LTC1703CG#TRPBF Linear Technology, LTC1703CG#TRPBF Datasheet - Page 25

• Manufacturer Part Number: LTC1703CG#TRPBF
• Description: IC REG SW DUAL SYNC VID 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1703CG#TRPBF

• Applications: Controller, Mobile Intel Pentium® III
• Voltage - Input: 3 ~ 7 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 2 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
Accuracy Trade-Offs
The V
particularly accurate, primarily due to uncertainty in the
R
arises from the ringing present at the SW pin, which
causes the V
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 R
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
DS(ON)
DS
from MOSFET to MOSFET. A second error term
sensing scheme used in the LTC1703 is not
DS
to look larger than (I
DS(ON)
U
spec.
U
W
LOAD
)(R
DS(ON)
U
) at the
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.
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
10µA pull-up to V
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.
Figure 13. Regulating an Auxiliary Output with the FCB Pin
LTC1703
FCB
BG
TG
CC
V
IN
QT
QB
and does not require an external pull-
+
C
IN
R
R
FCB1
FCB2
LTC1703
+
+
C
C
1703 F13
OUT(AUX)
V
OUT
OUT(AUX)
25
V
OUT
1703fa