LTM4657 datasheet(15/28 Pages) AD

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LTM4657

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Pre-Biased Output Start-Up

There may be situations that require the power supply to

start up with some charge on the output capacitors. The

LTM4657 can safely power up into a pre-biased output

without discharging it.

The LTM4657 accomplishes this by forcing discontinuous

mode (DCM) operation until the TRACK/SS pin voltage

reaches 0.5V reference voltage. This will prevent the BG

from turning on during the pre-biased output start-up

which would discharge the output.

SW Pins and Snubbering Circuit

The SW pin is generally for testing purposes by moni-

toring the pin. The SW pin can also be used to dampen

out switch node ringing caused by LC parasitic in the

switched current path. Usually a series R-C combination

is used called a snubber circuit. The resistor will dampen

the resonance and the capacitor is chosen to only affect

the high frequency ringing across the resistor.

If the stray inductance or capacitance can be measured

or approximated then a somewhat analytical technique

can be used to select the snubber values. The inductance

is usually easier to predict. It combines the power path

board inductance in combination with the MOSFET inter-

connect bond wire inductance.

First the SW pin can be monitored with a wide bandwidth

scope with a high frequency scope probe. The ring fre-

quency can be measured for its value. The impedance Z

can be calculated:

ZL = 2π • f • L

where f is the resonant frequency of the ring, and L is the

total parasitic inductance in the switch path. If a resistor

is selected that is equal to Z, then the ringing should be

dampened. The snubber capacitor value is chosen so that

its impedance is equal to the resistor at the ring frequency.

Calculated by:

ZC =

π • f •C

These values are a good place to start. Modification to

these components should be made to attenuate the ring-

ing with the least amount the power loss.

Stability Compensation

The LTM4657 has already been internally optimized and

compensated for all output voltages and capacitor combi-

nations including all ceramic capacitor applications when

COMPb is tied to COMPa. Please note that a 22pF to

47pF feedforward capacitor (CFF) is required connecting

from VOUT to VFB pin for all ceramic capacitor application

to achieve high bandwidth control loop compensation

with enough phase margin. Table 7 is provided for most

application requirements using the optimized internal

compensation. For specific optimized requirement, dis-

connect COMPb from COMPa and apply a Type II C-R-C

compensation network from COMPa to GND to achieve

external compensation. The LTpowerCAD design tool is

available to download online to perform specific control

loop optimization and analyze the control stability and

load transient performance.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is built into

the LTM4657 to sense output voltages accurately at the

remote load points. This is especially true for high current

loads. It is very important that the VOSNS+ and VOSNS– are

connected properly at the remote output sense point, and

the feedback resistor RFB is connected to between VFB

pin to VOSNS– pin. Review the schematics in Figure 23

for reference.

In multiphase single output application. Only one set of

differential sensing amplifier and one set of feedback

resistor are required while connecting RUN, TRACK/SS,

VOUT, VFB and COMPa of different channels together. See

Figure 25 for paralleling application.

Input Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTM4657 constantly

monitors each VIN pin for an overvoltage condition. When

VIN rises above 24.5V, the regulator suspends operation

by shutting off both power MOSFETs on the correspond-

ing channel. Once VIN drops below 21.5V, the regulator

immediately resumes normal operation. The regulator

executes its soft-start function when exiting an overvolt-

age condition.

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