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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

Run Enable

The RUN pin has an enable threshold of 1.40V maximum,

typically 1.25V with 150mV of hysteresis. It controls the

turn-on of the µModule. The RUN pin can be pulled up to

VIN for 5V operation, or a 5V Zener diode can be placed

on the pin and a 10k to 100k resistor can be placed up to

higher than 5V input for enabling the µModule. The RUN

pin can also be used for output voltage sequencing.

In parallel operation the RUN pins can be tied together and

controlled from a single control. See the Typical Applica-

tion circuits in Figures 20 and 21. The RUN pin can also

be left floating. The RUN pin has a 1µA pull-up current

source that increases to 4.5µA during ramp-up.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is provided

in the LTM4648 to sense low output voltages accurately

at the remote load points. This is especially true for high

current loads. It is very important that the DIFFP and

DIFFN are connected properly at the output, and DIFFOUT

is connected to VOUT_LCL. Review the parallel schematics

in Figures 20 and 21.

SW Pins

The SW pin is generally for testing purposes by monitor-

ing 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 interconnect

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 =

2π • f •C

These values are a good place to start with. Modification

to these components should be made to attenuate the

ringing with the least amount the power loss.

Temperature Monitoring

Measuring the absolute temperature of a diode is pos-

sible due to the relationship between current, voltage

and temperature described by the classic diode equation:

ID = IS • e

η • VT

⎛

⎝⎜

⎞

⎠⎟

VD = η• VT •ln

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

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

tion current) is a process dependent parameter. VT can

be broken out to:

VT =

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

temperature 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:

KD =

η • k

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