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LTC1703

drops, the FCB pin will trip and the LTC1703 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 LTC1703

is already running in continuous mode and the auxiliary

output drops due to excessive loading, no additional

action can be taken by the LTC1703 to regulate the

auxiliary output.

Accuracy Trade-Offs

The VDS sensing scheme used in the LTC1703 is not

particularly accurate, primarily due to uncertainty in the

RDS(ON) from MOSFET to MOSFET. A second error term

arises from the ringing present at the SW pin, which

causes the VDS to look larger than (ILOAD)(RDS(ON)) at the

beginning of QB’s on-time. These inaccuracies do not

prevent the LTC1703 current limit circuit from protecting

itself and the load from damaging overcurrent conditions,

but they do prevent the user from setting the current limit

to a tight tolerance if more than one copy of the circuit is

being built. The 50% factor in the current setting equation

above reflects the margin necessary to ensure that the

circuit will stay out of current limit at the maximum normal

load, even with a hot MOSFET that is running quite a bit

higher than its RDS(ON) spec.

FCB OPERATION/SECONDARY WINDINGS

The FCB pin can be used in conjunction with a secondary

winding on one side of the LTC1703 to generate a third

regulated voltage output. This output can be directly

regulated at the FCB pin. In theory, a fourth output could

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 LTC1703 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 is 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 20mV of hysteresis, allowing fairly precise control of

the auxiliary voltage. If the LTC1703 is in discontinuous or

Burst Mode operation and the auxiliary output voltage

Figure 13. Regulating an Auxiliary Output with the FCB Pin

LTC1703

FCB

COUT

RFCB1

COUT(AUX)

VOUT(AUX)

1703 F13

CIN

VOUT

VIN

RFCB2

FAULT FLAG

The FAULT pin is an open-drain output that indicates if one

or both of the outputs has exceeded 15% of its pro-

grammed 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 LTC1703 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 N-channel MOSFET or

NPN, which will allow the LTC1703 to resume normal

operation, but will not reset the latch. If the pull-down is

later removed, the LTC1703 will latch off again unless the

latch is reset by cycling the power or RUN/SS pins.

APPLICATIO S I FOR ATIO

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