LTC3815_15 Datasheet, PDF(14/42 Page) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

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LTC3815_15 Datasheet, PDF (14/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

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LTC3815

Operation

Main Control Loop

The LTC3815 is a 6A current mode monolithic step-down

regulator with PMBus Interface. The accurate 100ÂµA cur-

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

external resistor to program the output voltage. In normal

operation, 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 MOSFETâs 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,

VFB, from the output voltage 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 disabled by tying the

MODE pin to VIN, which forces continuous synchronous

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 switching

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 LTC3815 has an internal 9-bit DAC that provides up

to Â±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

reference 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 eliminate

the latency of the PMBus transaction when faster changes

are required, the LTC3815 can be pre-loaded with two

additional offsets with the MFR_VOUT_MARGIN_HIGH

and MFR_VOUT_MARGIN_LOW commands. The refer-

ence 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 LTC3815 has an integrated 13-bit ADC that monitors

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 telemetry

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.

Output Voltage Tracking and Soft-Start

The LTC3815 allows the user to program its output volt-

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

5ÂµA pulls up the TRACK/SS pin to VIN. Putting an external

capacitor on TRACK/SS enables soft starting the output to

prevent current surge on the input supply. If no capacitor

is connected or TRACK/SS pin is connected to VIN, the

ramp rate defaults to 1.1 volts/ms. For output tracking ap-

plications, TRACK/SS can be externally driven by another

voltage source. For TRACK/SS less than the output voltage

reference (set by the IREF resistor and margin register), the

TRACK/SS voltage will override the reference input to the

error amplifier, thus regulating the feedback voltage to that

of TRACK/SS pin. During this start-up time, the LTC3815

will operate in discontinuous mode. When TRACK/SS is

3815p

For more information www.linear.com/LTC3815

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