LTC4261 datasheet(17/74 Pages) AD

17 / 74 page

LTC4283

Rev. B

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OPERATION

The LTC4283 is designed to turn a board’s supply voltage

on and off in a controlled manner, allowing the board to

be safely inserted or removed from a live, power sys-

tem. In normal operation after a startup debounce delay,

the LTC4283 turns on the external N-channel MOSFET,

passing the power to the load. The inrush control dur-

ing startup is configurable between two methods. One is

programmable active current limiting with an adjustable

foldback factor. The other is constant dV/dt ramp control

of the output voltage using a capacitor connected between

RAMP and VEE. The inrush current is a function of the

RAMP capacitor, the load capacitor and the attenuated

load voltage seen between RTNS and DRNS.

An 11.5V shunt regulator on VIN powers the LTC4283

with an external dropping resistor from the system RTN

node. It also provides the gate drive. An optional buffer

transistor driven by VZ boosts sourcing capability to sup-

ply external loads.

An internally generated 5V supply on INTVCC supplies

the logic control circuits, communication interface, data

converters and EEPROM. Prior to turning on the MOSFET,

both VIN and INTVCC voltages must exceed their under-

voltage lockout thresholds. In addition, the control inputs

UVH, UVL, OV, EN#, PGIO3 and PGIO4 are monitored by

comparators. The MOSFET is held off until all startup con-

ditions are met.

The DRAIN, RTNS − DRNS and GATE voltages are moni-

tored to determine if power is available for the load. Two

power good signals are sequenced on PGIO1 and PGIO2,

each with a delay that is twice the startup debounce delay.

Additionally, PGIO3 serves as a watchdog input to monitor

the output of the DC/DC module. If the module output fails

to come up, the LTC4283 turns off the MOSFET. PGIO4

defaults as an external fault input (inverted). PGIO1-4 can

also be configured into general purpose inputs or outputs.

An overcurrent fault at the output may result in excessive

MOSFET power dissipation during Active Current Limiting

(ACL). To limit this power, the ACL amplifier regulates

the SENSE+ – SENSE− voltage at precise, programmable

values (15mV to 30mV in 1mV steps). When the output

voltage is low, power dissipation is further reduced by

folding back the current limit, with the foldback ratio con-

figurable to 10%, 20%, or 50% of nominal. In the event

of a catastrophic output short when the sensed current

is twice of the current limit, fast response comparator

immediately pulls the GATE pin down with 2.3A.

When active current limiting is engaged, TMR is pulled up

by a current that is proportional to the power dissipation

in the MOSFET. With an RC network representing the ther-

mal behavior of the MOSFET connected between TMR and

VEE, the TMR voltage is proportional to the temperature

rise in the MOSFET. When TMR voltage reaches its thresh-

old of 2.048V (representing TJ(MAX) of the MOSFET), the

overcurrent fault is logged and GATE turns off, allowing

protection of the MOSFET based on true SOA. TMR can

also be configured to drive a single capacitor. Following

the overcurrent fault, the LTC4283 can either latch off the

MOSFET or auto-retry after a cooling delay. Both the retry

delay and the number of retries are configurable, too. The

LTC4283 also logs and responds to other faults including

overvoltage, undervoltage, FET bad, Power Good Input

(PGI) fault, FET short and external fault.

Included in the LTC4283 is a pair of analog to digital con-

verters (ADCs). The ADCs are configurable from 8-bit at

1kHz to 16-bit at 1Hz in five settings. As shown in the

Block Diagram, ADC1 continuously monitors the current

sense voltage between SENSE+ and SENSE−. ADC2 is syn-

chronized to ADC1 and measures the attenuated input

voltage at RTNS or the attenuated MOSFET drain voltage

at DRNS plus one of the fourteen auxiliary inputs. Every

time the ADCs finish taking a measurement, the current

sense voltage is multiplied by the measurement of the

RTNS or DRNS voltage to provide a power measurement.

Every time power is measured, it is added to an energy

accumulator that tracks the input energy or the energy

consumption of the MOSFET. The energy accumulator

can generate an optional alert upon overflow, and can

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