LTC2925I(RevD) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC2925I
(Rev.:RevD)

Multiple Power Supply Tracking Controller with Power Good Timeout

Linear Technology 

LTC2925

Applications Information

Optional External FET

Figure 7 illustrates how an optional external N-channel

FET can ramp up a single supply that becomes the mas-

ter signal. When used, the FET’s gate is tied to the GATE

pin and its source is tied to the RAMP pin. Under normal

operation, the GATE pin sources or sinks 10µA to ramp

the FET’s gate up or down at a rate set by the external

capacitor connected to the GATE pin.

The series FET easily controls any supply with an output

voltage between 0V and VCC. See the Typical Applications

section for examples.

RSENSE

Q1

VIN

0.1µF

10Ω

CGATE

RONB

RONA

VIN

VCC SENSEP SENSEN GATE

ON

REMOTE

RAMP

PGI

SD1

FB1

MASTER

SUPPLY

MONITOR

RST

3.3V

RUN/SS IN

DC/DC

FB = 1.235V OUT

10k

VIN

10k

RTB1

RTA1

RTB2

RTA2 RTB3

RTA3

STATUS

LTC2925

SD2

FAULT

FB2

RAMPBUF

TRACK1

TRACK2

SD3

FB3

TRACK3

GND SCTMR SDTMR PGTMR

CSCTMR

CSDTMR

CPGTMR

RFA1

RFB1

3.3V

RUN/SS IN

DC/DC

FB = 0.8V OUT

RFA2

RFB2

3.3V

RUN/SS IN

DC/DC

FB = 0.8V OUT

RFB3

RFA3

2925 F07

Figure 7. Typical Application with External FET

1.8V

SLAVE1

2.5V

SLAVE2

1.5V

SLAVE3

Electronic Circuit Breaker

The LTC2925 features an electronic circuit breaker function

that protects the optional series FET against short circuits.

An external sense resistor is used to measure the current

flowing in the FET. If the voltage across the sense resistor

exceeds 50mV for more than a short-circuit timer cycle,

the gate of the FET is pulled low with 20mA, turning it off.

The short-circuit timer duration is configured by a capaci-

tor tied between SCTMR and GND. SCTMR will pull up

with 50µA when SENSEP – SENSEN > 50mV. Otherwise,

it pulls down with 2µA. When the voltage at SCTMR

exceeds 1.23V, the GATE will be pulled to ground with

20mA and the FAULT pin will be pulled low. Thus, the

capacitor, CSCTMR, required to configure the short-circuit

timer duration, tSCTMR is determined from:

CSCTMR = 50

A • tSCTMR

1.23V

Because the slave supplies track the RAMP pin which

is driven by the external FET, they are pulled low by the

tracking circuit when a short-circuit fault occurs. Following

a short-circuit fault, the FET is latched off and FAULT is

pulled low until the fault is cleared by pulling the ON pin

below 0.4V. Note that the supplies will not be allowed to

ramp up again until SCTMR has been pulled below about

100mV by the 2µA pull-down current source. The electronic

circuit breaker supports any supply voltage between 0V

and VCC. Although it is normally used to monitor current

through the optional series FET, it is capable of monitoring

other currents, including the current from a slave supply.

The Typical Applications section shows one such example.

If the electronic circuit breaker is not used, tie SENSEP

and SENSEN to VCC and SCTMR to GND.

Power Good Timeout

The power good timeout circuit turns off the supplies if an

external supply monitor, connected to the PGI pin, fails to

indicate that all supplies have entered regulation in time

after power up begins. After power up is complete, it turns

off the supplies if any supply exits regulation.

The power good timer duration is configured by a capacitor

tied between PGTMR and GND. PGTMR will pull up the

CPGTMR capacitor with 10µA starting when the ON pin

is driven above 1.23V. Once the voltage at the PGTMR

exceeds 1.23V, a fault will trip if the PGI pin is low. When

the power good timeout circuit detects a fault, the GATE

For more information www.linear.com/LTC2925

2925fd

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