LTM4645 datasheet(20/34 Pages) LINER

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LTM4646

4646f

For more information www.linear.com/LTM4646

APPLICATIONS INFORMATION

The LTM4646 differential output sensing can correct for

up to ±300mV of common-mode deviation in the output’s

power and ground lines on channel 1, and ±200mV on

channel 2.

To avoid noise coupling into the feedback voltages, the

resistor dividers should be placed close to the VOUTS1 and

VOUTS1–, or VOUTS2 and VOUTS2– pins. Remote output and

ground traces should be routed together as a differential

pair to the remote output. For best accuracy, these traces

to the remote output and ground should be connected as

close as possible to the desired regulation point. Review

the parallel schematics in Figure 20.

OUTPUT CURRENT RANGE PIN (VRNG)

Tying the VRNG pin to SGND will set the output current to

5A, and ~9A current limit. Tying the VRNG pin to INTVCC

will set the output current to 10A, and ~20A current limit.

SW Pins

The SW pins are generally for testing purposes by moni-

toring these pins. These pins can also be used to dampen

out switch node ringing caused by LC parasitic in the

switched current paths. 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.

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 fre-

quency scope probe. The ring frequency can be measured

for its value. The impedance Z can be calculated:

Z(L) = 2πfL

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: Z(C) = 1/(2πfC). These values are a good

place to start with. Modification to these components

should be made to attenuate the ringing with the least

amount of power loss.

Temperature Monitoring (TEMP1 and TEMP2)

A diode connected PNP transistor is used for tempera-

ture monitoring. Measuring the absolute temperature of a

diode is possible due to the relationship between current,

voltage and temperature described by the classic diode

equation:

ID IS • e

• VT

• VT • ln

where ID is the diode current, VD is the diode voltage,

is the ideality factor (typically close to 1.0) and IS (satu-

ration current) is a process dependent parameter. VT can

be broken out to:

k • T

where T is the diode junction temperature in Kelvin, q is

the electron charge and k is Boltzmann’s constant. VT is

approximately 26mV at room temperature (298K) and

scales linearly with Kelvin temperature. It is this linear

temperature relationship that makes diodes suitable tem-

perature sensors. The IS term in the equation above is the

extrapolated current through a diode junction when the

diode has zero volts across the terminals. The IS term

varies from process to process, varies with temperature,

and by definition must always be less than ID. Combining

all of the constants into one term:

• k

where KD = 8.62 – 5, and knowing ln(ID/IS) is always

positive because ID is always greater than IS, leaves us

with the equation that:

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