LTC3613 datasheet(14/44 Pages) AD

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LTC7131-1

Rev. 0

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OPERATION

Main Control Loop

The LTC7131-1 is a 25A current mode monolithic step-down

regulator with PMBus Interface. The accurate 100µA current

source on the REF pin allows the user to use just one external

resistor to program the output voltage. In normal opera-

tion, the internal top power MOSFET is turned on for a fixed

interval determined by a one-shot timer, OST. When the top

power MOSFET turns off, the bottom power MOSFET turns

on until the current comparator, ICMP, trips, restarting the

one-shot timer and initiating the next cycle. Inductor current

is determined by sensing the voltage drop across the bottom

power MOSFET’s VDS. The voltage on the ITH pin sets the

comparator threshold corresponding to the inductor valley

current. The error amplifier, EA, adjusts this ITH voltage by

comparing the feedback signal, FB, from the output volt-

age with that of voltage on the REF pin. If the load current

increases, it causes a drop in the feedback voltage relative

to the internal reference. The ITH voltage then rises until the

average inductor current matches that of the load current.

At low load current, the inductor current can drop to zero

and become negative. This is detected by current reversal

comparator, IREV, which then shuts off the bottom power

MOSFET, resulting in discontinuous operation. Both power

MOSFETs will remain off with the output capacitor sup-

plying the load current until the ITH voltage rises above

the zero current level (~0.6V) to initiate another cycle.

Discontinuous mode operation is enabled by tying the

MODE pin to INTVCC, which forces continuous synchro-

nous operation regardless of output load.

The operating frequency is determined by the value of the

RT resistor, which programs the current for the internal

oscillator. The internal phase-lock loop servos the switch-

ing regulator on-time to track the internal oscillator to

force constant switching frequency. If an external clock

signal is detected on the MODE/SYNC pin, the phase-lock

loop will servo the on-time to track the external clock

signal instead.

VOUT Margining

The LTC7131-1 has an internal 9-bit DAC that provides

up to ±25% adjustment at 0.1%/bit resolution around the

reference voltage set at the REF pin. The digital offset value

is changed with the MFR_VOUT_COMMAND command

through the PMBus interface. When a change in the ref-

erence is detected, the reference is ramped (0.1%/step)

from its current value to the new value at a rate set by the

capacitor value connected to the CSLEW pin, thus providing

programmable slew rate of the VOUT transition. To elim-

inate the latency of the PMBus transaction when faster

changes are required, the LTC7131-1 can be pre-loaded

with two additional offsets with the MFR_VOUT_MARGIN_

HIGH and MFR_VOUT_MARGIN_LOW commands. The

reference offset can then be switched between any of these

three register values with the 3-state MARGIN pin. When

using the MARGIN pin, the latency of the VOUT transition

is limited only by the chosen CSLEW capacitor and the loop

bandwidth of the power supply. Changes to these registers

are prevented by pulling the write protect (WP) pin high.

Telemetry Readback

The LTC7131-1 has an integrated 13-bit ADC that mon-

itors and performs conversions on the input and output

voltage, input and output current, and die temperature.

The values are refreshed at a 25Hz rate and are readable

through the PMBus interface.

A peak monitor is also available for each of these teleme-

try measurements to provide that highest value measured

since the start of the monitor. The monitor is reset by the

MFR_CLEAR_PEAKS command, writing to the individual

peak register, or de-asserting RUN_MSTR.

INTVCC Regulator

Power for the top and bottom MOSFET and most other

internal circuitry is derived from the INTVCC pin. The

regulated 5.0V on this pin is generated from an internal

low dropout regulator. The top MOSFET driver is biased

from the floating bootstrap capacitor, CB, which normally

recharges during the off cycle through an external diode

when the top MOSFET turns off.

Output Voltage Tracking and Soft-Start

The LTC7131-1 allows the user to program its output volt-

age ramp rate by means of the TRACK/SS pin. An inter-

nal 5µA pulls up the TRACK/SS pin to INTVCC. Putting an

external capacitor on TRACK/SS enables soft starting the

output to prevent current surge on the input supply. If no

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