSFET gate drive output. Connect to the gate of the channel2 high-side MOSFET

through a short, low inductance path.

27 HB2 High-side driver supply for bootstrap gate drive. Connect to the cathode of the channel2 external

bootstrap diode and to the bootstrap capacitor. The bootstrap capacitor supplies current to charge

the high side MOSFET gate and should be placed as close to the controller as possible.

28 UVLO Under-voltage lockout programming pin. If the UVLO pin is below 0.4V, the regulator will be in the

shutdown mode with all function disabled. If the UVLO pin is greater than 0.4V and below 1.25V, the

regulator will be in standby mode with the VCC regulators operational, the SS pins grounded and no

switching at the HO and LO outputs. If the UVLO pin voltage is above 1.25V, the SS pins are allowed

to ramp and pulse width modulated gate drive signals are delivered at the LO and HO pins. A 20µA

current source is enabled when UVLO exceeds 1.25V and flows through the external UVLO resistors

to provide hysteresis.

29 VIN Supply voltage input source for the VCC regulators.

30 HB1 High-side driver supply for bootstrap gate drive. Connect to the cathode of the channel1 external

bootstrap diode and to the bootstrap capacitor. The bootstrap capacitor supplies current to charge

the high side MOSFET gate and should be placed as close to controller as possible.

31 HO1 High side MOSFET gate drive output. Connect to the gate of the channel1 high-side MOSFET

through a short, low inductance path.

32 SW1 Switching node of the buck regulator. Connect to channel1 bootstrap capacitor, the source terminal

of the high-side MOSFET and the drain terminal of the low-side MOSFET.

EP EP Exposed pad of LLP package. No internal electrical connections. Solder to the ground plane to reduce

thermal resistance.

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LM25119

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN to AGND -0.3 to 45V

SW1, SW2 to AGND -3.0 to 45V

HB1 to SW1, HB2 to SW2 -0.3 to 15V

VCC1, VCC2 to AGND

(Note 2)

-0.3 to 15V

FB1, FB2, DEMB, RES,

VCCDIS, UVLO to AGND

-0.3 to 15V

HO1 to SW1, HO2 to SW2 -0.3 to HB+0.3V

LO1, LO2 to AGND -0.3 to VCC+0.3V

SS1, SS2 to AGND -0.3 to 7V

EN2, RT to AGND -0.3 to 7V

CS1, CS2, CSG1, CSG2 to

AGND

-0.3V to 0.3V

PGND to AGND -0.3V to 0.3V

ESD Rating HBM (Note 3) 2kV

Storage Temperature -55°C to +150°C

Junction Temperature +150°C

Operating Ratings (Note 1)

VIN 4.5V to 42V

VCC 4.5V to 14V

HB to SW 4.5V to 14V

Junction Temperature -40°C to +125°C

Note: COMP1, COMP2, RAMP1, and RAMP2 are output pins. As such they

are not specified to have an external voltage applied.

Electrical Characteristics Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the

junction temperature range of -40°C to +125°C. Unless otherwise specified, the following conditions apply: VIN = 12V, VCC = 8V,

VCCDIS = 0V, EN2 = 5V, RT = 25kΩ, no load on LO and HO. Electrical characteristics are per channel where applicable. See

(Note 4) and (Note 5).

Symbol Parameter Conditions Min Typ Max Units

VIN Supply

IBIAS VIN Operating Current SS1 = SS2 = 0V 6 7.3 mA

VCCDIS = 2V, SS1 = SS2 = 0V 340 500 µA

IVCC VCC1 Operating Current VCCDIS = 2V, SS1 = SS2 = 0V 3.9 4.5 mA

VCC2 Operating Current VCCDIS = 2V, SS1 = SS2 = 0V 1.4 2.0 mA

ISHUTDOWN VIN Shutdown Current UVLO = 0V, SS1 = SS2 = 0V 15 33 µA

VCC Regulator (Note 6)

VCC(REG) VCC Regulation 6.77 7.6 8.34 V

VCC Regulation VIN = 4.5V, No external load 4.4 4.46 V

VCC Sourcing Current Limit VCC = 0V 25 40 mA

VCCDIS Switch Threshold VCCDIS Rising 1.19 1.25 1.29 V

VCCDIS Switch Hysteresis 0.07 V

VCCDIS Input Current VCCDIS = 0V -20 nA

VCC Under-voltage Threshold Positive going VCC 3.8 4.0 4.2 V

VCC Under-voltage Hysteresis 0.2 V

EN2 Input

VIL EN2 Input Low Threshold 2.0 1.5 V

VIH EN2 Input High Threshold 2.9 2.5 V

EN2 Input pull-up resistor 50 kΩ

UVLO

UVLO Threshold UVLO Rising 1.20 1.25 1.29 V

UVLO Hysterisis Current UVLO = 1.4V 15 20 25 µA

UVLO Shutdown Threshold 0.4 V

UVLO Shutdown Hysterisis Voltage 0.1 V

Soft Start

SS Current Source SS = 0V 710 13 µA

SS Pull Down RDSON 10 Ω

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LM25119

Symbol Parameter Conditions Min Typ Max Units

Error Amplifier

VREF FB Reference Voltage Measured at FB pin, FB = COMP 0.788 0.8 0.812 V

FB Input Bias Current FB = 0.8V 1 nA

FB Disable Threshold Interleaved Threshold 2.5 V

COMP VOH Isource = 3mA 2.8 V

COMP VOL Isink = 3mA 0.31 V

AOL DC Gain 80 dB

fBW Unity Gain Bandwidth 3 MHz

PWM Comparators

tHO(OFF) Forced HO Off-time 220 320 430 ns

tON(min) Minimum HO On-time CRAMP = 50pF 100 ns

Oscillator

fSW1 Frequency 1 RT = 25kΩ180 200 220 kHz

fSW2 Frequency 2 RT = 10kΩ430 480 530 kHz

RT Output Voltage 1.25 V

RT Sync Positive Threshold 2.5 3.2 4V

Sync Pulse Minimum Width 100 ns

Current Limit

VCS(TH) Cycle-by-cycle Sense Voltage

Threshold (CS - CSG)

RAMP = 0 106 120 134 mV

CS Bias Current CS = 0V -70 -95 µA

Hiccup Mode Fault Timer 256 Cycles

RES

IRES RES current Source 9.7 µA

VRES RES threshold CRES Charging 1.20 1.25 1.30 V

Diode Emulation

VIL DEMB Input Low Threshold 2.0 1.65 V

VIH DEMB Input High Threshold 2.9 2.6 V

DEMB Input Pull-Down Resistance 50 kΩ

SW Zero Cross Threshold -5 mV

LO Gate Driver

VOLL LO Low-state Output Voltage ILO = 100mA 0.1 0.18 V

VOHL LO High-state Output Voltage ILO = -100mA, VOHL = VCC - VLO 0.17 0.26 V

LO Rise Time C-load = 1000pF 6 ns

LO Fall Time C-load = 1000pF 5 ns

IOHL Peak LO Source Current VLO = 0V 2.5 A

IOLL Peak LO Sink Current VLO = VCC 3.3 A

HO Gate Driver

VOLH HO Low-state Output Voltage IHO = 100mA 0.11 0.19 V

VOHH HO High-state Output Voltage IHO = -100mA, VOHH = VHB - VHO 0.18 0.27 V

HO Rise Time C-load = 1000pF 6 ns

HO Fall Time C-load = 1000pF 5 ns

IOHH Peak HO Source Current VHO = 0V, SW = 0, HB = 8V 2.2 A

IOLH Peak HO Sink Current VHO = VHB = 8V 3.3 A

HB to SW Under-voltage 3 V

HB DC Bias Current HB - SW = 8V 70 100 µA

SWITCHING CHARACTERISTICS

LO Fall to HO Rise Delay No load 70 ns

HO Fall to LO Rise Delay No load 60 ns

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LM25119

Symbol Parameter Conditions Min Typ Max Units

THERMAL

TSD Thermal Shutdown Rising 165 °C

Thermal Shutdown Hysteresis 25 °C

θJA Junction to Ambient 40 °C/W

θJC Junction to Case 4 °C/W

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and test conditions see the Electrical Characteristics

Table.

Note 2: These pins must not exceed VIN.

Note 3: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.

Note 4: All limits are guaranteed. All electrical characteristics having room temperature limits are tested during production at TA = 25°C. All hot and cold limits

are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control.

Note 5: Typical specifications represent the most likely parametric normal at 25°C operation.

Note 6: Per VCC Regulator

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LM25119

Typical Performance Characteristics

HO Peak Driver Current vs Output Voltage

30126203

LO Peak Driver Current vs Output Voltage

30126204

Driver Dead Time vs VCC

30126205

Driver Dead Time vs Temperature

30126206

VCC vs IVCC

30126207

Switching Frequency vs RT

30126208

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LM25119

Error Amp Gain and Phase vs Frequency

30126209

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LM25119

Block Diagram

30126210

FIGURE 1. Block Diagram

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LM25119

Detailed Operating Description

The LM25119 high voltage switching regulator features all of

the functions necessary to implement an efficient dual chan-

nel buck regulator that operates over a very wide input voltage

range. The LM25119 may be configured as two independent

regulators or as a single high current regulator with two inter-

leaved channels. This easy to use regulator integrates high-

side and low-side MOSFET drivers capable of supplying peak

currents of 2.5 Amps (VCC=8V). The regulator control

method is based on current mode control utilizing an emulat-

ed current ramp. Emulated peak current mode control pro-

vides inherent line feed-forward, cycle-by-cycle current

limiting and ease of loop compensation. The use of an emu-

lated control ramp reduces noise sensitivity of the pulse-width

modulation circuit, allowing reliable processing of the very

small duty cycles necessary in high input voltage applications.

The switching frequency is user programmable from 50kHz

to 750kHz. An oscillator/synchronization pin allows the oper-

ating frequency to be set by a single resistor or synchronized

to an external clock. An under-voltage lockout and channel2

enable pin allows either both regulators to be disabled or

channel2 to be disabled with full operation of channel1. Fault

protection features include current limiting, thermal shutdown

and remote shutdown capability. The under-voltage lockout

input enables both channels when the input voltage reaches

a user selected threshold and provides a very low quiescent

shutdown current when pulled low. The LLP32 package fea-

tures an exposed pad to aid in thermal dissipation.

High Voltage Start-Up Regulator

The LM25119 contains two internal high voltage bias regula-

tors, VCC1 and VCC2, that provide the bias supply for the

PWM controllers and gate drive for the MOSFETs of each

regulator channel. The input pin (VIN) can be connected di-

rectly to an input voltage source as high as 42 volts. The

outputs of the VCC regulators are set to 7.6V. When the input

voltage is below the VCC set-point level, the VCC output will

track the VIN with a small dropout voltage. If VCC1 is in an

under voltage condition, channel2 will be disabled. This in-

terdependence is necessary to prevent channel2 from run-

ning open loop in the single output interleaved mode when

the channel2 error amplifier is disabled (if either VCC is in UV,

both channels are disabled).

The outputs of the VCC regulators are current limited at 25mA

(minimum) output capability. Upon power-up, the regulators

source current into the capacitors connected to the VCC pins.

When the voltage at the VCC pins exceed 4.0V and the UVLO

pin is greater than 1.25V, both channels are enabled and a

soft-start sequence begins. Both channels remain enabled

until either VCC pin falls below 3.8V, the UVLO pin falls below

1.25V or the die temperature exceeds the thermal limit thresh-

old.

When operating at higher input voltages the bias power dis-

sipation within the controller can be excessive. An output

voltage derived bias supply can be applied to a VCC pins to

reduce the IC power dissipation. The VCCDIS input can be

used to disable the internal VCC regulators when external bi-

asing is supplied. If VCCDIS >1.25V, the internal VCC regu-

lators are disabled. The externally supplied bias should be

coupled to the VCC pins through a diode, preferably a Schot-

tky (low forward voltage). VCCDIS has a 500kΩ internal pull-

down resistance to ground for normal operation with no

external bias. The internal pull-down resistance can be over-

ridden by pulling VCCDIS above 1.25V through a resistor

divider connected to an external bias supply.

The VCC regulator series pass transistor includes a diode

between VCC and VIN that should not be forward biased in

normal operation.

If the external bias winding can supply VCC greater than VIN,

an external blocking diode is required from the input power

supply to the VIN pin to prevent the external bias supply from

passing current to the input supply through the VCC pins. For

VOUT between 5V and 14.5V, VOUT can be connected di-

rectly to VCC through a diode. For VOUT < 5V, a bias winding

on the output inductor can be added as shown in Figure 2.

30126211

FIGURE 2. VCC Bias Supply with Additional Inductor

Winding

In high voltage applications extra care should be taken to en-

sure the VIN pin does not exceed the absolute maximum

voltage rating of 45V. During line or load transients, voltage

ringing on the VIN line that exceeds the Absolute Maximum

Rating can damage the IC. Both careful PC board layout and

the use of quality bypass capacitors located close to the VIN

and AGND pins are essential.

UVLO

The LM25119 contains a dual level under-voltage lockout

(UVLO) circuit. When the UVLO pin is less than 0.4V, the

LM25119 is in shutdown mode. The shutdown comparator

provides 100mV of hysteresis to avoid chatter during transi-

tions. When the UVLO pin voltage is greater than 0.4V but

less than 1.25V, the controller is in standby mode. In the

standby mode the VCC bias regulators are active but the

controller outputs are disabled. This feature allows the UVLO

pin to be used as a remote enable/disable function. When the

VCC outputs exceed their respective under-voltage thresh-

olds (4V) and the UVLO pin voltage is greater than 1.25V, the

outputs are enabled and normal operation begins.

An external set-point voltage divider from the VIN to GND is

used to set the minimum VIN operating voltage of the regu-

lator. The divider must be designed such that the voltage at

the UVLO pin will be greater than 1.25V when the input volt-

age is in the desired operating range. UVLO hysteresis is

accomplished with an internal 20μA current source that is

switched on or off into the impedance of the set-point divider.

When the UVLO pin voltage exceeds 1.25V threshold, the

current source is activated to quickly raise the voltage at the

UVLO pin. When the UVLO pin voltage falls below the 1.25V

threshold, the current source is turned off causing the voltage

at the UVLO pin to quickly fall. The UVLO pin should not be

left floating.

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LM25119

Enable 2

The LM25119 contains an enable function allowing shutdown

control of channel2, independent of channel1. If the EN2 pin

is pulled below 2.0V, channel2 enters shutdown mode. If the

EN2 input is greater than 2.5V, channel2 returns to normal

operation. An internal 50kΩ pull-up resistor on the EN2 pin

allows this pin to be left floating for normal operation. The EN2

input can be used in conjunction with the UVLO pin to se-

quence the two regulator channels. If EN2 is held low as the

UVLO pin increases to a voltage greater than the 1.25V UVLO

threshold, channel1 will begin operation while channel2 re-

mains off. Both channels become operational when the UV-

LO, EN2, VCC1, and VCC2 pins are above their respective

operating thresholds. Either channel of the LM25119 can also

be disabled independently by pulling the corresponding SS

pin to AGND.

Oscillator and Sync Capability

The LM25119 switching frequency is set by a single external

resistor connected between the RT pin and the AGND pin

(RT). The resistor should be located very close to the device

and connected directly to the pins of the IC (RT and AGND).

To set a desired switching frequency (fSW) of each channel,

the resistor can be calculated from the following equation:

(1)

Where RT is in ohms and fSW is in Hertz. The frequency fSW

is the output switching frequency of each channel. The inter-

nal oscillator runs at twice the switching frequency and an

internal frequency divider interleaves the two channels with

180° phase shift between PWM pulses at the HO pins.

The RT pin can be used to synchronize the internal oscillator

to an external clock. The internal oscillator can be synchro-

nized by AC coupling a positive edge into the RT pin. The

voltage at the RT pin is nominally 1.25V and the voltage at

the RT pin must exceed 4V to trip the internal synchronization

pulse detector. A 5V amplitude signal and 100pF coupling

capacitor are recommended. Synchronizing at greater than

twice the free-running frequency may result in abnormal be-

havior of the pulse width modulator. Also, note that the output

switching frequency of each channel will be one-half the ap-

plied synchronization frequency.

Error Amplifiers and PWM

Comparators

Each of the two internal high-gain error amplifiers generates

an error signal proportional to the difference between the reg-

ulated output voltage and an internal precision reference

(0.8V). The output of each error amplifier is connected to the

COMP pin allowing the user to provide loop compensation

components. Generally a Type II network is recommended.

This network creates a pole at 0Hz, a mid-band zero, and a

noise reducing high frequency pole. The PWM comparator

compares the emulated current sense signal from the RAMP

generator to the error amplifier output voltage at the COMP

pin. Only one error amplifier is required when configuring the

controller as a two channel, single output interleaved regula-

tor. For these applications, the channel1 error amplifier (FB1,

COMP1) is configured as the master error amplifier. The

channel2 error amplifier must be disabled by connecting the

FB2 pin to the VCC2 pin. When configured in this manner the

output of the channel2 error amplifier (COMP2) will be dis-

abled and have a high output impedance. To complete the

interleaved configuration the COMP1 and the COMP2 pins

should be connected together to facilitate PWM control of

channel2 and current sharing between channels.

Ramp Generator

The ramp signal used in the pulse width modulator for current

mode control is typically derived directly from the buck switch

current. This switch current corresponds to the positive slope

portion of the inductor current. Using this signal for the PWM

ramp simplifies the control loop transfer function to a single

pole response and provides inherent input voltage feed-for-

ward compensation. The disadvantage of using the buck

switch current signal for PWM control is the large leading

edge spike due to circuit parasitics that must be filtered or

blanked. Also, the current measurement may introduce sig-

nificant propagation delays. The filtering, blanking time and

propagation delay limit the minimum achievable pulse width.

In applications where the input voltage may be relatively large

in comparison to the output voltage, controlling small pulse

widths and duty cycles are necessary for regulation. The

LM25119 utilizes a unique ramp generator which does not

actually measure the buck switch current but rather recon-

structs the signal. Representing or emulating the inductor

current provides a ramp signal to the PWM comparator that

is free of leading edge spikes and measurement or filtering

delays. The current reconstruction is comprised of two ele-

ments; a sample-and-hold DC level and the emulated induc-

tor current ramp as shown in Figure 3.

30126212

FIGURE 3. Composition of Current Sense Signal

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LM25119

The sample-and-hold DC level is derived from a measure-

ment of the recirculating current flowing through the current

sense resistor. The voltage across the sense resistor is sam-

pled and held just prior to the onset of the next conduction

interval of the buck switch. The current sensing and sample-

and-hold provide the DC level of the reconstructed current

signal. The positive slope inductor current ramp is emulated

by an external capacitor connected from RAMP pin to AGND

and a series resistor connected between SW and RAMP. The

ramp resistor should not be connected to VIN directly be-

cause the RAMP pin voltage rating could be exceeded under

high VIN conditions. The ramp created by the external resistor

and capacitor will have a slope proportional to the rising in-

ductor current plus some additional slope required for slope

compensation. Connecting the RAMP pin resistor to SW pro-

vides optimum slope compensation with a RAMP capacitor

slope that is proportional to VIN. This “adaptive slope com-

pensation” eliminates the requirement for additional slope

compensation circuitry with high output voltage set points and

frees the user from additional concerns in this area. The em-

ulated ramp signal is approximately linear and the ramp slope

is given by:

(2)

The factor of 10 in equation (2) corresponds to the internal

current sense amplifier gain of the LM25119. The K factor is

a constant which adds additional slope for robust pulse-width

modulation control at lower input voltages. In practice this

constant can be varied from 1 to 3. RS is the external sense

resistor value.

The voltage on the ramp capacitor is given by:

(3)

(4)

The approximation is the first order term in a Taylor Series

expansion of the exponential and is valid since tPERIOD is small

relative to the RAMP pin R-C time constant.

Multiplying (2) by tPERIOD to convert the slope to a peak volt-

age, and then equating (2) with (4) allows us to solve for

CRAMP:

(5)

Choose either CRAMP or RRAMP and use (5) to calculate the

other component.

The difference between the average inductor current and the

DC value of the sampled inductor current can cause instability

for certain operating conditions. This instability is known as

sub-harmonic oscillation, which occurs when the inductor rip-

ple current does not return to its initial value by the start of

next switching cycle. Sub-harmonic oscillation is normally

characterized by alternating wide and narrow pulses at the

switch node. The ramp equation above contains the optimum

amount of slope compensation, however extra slope com-

pensation is easily added by selecting a lower value for

RRAMP or CRAMP.

Current Limit

The LM25119 contains a current limit monitoring scheme to

protect the regulator from possible over-current conditions.

When set correctly, the emulated current signal is proportion-

al to the buck switch current with a scale factor determined by

the current limit sense resistor, RS, and current sense ampli-

fier gain. The emulated signal is applied to the current limit

comparator. If the emulated ramp signal exceeds 1.2V, the

present cycle is terminated (cycle-by-cycle current limiting).

Shown in Figure 4 is the current limit comparator and a sim-

plified current measurement schematic. In applications with

small output inductance and high input voltage, the switch

current may overshoot due to the propagation delay of the

current limit comparator. If an overshoot should occur, the

sample-and-hold circuit will detect the excess recirculating

current before the buck switch is turned on again. If the sam-

ple-and-hold DC level exceeds the internal current limit

threshold, the buck switch will be disabled and skip pulses

until the current has decayed below the current limit threshold.

This approach prevents current runaway conditions due to

propagation delays or inductor saturation since the inductor

current is forced to decay to a controlled level following any

current overshoot.

30126213

FIGURE 4. Current Limit and Ramp Circuit

13 www.national.com

LM25119

Hiccup Mode Current Limiting

To further protect the regulator during prolonged current limit

conditions, an internal counter counts the PWM clock cycles

during which cycle-by-cycle current limiting occurs. When the

counter detects 256 consecutive cycles of current limiting, the

regulator enters a low power dissipation hiccup mode with the

HO and LO outputs disabled. The restart timer pin, RES, and

an external capacitor configure the hiccup mode current lim-

iting. A capacitor on the RES pin (CRES) determines the time

the controller will remain in low power standby mode before

automatically restarting. A 10µA current source charges the

RES pin capacitor to the 1.25V threshold which restarts the

overloaded channel. The two regulator channels operate in-

dependently. One channel may operate normally while the

other is in the hiccup mode overload protection. The hiccup

mode commences when either channel experiences 256 con-

secutive PWM cycles with cycle-by-cycle current limiting. If

that occurs, the overloaded channel will turn off and remain

off for the duration of the RES pin timer.

The hiccup mode current limiting function can be disabled.

The RES configuration is latched during initial power-up when

UVLO is above 1.25V and VCC1 and VCC2 are above their

UV thresholds, determining hiccup or non-hiccup current lim-

iting. If the RES pin is tied to VCC at initial power-on, hiccup

current limit is disabled.

Soft-Start

The soft-start feature allows the regulator to gradually reach

the steady state operating point, thus reducing start-up

stresses and surges. The LM25119 will regulate the FB pin to

the SS pin voltage or the internal 0.8V reference, whichever

is lower. At the beginning of the soft-start sequence when SS

= 0V, the internal 10µA soft-start current source gradually in-

creases the voltage on an external soft-start capacitor (CSS)

connected to the SS pin resulting in a gradual rise of the FB

and output voltages.

Either regulator channel of the LM25119 can be disabled by

pulling the corresponding SS pin to AGND.

Diode Emulation

A fully synchronous buck regulator implemented with a free-

wheel MOSFET rather than a diode has the capability to sink

current from the output in certain conditions such as light load,

over-voltage or pre-bias startup. The LM25119 provides a

diode emulation feature that can be enabled to prevent re-

verse (drain to source) current flow in the low side free-wheel

MOSFET. When configured for diode emulation, the low side

MOSFET is disabled when reverse current flow is detected.

The benefit of this configuration is lower power loss at no load

or light load conditions and the ability to turn on into a pre-

biased output without discharging the output. The diode em-

ulation mode allows for start-up into pre-biased loads, since

it prevents reverse current flow as the soft-start capacitor

charges to the regulation level during startup. The negative

effect of diode emulation is degraded light load transient re-

sponse times. Enabling the diode emulation feature is rec-

ommended and allows discontinuous conduction operation.

The diode emulation feature is configured with the DEMB pin.

To enable diode emulation, connect the DEMB pin to ground

or leave the pin floating. If continuous conduction operation is

desired, the DEMB pin should be tied to either VCC1 or VCC2.

HO and LO Output Drivers

The LM25119 contains a high current, high-side driver and

associated high voltage level shift to drive the buck switch of

each regulator channel. This gate driver circuit works in con-

junction with an external diode and bootstrap capacitor. A

0.1µF or larger ceramic capacitor, connected with short traces

between the HB pin and SW pin, is recommended. During the

off-time of the high-side MOSFET, the SW pin voltage is ap-

proximately 0V and the bootstrap capacitor charges from

VCC through the external bootstrap diode. When operating

with a high PWM duty cycle, the buck switch will be forced off

each cycle for 320ns to ensure that the bootstrap capacitor is

recharged.

The LO and HO outputs are controlled with an adaptive dead-

time methodology which insures that both outputs are never

enabled at the same time. When the controller commands HO

to be enabled, the adaptive dead-time logic first disables LO

and waits for the LO voltage to drop. HO is then enabled after

a small delay. Similarly, the LO turn-on is disabled until the

HO voltage has discharged. This methodology insures ade-

quate dead-time for any size MOSFET.

Care should be exercised in selecting an output MOSFET

with the appropriate threshold voltage, especially if VCC is

supplied from the regulator output. During startup at low input

voltages the MOSFET threshold should be lower than the 4V

VCC under-voltage lockout threshold. Otherwise, there may

be insufficient VCC voltage to completely turn on the MOS-

FET as VCC under-voltage lockout is released during startup.

If the buck switch MOSFET gate drive is not sufficient, the

regulator may not start or it may hang up momentarily in a

high power dissipation state. This condition can be avoided

by selecting a MOSFET with a lower threshold voltage or if

VCC is supplied from an external source higher than the out-

put voltage. If the minimum input voltage programmed by the

UVLO pin resistor divider is above the VCC regulation level,

this precaution is of no concern.

Maximum Duty Cycle

When operating with a high PWM duty cycle, the buck switch

will be forced off each cycle for 320ns to ensure the boot-strap

capacitor is recharged and to allow time to sample and hold

the current in the low side MOSFET. This forced off-time limits

the maximum duty cycle of the controller. When designing a

regulator with high switching frequency and high duty cycle

requirements, a check should be made of the required maxi-

mum duty cycle (including losses) against the graph shown

in Figure 5.

The actual maximum duty cycle will vary with the operating

frequency as follows:

(6)

www.national.com 14

LM25119

30126214

FIGURE 5. Maximum Duty Cycle vs Switching Frequency

Thermal Protection

Internal thermal shutdown circuitry is provided to protect the

integrated circuit in the event the maximum junction temper-

ature is exceeded. When activated, typically at 165°C, the

controller is forced into a low power reset state, disabling the

output driver and the VCC bias regulators. This feature is de-

signed to prevent catastrophic failures from overheating and

destroying the device.

Application Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is il-

lustrated with the following design example. Only the values

for the 3.3V output are calculated since the procedure is the

same for the 1.8V output. The circuit shown in Figure 14 is

configured for the following specifications:

•CH1 output voltage, VOUT1 = 3.3V

•CH2 output voltage, VOUT2 = 1.8V

•CH1 maximum load current, IOUT1 = 8A

•CH2 maximum load current, IOUT2 = 8A

•Minimum input voltage, VIN(MIN) = 6V

•Maximum input voltage, VIN(MAX) = 36V

•Switching frequency, fSW = 230kHz

Some component values were chosen as a compromise be-

tween the 3.3V and 1.8V outputs to allow identical compo-

nents to be used on both outputs. This design can be

reconfigured in a dual-channel interleaved configuration with

a single 3.3V output which requires identical power channels.

TIMING RESISTOR

RT sets the switching frequency of each regulator channel.

Generally, higher frequency applications are smaller but have

higher losses. Operation at 230kHz was selected for this ex-

ample as a reasonable compromise between small size and

high efficiency. The value of RT for 230kHz switching fre-

quency can be calculated as follows:

(7)

A standard value or 22.1kΩ was chosen for RT. The internal

oscillator frequency is twice the switching frequency and is

about 460kHz.

OUTPUT INDUCTOR

The inductor value is determined based on the operating fre-

quency, load current, ripple current and the input and output

voltages.

30126215

FIGURE 6. Inductor Current

Knowing the switching frequency, maximum ripple current

(IPP), maximum input voltage and the nominal output voltage

(VOUT), the inductor value can be calculated:

(8)

The maximum ripple current occurs at the maximum input

voltage. Typically, IPP is 20% to 40% of the full load current.

When operating in the diode emulation mode configuration,

the maximum ripple current should be less than twice the

minimum load current. For full synchronous operation, higher

ripple current is acceptable. Higher ripple current allows for a

smaller inductor size, but places more of a burden on the out-

put capacitor to smooth the ripple current. For this example,

a ripple current of 25% of 8A was chosen as a compromise

for the 1.8V output.

(9)

The nearest standard value of 6.8μH was chosen for L. Using

the value of 6.8µH for L, calculate IPP again. This step is nec-

essary if the chosen value of L differs significantly from the

calculated value.

(10)

(11)

CURRENT SENSE RESISTOR

Before determining the value of current sense resistor (RS), it

is valuable to understand the K factor, which is the ramp slope

multiple chosen for slope compensation. The K factor can be

varied from 1 to 3 in practice and is defined as:

15 www.national.com

LM25119

(12)

The performance of the converter will vary depending on the

selected K value (See Table 1). For this example, 3 was cho-

sen as the K factor to minimize the power loss in sense

resistor RS and the cross-talk between channels. Crosstalk

between the two regulators under certain conditions may be

observed on the output as switch jitter.

The maximum output current capability (IOUT(MAX)) should be

20~50% higher than the required output current, (8A at

VOUT1) to account for tolerances and ripple current. For this

example, 130% of 8A was chosen (10.4A). The current sense

resistor value can be calculated as:

(13)

(14)

Where VCS(TH) is the current limit threshold voltage (120mV).

A value of 8mΩ was chosen for RS. The sense resistor must

be rated to handle the power dissipation at maximum input

voltage when current flows through the free-wheel MOSFET

for the majority of the PWM cycle. The maximum power dis-

sipation of RS can be calculated:

(15)

(16)

During output short condition, the worst case peak inductor

current is limited to:

(17)

(18)

Where tON(MIN) is the minimum HO on-time which is nominally

100ns. The chosen inductor must be evaluated for this con-

dition, especially at elevated temperature where the satura-

tion current rating of the inductor may drop significantly. At the

maximum input voltage with a shorted output, the valley cur-

rent must fall below VCS(TH) / RS before the high-side MOSFET

is allowed to turn on.

RAMP RESISTOR AND RAMP CAPACITOR

The value of ramp capacitor (CRAMP) should be less than 2nF

to allow full discharge between cycles by the discharge switch

internal to the LM25119. A good quality, thermally stable ce-

ramic capacitor with 5% or less tolerance is recommended.

For this design the value of CRAMP was set at the standard

capacitor value of 820pF. With the inductor, sense resistor

and the K factor selected, the value of the ramp resistor

(RRAMP) can be calculated as:

(19)

(20)

The standard value of 34kΩ was selected.

OUTPUT CAPACITORS

The output capacitors smooth the inductor ripple current and

provide a source of charge during transient loading condi-

tions. For this design example, a 680µF electrolytic capacitor

with 10mΩ ESR was selected as the main output capacitor.

The fundamental component of the output ripple voltage is

approximated as:

(21)

(22)

(23)

Two 22µF low ERS / ESL ceramic capacitors are placed in

parallel with the 680µF electrolytic capacitor, to further reduce

the output voltage ripple and spikes.

TABLE 1. Performance Variation by K Factor

K < 1 1 <— K —> 3 K > 3

Cross Talk

Sub-harmonic

oscillation may occur

Higher Lower

Introduces

additional pole near

cross-over

frequency

Peak Inductor Current with Short Output

Condition

Lower Higher

Inductor Size Smaller Larger

Power Dissipation of Rs Higher Lower

Efficiency Lower Higher

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LM25119

INPUT CAPACITORS

The regulator input supply voltage typically has high source

impedance at the switching frequency. Good quality input ca-

pacitors are necessary to limit the ripple voltage at the VIN

pin while supplying most of the switch current during the on-

time. When the buck switch turns on, the current into the buck

switch steps to the valley of the inductor current waveform,

ramps up to the peak value, and then drops to the zero at turn-

off. The input capacitance should be selected for RMS current

rating and minimum ripple voltage. A good approximation for

the required ripple current rating necessary is IRMS > IOUT / 2.

Seven 2.2μF ceramic capacitors were used for each channel.

With ceramic capacitors, the input ripple voltage will be trian-

gular. The input ripple voltage with one channel operating is

approximately:

(24)

(25)

The ripple voltage of the input capacitors will be reduced sig-

nificantly with dual channel operation since each channel

operates 180 degrees out of phase from the other. Capacitors

connected in parallel should be evaluated for RMS current

rating. The current will split between the input capacitors

based on the relative impedance of the capacitors at the

switching frequency.

When the converter is connected to an input power source, a

resonant circuit is formed by the line inductance and the input

capacitors. To minimize overshoot make CIN > 10 x LIN. The

characteristic source impedance (ZS) and resonant frequency

(fS) are:

(26)

(27)

Where LIN is the inductance of the input wire. The converter

exhibits negative input impedance which is lowest at the min-

imum input voltage:

(28)

The damping factor for the input filter is given by:

(29)

Where RIN is the input wiring resistance and ESR is the equiv-

alent series resistance of the input capacitors. When δ = 1,

the input filter is critically damped. This may be difficult to

achieve with practical component values. With δ < 0.2, the

input filter will exhibit significant ringing. If δ is zero or nega-

tive, there is not enough resistance in the circuit and the input

filter will sustain an oscillation. When operating near the min-

imum input voltage, a bulk aluminum electrolytic capacitor

across CIN may be needed to damp the input for a typical

bench test setup.

VCC CAPACITOR

The primary purpose of the VCC capacitor (CVCC) is to supply

the peak transient currents of the LO driver and bootstrap

diode as well as provide stability for the VCC regulator. These

peak currents can be several amperes. The recommended

value of CVCC should be no smaller than 0.47µF, and should

be a good quality, low ESR, ceramic capacitor located at the

pins of the IC to minimize potentially damaging voltage tran-

sients caused by trace inductance. A value of 1μF was se-

lected for this design.

BOOTSTRAP CAPACITOR

The bootstrap capacitor between the HB and SW pins sup-

plies the gate current to charge the high-side MOSFET gate

at each cycle’s turn-on and recovery charge for the bootstrap

diode. These current peaks can be several amperes. The

recommended value of the bootstrap capacitor is at least

0.1μF, and should be a good quality, low ESR, ceramic ca-

pacitor located at the pins of the IC to minimize potentially

damaging voltage transients caused by trace inductance. The

absolute minimum value for the bootstrap capacitor is calcu-

lated as:

(30)

Qg is the high-side MOSFET gate charge and ΔVHB is the

tolerable voltage droop on CHB, which is typically less than

5% of VCC. A value of 0.47μF was selected for this design.

SOFT START CAPACITOR

The capacitor at the SS pin (CSS) determines the soft-start

time (tSS), which is the time for the output voltage to reach the

final regulated value. The value of CSS for a given time is de-

termined from:

(31)

For this application, a value of 0.047μF was chosen for a soft-

start time of 3.8ms.

RESTART CAPACITOR

The restart pin sources 10µA into the external restart capac-

itor (CRES). The value of the restart capacitor is given by:

(32)

Where tRES is the time the LM25119 remains off before a

restart attempt in hiccup mode current limiting. For this appli-

cation, a value of 0.47µF was chosen for a restart time of

59ms.

OUTPUT VOLTAGE DIVIDER

RFB1 and RFB2 set the output voltage level, the ratio of these

resistors is calculated from:

(33)

2.21kΩ was chosen for RFB1 in this design which results in a

RFB2 value of 6.98kΩ for VOUT1 of 3.3V. A reasonable guide

is to select the value of RFB1 in the range between 500Ω and

10kΩ. The value of RFB1 should be large enough to keep the

total divider power dissipation small.

17 www.national.com

LM25119

30126259

FIGURE 7. Feedback Configuration

UVLO DIVIDER

The UVLO threshold is internally set to 1.25V at the UVLO

pin. The LM25119 is enabled when the system input voltage

VIN causes the UVLO pin to exceed the threshold voltage of

1.25V. When the UVLO pin voltage is below the threshold, the

internal 20μA current source is disabled. When the UVLO pin

voltage exceeds the 1.25V threshold, the 20μA current source

is enabled causing the UVLO pin voltage to increase, provid-

ing hysteresis. The values of RUV1 and RUV2 can be deter-

mined from the following equation:

(34)

(35)

VHYS is the desired UVLO hysteresis at VIN, and VIN in the

second equation is the desired UVLO release (turn-on) volt-

age. For example, if it is desired for the LM25119 to be

enabled when VIN reaches 5.6V, and the desired hysteresis

is 1.05V, then RUV2 should be set to 52.5kΩ and RUV1 should

be set to 15.1kΩ. For this application RUV2 was selected to be

52.3kΩ, RUV1was selected to be 15kΩ. The LM25119 can be

remotely shutdown by taking the UVLO pin below 0.4V with

an external open collector or open drain device. The outputs

and the VCC regulator are disabled in shutdown mode. Ca-

pacitor CFT provides filtering for the divider. A value of 100pF

was chosen for CFT. The voltage at the UVLO pin should nev-

er exceed 15V when using the external set-point divider. It

may be necessary to clamp the UVLO pin at high input volt-

ages.

30126257

FIGURE 8. UVLO Configuration

MOSFET SELECTION

Selection of the power MOSFETs is governed by the same

tradeoffs as switching frequency. Breaking down the losses

in the high-side and low-side MOSFETs is one way to com-

pare the relative efficiencies of different devices. When using

discrete SO-8 MOSFETs, generally the output current capa-

bility range is 2A to 10A. Losses in the power MOSFETs can

be broken down into conduction loss, gate charging loss, and

switching loss. Conduction loss PDC is approximately:

(36)

(37)

Where, D is the duty cycle and the factor of 1.3 accounts for

the increase in MOSFET on-resistance due to heating. Alter-

natively, the factor of 1.3 can be eliminated and the high

temperature on-resistance of the MOSFET can be estimated

using the RDS(ON) vs Temperature curves in the MOSFET

datasheet. Gate charging loss, PGC, results from the current

driving the gate capacitance of the power MOSFETs and is

approximated as:

(38)

Where Qg refers to the total gate charge of an individual

MOSFET, and ‘n’ is the number of MOSFETs. Gate charge

loss differs from conduction and switching losses in that the

actual dissipation occurs in the LM25119 and not in the MOS-

FET itself. Further loss in the LM25119 is incurred if the gate

driving current is supplied by the internal linear regulator.

Switching loss occurs during the brief transition period as the

MOSFET turns on and off. During the transition period both

current and voltage are present in the channel of the MOS-

FET. The switching loss can be approximated as:

(39)

Where tR and tF are the rise and fall times of the MOSFET.

The rise and fall times are usually mentioned in the MOSFET

datasheet or can be empirically observed with an oscillo-

scope. Switching loss is calculated for the high-side MOSFET

only. Switching loss in the low-side MOSFET is negligible be-

cause the body diode of the low-side MOSFET turns on

before the MOSFET itself, minimizing the voltage from drain

to source before turn-on. For this example, the maximum

drain-to-source voltage applied to either MOSFET is 36V.The

selected MOSFETs must be able to withstand 36V plus any

ringing from drain to source, and be able to handle at least

the VCC voltage plus any ringing from gate to source. A good

choice of MOSFET for the 36V input design example is the

SI7884. It has an RDS(ON) of 7.5mΩ and total gate charge of

21nC. In applications where a high step-down ratio is main-

tained in normal operation, efficiency may be optimized by

choosing a high-side MOSFET with lower Qg, and low-side

MOSFET with lower RDS(ON).

MOSFET SNUBBER

A resistor-capacitor snubber network across the low-side

MOSFET reduces ringing and spikes at the switching node.

Excessive ringing and spikes can cause erratic operation and

couple noise to the output. Selecting the values for the snub-

ber is best accomplished through empirical methods. First,

make sure the lead lengths for the snubber connections are

very short. Start with a resistor value between 5 and 50Ω.

Increasing the value of the snubber capacitor results in more

damping, but higher snubber losses. Select a minimum value

for the snubber capacitor that provides adequate damping of

www.national.com 18

LM25119

the spikes on the switch waveform at high load. A snubber

may not be necessary with an optimized layout.

ERROR AMPLIFIER COMPENSATION

RCOMP, CCOMP and CHF configure the error amplifier gain

characteristics to accomplish a stable voltage loop gain. One

advantage of current mode control is the ability to close the

loop with only two feedback components, RCOMP and CCOMP.

The voltage loop gain is the product of the modulator gain and

the error amplifier gain. For the 3.3V output design example,

the modulator is treated as an ideal voltage-to-current con-

verter. The DC modulator gain of the LM25119 can be mod-

eled as:

(40)

Note that A is the gain of the current sense amplifier which is

10 in the LM25119. The dominant low frequency pole of the

modulator is determined by the load resistance (RLOAD) and

output capacitance (COUT). The corner frequency of this pole

is:

(41)

For RLOAD = 3.3V / 8A = 0.413Ω and COUT = 724μF (effective)

then fP(MOD) = 532Hz

DC Gain(MOD) = 0.413Ω / (10 x 8mΩ) = 5.16 = 14.2dB

For the 3.3V design example, the modulator gain vs. frequen-

cy characteristic is shown in Figure 9.

30126216

FIGURE 9. Modulator Gain and Phase

Components RCOMP and CCOMP configure the error amplifier

as a Type II configuration. The DC gain of the amplifier is

80dB with a pole at 0Hz and a zero at fZEA = 1 / (2π x

RCOMP x CCOMP). The error amplifier zero cancels the modu-

lator pole leaving a single pole response at the crossover

frequency of the voltage loop. A single pole response at the

crossover frequency yields a very stable loop with 90 degrees

of phase margin. For the design example, a conservative tar-

get loop bandwidth (crossover frequency) of 11kHz was se-

lected. The compensation network zero (fZEA) should be

selected at least an order of magnitude less than the target

crossover frequency. This constrains the product of RCOMP

and CCOMP for a desired compensation network zero 1 / (2π

x RCOMP x CCOMP) to be about 1.1kHz. Increasing RCOMP,

while proportionally decreasing CCOMP, increases the error

amp gain. Conversely, decreasing RCOMP while proportionally

increasing CCOMP, decreases the error amp gain. For the de-

sign example CCOMP was selected as 6800pF and RCOMP was

selected as 36.5kΩ. These values configure the compensa-

tion network zero at 640Hz. The error amp gain at frequencies

greater than fZEA is: RCOMP / RFB2, which is approximately 5.22

(14.3dB).

30126217

FIGURE 10. Error Amplifier Gain and Phase

The overall voltage loop gain can be predicted as the sum (in

dB) of the modulator gain and the error amp gain.

30126218

FIGURE 11. Overall Voltage Loop Gain and Phase

If a network analyzer is available, the modulator gain can be

measured and the error amplifier gain can be configured for

the desired loop transfer function. If the K factor is between 2

and 3, the stability should be checked with the network ana-

lyzer. If a network analyzer is not available, the error amplifier

compensation components can be designed with the guide-

lines given. Step load transient tests can be performed to

verify acceptable performance. The step load goal is mini-

mum overshoot with a damped response. CHF can be added

to the compensation network to decrease noise susceptibility

of the error amplifier. The value of CHF must be sufficiently

small since the addition of this capacitor adds a pole in the

19 www.national.com

LM25119

error amplifier transfer function. This pole must be well be-

yond the loop crossover frequency. A good approximation of

the location of the pole added by CHF is: fP2 = fZEA x CCOMP /

CHF. The value of CHF was selected as 100pF for the design

example.

Miscellaneous Functions

EN2 is left floating which allows channel2 to always remain

enabled. If EN2 is pulled below 2V, channel2 is disabled.

The DEMB pin is left floating since this design uses diode

emulation. For fully synchronous (continuous conduction) op-

eration, connect the DEMB to a voltage greater than 2.6V.

VCCDIS is left floating to enable the internal VCC regulators.

To disable the internal VCC regulators, connect this pin to a

voltage greater than 1.25V.

Interleaved Operation

Interleaved operation can offer many advantages in single

output, high current applications. The output power path is

split between two identical channels reducing the current in

each channel by one-half. Ripple current reduction in the out-

put capacitors is reduced significantly since each channel

operates 180 degrees out of phase from the other. Ripple re-

duction is greatest at 50% duty cycle and decreases as the

duty cycle varies away from 50%.

Refer to Figure 12 to estimate the ripple current reduction.

Also, the effective ripple in the input and output capacitors

occurs at twice the frequency of a single channel design due

to the combining of the two channels. All of these factors are

advantageous in managing the higher currents and their ef-

fects in a high power design.

30126219

FIGURE 12. Cancellation Factor vs. Duty Cycle for Output

Capacitor

To begin an interleaved design, use the previous equations

in this datasheet to first calculate the required value of com-

ponents using one-half the current in the output power path.

The Attenuation Factor in Figure 12 is the ratio of the output

capacitor ripple to the inductor ripple vs. duty cycle. The in-

ductor ripple used in this calculation is the ripple in either

inductor in a two phase design, not the ripple calculated for a

single phase design of the same output power. It can be ob-

served that operation around 50% duty cycle results in almost

complete ripple attenuation in the output capacitor. Figure

12 can be used to calculate the amount of ripple attenuation

in the output capacitors.

30126220

FIGURE 13. Normalized Input Capacitor RMS Ripple

Current vs. Duty Cycle

Figure 13 illustrates the ripple current reduction in the input

capacitors due to interleaving. As with the output capacitors,

there is near perfect ripple reduction near 50% duty cycle.

This plot can be used to calculate the ripple in the input ca-

pacitors at any duty cycle. In designs with large duty cycle

swings, use the worst case ripple reduction for the design.

To configure the LM25119 for interleaved operation, connect

COMP1 and COMP2 pins together at the IC. Connecting the

FB2 pin to VCC2 pin will disable the channel2 error amplifier

with a high output impedance at COMP2. Connect the com-

pensation network between FB1 and the common COMP

pins. Connect the two power stages together at the output

capacitors. Finally use the plots in Figure 12 and Figure 13

along with the duty cycle range to determine the amount of

output and input capacitor ripple reduction. Frequently more

capacitance than necessary is used in a design just to meet

ESR requirements. Reducing the capacitance based solely

on ripple reduction graphs alone may violate this requirement.

In the LM25119 evaluation board (schematic shown in Figure

14) interleaved operation can be enabled by shorting both

outputs together (with identical components in the power

train), and using zero ohm resistors for R22 and R21. This

shorts VCC2 to FB2 and COMP2 to COMP1 respectively. Al-

so the channel2 feedback network C14, R6, and C15 should

be removed. The easy re-configuration between two channel

and single channel operation will allow insight into the benefits

of interleaved operation.

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LM25119

30126221

FIGURE 14. 3.3V 8A, 1.8V 8A Typical Application Schematic

21 www.national.com

LM25119

PCB Board Layout

Recommendations

The LM25119 consists of two integrated regulators operating

almost independently. Crosstalk between the two regulators

under certain conditions may be observed as switch jitter.

This effect is common for any dual channel regulator. Cross-

talk effects are usually most severe when one channel is

operating around 50% duty cycle. Careful layout practices

help to minimize this effect. The following board layout guide-

lines apply specifically to the LM25119 and should be fol-

lowed for best performance.

1. Keep the Loop1 and Loop2, shown in Figure 15, as small

as possible

2. Keep the signal and power grounds separate

3. Place VCC capacitors (C6, C7) and VIN capacitor (C9)

as closes as possible to the LM25119

4. Route CS and CSG traces together with Kelvin

connection to the sense resistor

5. Connect AGND and PGND directly to the underside

exposed pad

6. Ensure there are no high current paths beneath the

underside exposed pad

Switching Jitter Root Causes and Solutions

1. Noise coupling of the high frequency switching between

two channels through the input power rail

A. Keep the high current path as short as possible

B. Choose a FET with minimum lead inductance

C. Place local bypass capacitors (CIN1, CIN2) as close

as possible to the high-side FETs to isolate one

channel from the high frequency noise of the other

channel

D. Slow down the SW switching speed by increasing

gate resistors R29 and R30

E. Minimize the effective ESR/ESL of the input

capacitor by paralleling input capacitors

2. High frequency AC noise on FB, CS, CSG and COMP

A. Use the star ground PCB layout technique and

minimize the length of the high current path

B. Keep the signal traces away from the SW, HO, HB

traces and the inductor

C. Add an R-C filter between the CS and CSG pins

D. Place CS filter capacitor (C30, C31) next to the

LM25119 and on the same PCB layer as the

LM25119

3. Ground offset at the switching frequency

A. Use the star ground PCB layout technique and

minimize the length between the grounds of CIN1and

CIN2

30126222

FIGURE 15. Recommended PCB Layout

www.national.com 22

LM25119

Physical Dimensions inches (millimeters) unless otherwise noted

32 Pin LLP

NS Package Number SQA32A

23 www.national.com

LM25119

Notes

LM25119 Wide Input Range Dual Synchronous Buck Controller

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