LTC1702A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702A' PDF : 36 Pages View PDF
LTC1702A
APPLICATIONS INFORMATION
TG
LTC1702A
FCB BG
VIN
+
CIN
QT
QB
VOUT(AUX)
+
COUT(AUX)
+
VOUT
COUT
RFCB1
RFCB2
1702A F08
Figure 13. Regulating an Auxiliary Output with the FCB Pin
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.
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 100µs 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 10µA 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.
1702afa
25
Share Link: 
