LTC1702AIGN datasheet(25/36 Pages) LINER

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LTC1702A

1702afa

POWER GOOD/FAULT FLAGS

The PGOOD pins report the status of the output voltage at

their respective outputs. Each is an open-drain output that

pulls low until the FB pin rises to (VREF – 5%), indicating

that the output voltage has risen to within 5% of the

programmed output voltage. Each PGOOD pin can inter-

face directly to standard logic inputs if an appropriate pull-

up resistor is added, or the two pins can be tied together

with a single pull-up to give a “both good” signal. Each

PGOOD pin includes an internal 100

µs delay to prevent

glitches at the output from indicating false PGOOD

signals.

The FAULT pin is an additional open-drain output that

indicates if one or both of the outputs has exceeded 15%

of its programmed output voltage. FAULT includes an

internal 10

µA pull-up to VCC and does not require an

external pull-up to interface to standard logic. FAULT pulls

low in normal operation, and releases when a overvoltage

fault is detected.

When an overvoltage fault occurs, an internal latch sets

and FAULT goes high, disabling the LTC1702A until the

latch is cleared by recycling the power or pulling both

RUN/SS pins low simultaneously. Alternately, the FAULT

pin can be pulled back low externally with an open-

collector/open-drain device or an NFET or NPN, which will

allow the LTC1702A to resume normal operation, but will

not reset the latch. If the pull-down is later removed, the

LTC1702A will latch off again unless the latch is reset by

cycling the power or RUN/SS pins.

Note that both the PGOOD pins and the FAULT pin monitor

the output voltages by watching the FB pins. During

normal operation, each FB pin is held at a virtual ground by

the feedback amplifier, and changes at the output will not

appear at FB. This is not an issue with a properly designed

circuit, since the virtual ground at FB implies that the

output voltage is under control. If the feedback amplifier

loses control of the output, the virtual ground disappears

and the PGOOD circuit can see any output changes. This

occurs whenever the soft-start or current limit circuits are

active or any time the feedback amplifier output (the

COMP pin) hits a rail or is in slew limit.

APPLICATIONS INFORMATION

be added, either unregulated or with additional external

circuitry at the FCB pin.

The extra auxiliary output is taken from a second winding

on the core of the inductor on one channel, converting it

into a transformer (Figure 13). The auxiliary output voltage

is set by the main output voltage and the turns ratio of the

extra winding to the primary winding. Load regulation at

the auxiliary output will be relatively good as long as the

main output is running in continuous mode. As the load on

the main channel drops and the LTC1702A switches to

discontinuous or Burst Mode operation, the auxiliary

output will not be able to maintain regulation, especially if

the load at the auxiliary output remains heavy.

To avoid this, the auxiliary output voltage can be divided

down with a conventional feedback resistor string with the

divided auxiliary output voltage fed back to the FCB pin

(Figure 13). The FCB pin threshold is trimmed to 800mV

with 10mV of hysteresis, allowing fairly precise control of

the auxiliary voltage. If the LTC1702A is in discontinuous

or Burst Mode operation and the auxiliary output voltage

drops, the FCB pin will trip and the LTC1702A will resume

continuous operation regardless of the load on the main

output. The FCB pin removes the requirement that power

must be drawn from the inductor primary in order to

extract power from the auxiliary windings. With the loop in

continuous mode, the auxiliary outputs may be loaded

without regard to the primary load. Note that if the LTC1702A

is already running in continuous mode and the auxiliary

output drops due to excessive loading, no additional

action can be taken by the LTC1702A to regulate the

auxiliary output.

Figure 13. Regulating an Auxiliary Output with the FCB Pin

LTC1702A

FCB

COUT

RFCB1

COUT(AUX)

VOUT(AUX)

1702A F08

CIN

VOUT

VIN

RFCB2

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