LTC2923IDE datasheet(9/20 Pages) LINER

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LTC2923

2923fa

Figure 6. Simplified Functional Diagram

The LTC2923 features an electronic circuit breaker func-

tion that protects the optional series FET against short

circuits. When the FET is fully enhanced (GATE > RAMP +

4.9V), the electronic circuit breaker is enabled. Then, if the

voltage across the FET (VDS) exceeds 200mV as measured

from VCC to the RAMP pin for more than about 10µs the

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

Because the slaved supplies track the RAMP pin, they are

pulled low by the tracking circuit when a short-circuit fault

occurs. Following a short-circuit fault, the FET is latched

off until the fault is cleared by pulling the ON pin below

0.4V.

Ramp Buffer

The RAMPBUF pin provides a buffered version of the

RAMP pin voltage that drives the resistive dividers on the

TRACKx pins. When there is no external FET, it provides up

to 2mA to drive the resistors even though the GATE pin

only supplies 10µA. The RAMPBUF pin also proves useful

in systems with an external FET. Since the track cell in the

simplified functional diagram above drives 0.8V on the

TRACKx pins, if RTBx is connected directly to the FET’s

source, the TRACKx pin could potentially pull up the FET’s

source towards 0.8V when the FET is off. RAMPBUF

blocks this path.

APPLICATIO S I FOR ATIO

–

+ –

VCC

200mV

GATE

MASTER

SLAVE1

SLAVE2

CGATE

1.2V

RAMPBUF

10µA

VCC

RONB

RONA

RTA1

RTB1

RTA2

RTB2

10µA

RAMP

FB1

TRACK1

0.8V

–

FB2

TRACK2

0.8V

RFA1

RFB1

DC/DC

RFA2

2923 F06

RFB2

DC/DC

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