18
LTC1735
1735fc
Diode D1 in Figure 5 reduces the start delay while allowing
C
SS
to charge up slowly for the soft-start function. This
diode and C
SS
can be deleted if soft-start is not needed.
The RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
capacitor during a severe overcurrent and/or short-circuit
condition. When deriving the 5µA current from V
IN
as in
Figure␣ 6a, current latchoff is always defeated. A diode
connecting this pull-up resistor to INTV
CC
, as in Figure␣ 6b,
eliminates any extra supply current during controller shut-
down while eliminating the INTV
CC
loading from prevent-
ing controller start-up. If the voltage on C
SS
does not
exceed 4.1V, the overcurrent latch is not armed and the
function is disabled.
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off. Defeating this feature will easily allow trouble-
shooting of the circuit and PC layout. The internal short-
circuit and foldback current limiting still remains active,
thereby protecting the power supply system from failure.
After the design is complete, a decision can be made
whether to enable the latchoff feature.
The value of the soft-start capacitor C
SS
will need to be
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C
SS
> (C
OUT
)(V
OUT
)(10
–4
)(R
SENSE
)
The minimum recommended soft-start capacitor of
C
SS
␣=␣0.1µF will be sufficient for most applications.
Fault Conditions: Current Limit and Current Foldback
The LTC1735 current comparator has a maximum sense
voltage of 75mV resulting in a maximum MOSFET current
of 75mV/R
SENSE
.
The LTC1735 includes current foldback to help further
limit load current when the output is shorted to ground.
The foldback circuit is active even when the overload
shutdown latch described above is defeated. If the output
falls by more than half, then the maximum sense voltage
is progressively lowered from 75mV to 30mV. Under
short-circuit conditions with very low duty cycle, the
LTC1735 will begin cycle skipping in order to limit the
short-circuit current. In this situation the bottom MOSFET
will be conducting the peak current. The short-circuit
ripple current is determined by the minimum on-time
APPLICATIO S I FOR ATIO
WUU
U
Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated
Figure 5. RUN/SS Pin Interfacing
3.3V OR 5V RUN/SS RUN/SS
D1
C
SS
C
SS
1735 F05
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
D1
C
SS
R
SS
C
SS
R
SS
1735 F06
(a) (b)
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to shut off the
controller and latch off when an overcurrent condition is
detected. The RUN/SS capacitor, C
SS
,
is used initially to
turn on and limit the inrush current of the controller. After
the controller has been started and given adequate time to
charge up the output capacitor and provide full load
current, C
SS
is used as a short-circuit timer. If the output
voltage falls to less than 70% of its nominal output voltage
after C
SS
reaches 4.1V
, the assumption is made that the
output is in a severe overcurrent and/or short-circuit
condition, so C
SS
begins discharging. If the condition lasts
for a long enough period as determined by the size of C
SS
,
the controller will be shut down until the RUN/SS pin
voltage is recycled.
This built-in latchoff can be overridden by providing a
current >5µA at a compliance of 5V to the RUN/SS pin as
shown in Figure␣ 6. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
