LTM4645 datasheet(16/34 Pages) LINER

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LTM4645

4645f

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applicaTions inForMaTion

slave regulator. This will impose an offset on the TRACK/

SS pin input. Smaller value resistors with the same ratios

as the resistor values calculated from the above equation

can be used. For example, where the 60.4k is used then

a 6.04k can be used to reduce the TRACK/SS pin offset

to a negligible value.

The coincident output tracking can be recognized as a

special ratiometric output tracking in which the master’s

output slew rate (MR) is the same as the slave’s output

slew rate (SR), waveform as shown in Figure 7.

In parallel operation the RUN pins can be tie together and

controlled from a single control. The RUN pin can also be

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

that increases to 5µA during ramp-up. Please note that

the RUN pin has an ABSMAX voltage of 6V.

Differential Remote Sense Amplifier

Anaccuratedifferentialremotesenseamplifierisbuildinto

the LTM4645 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.

Power Good

The PGOOD pins are open-drain pins that can be used to

monitor valid output voltage regulation. This pin monitors

a ±7.5% window around the regulation point. A resistor

can be pulled up to a particular supply voltage no greater

than 6V maximum for monitoring.

Overvoltage and Overcurrent Protection

The LTM4645 has over current protection (OCP) in a

short circuit. The internal current comparator threshold

folds back during a short to reduce the output current. An

overvoltage condition (OVP) above 10% of the regulated

outputvoltagewillforcethetopMOSFEToffandthebottom

MOSFET on until the condition is cleared. Foldback cur-

rent limit is disabled during soft-start or tracking start-up.

Pre-Biased Output Start-Up

In the application that require the power supply to start

up with a pre-bias on the output capacitors, the LTM4645

module can safely power up into a pre-biased output

without discharging it.

TIME

MASTER OUTPUT

SLAVE OUTPUT

4645 F07

Figure 7. Output Coincident Tracking Waveform

From the equation, we could easily find that, in coincident

tracking,theslaveregulator’sTRACK/SSpinresistordivider

is always the same as its feedback divider:

RFB(SL)

RFB(SL) +60.4k

RTR(BOT)

RTR(TOP) +RTR(BOT)

For example, RTR(TOP) = 60.4k and RTR(BOT) = 60.4k is a

good combination for coincident tracking for a VOUT(MA)

=1 .5V and VOUT(SL) = 1.2V application.

Run Enable

The RUN pin has an enable threshold of 1.45V maximum,

typically 1.35V with 180mV 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.

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