17
LTC1735-1
APPLICATIO S I FOR ATIO
WUU
U
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 and 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 5a. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
conditions. When deriving the 5µA current from V
IN
as in
Figure 5a, current latchoff is always defeated. The diode
connecting this pull-up resistor to INTV
CC
, as in Figure 5b,
eliminates any extra supply current during shutdown
while eliminating the INTV
CC
loading from preventing
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 current 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 LTC1735-1 current comparator has a maximum sense
voltage of 75mV resulting in a maximum MOSFET current
of 75mV/R
SENSE
.
The LTC1735-1 includes current foldback to help further
limit load current when the output is shorted to ground. 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
cycles, the LTC1735-1 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 t
ON(MIN)
of the LTC1735-1 (less than 200ns), the
input voltage, and inductor value:
∆I
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short circuit-current is:
I
mV
R
I
SC
SENSE
LSC
=+
30 1
2
∆
()
The current foldback function is always active and is not
effected by the current latchoff function.
Fault Conditions: Output Overvoltage Protection
(Crowbar)
The output overvoltage crowbar is designed to blow a
system fuse in the input lead when the output of the
regulator rises much higher than nominal levels. This
condition causes huge currents to flow, much greater than
in normal operation. This feature is designed to protect
against a shorted top MOSFET; it does not protect against
a failure of the controller itself.
The comparator (OV in the Functional Diagram) detects
overvoltage faults greater than 7.5% above the nominal
output voltage. When this condition is sensed the top
MOSFET is turned off and the bottom MOSFET is forced
on. The bottom MOSFET remains on continuously for as
long as the OV condition persists; if V
OUT
returns to a safe
level, normal operation automatically resumes. Note that
dynamically changing the output voltage may cause over-
voltage protection to be momentarily activated during
output voltage decreases. This will not cause permanent
latchoff nor will it disrupt the desired voltage change.
With soft-latch overvoltage protection, dynamically chang-
ing the output voltage is allowed and the overvoltage
protection tracks the newly programmed output voltage,
always protecting the load (CPU).
