LTC1735 Linear Technology, LTC1735 Datasheet - Page 17

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Diode D1 in Figure 5 reduces the start delay while allowing
C
diode and C
The RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
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
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
voltage falls to less than 70% of it’s nominal output voltage
after C
output is in a severe overcurrent and/or short-circuit
condition and C
lasts for a long enough period as determined by the size of
C
voltage is recycled.
This built-in latchoff can be overridden by providing a
current >5 A 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
SS
SS
, the controller will be shut down until the RUN/SS pin
3.3V OR 5V
Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated
to charge up slowly for the soft-start function. This
3.3V OR 5V
SS
reaches 4.1V , the assumption is made that the
SS
SS
is used as a short-circuit timer. If the output
D1
Figure 5. RUN/SS Pin Interfacing
(a)
can be deleted if soft-start is not needed.
D1
V
SS
IN
R
U
begins discharging. If the condition
SS
RUN/SS
RUN/SS
C
SS
U
C
SS
SS
W
, is used initially to
INTV
CC
R
D1
SS
(b)
RUN/SS
RUN/SS
U
1735 F06
1735 F05
C
C
SS
SS
capacitor during a severe overcurrent and/or short-circuit
condition. When deriving the 5 A current from V
Figure 6a, current latchoff is always defeated. Diode con-
necting this pull-up resistor to INTV
eliminates any extra supply current during controller shut-
down while eliminating the INTV
ing controller start-up. If the voltage on C
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
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of
C
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
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
SS
C
SS
= 0.1 F will be sufficient for most applications.
> (C
SENSE
OUT
)(V
.
OUT
)(10
– 4
)(R
CC
SENSE
loading from prevent-
CC
)
SS
LTC1735
, as in Figure 6b,
will need to be
SS
does not
17
IN
as in