18
LTC1736
APPLICATIO S I FOR ATIO
W
U
U
U
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 voltage, 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
=
0.1
µ
F will be sufficient for most applications.
Fault Conditions: Current Limit and Current Foldback
The LTC1736 current comparator has a maximum sense
voltage of 75mV resulting in a maximum MOSFET current
of 75mV/R
SENSE
.
The LTC1736 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
LTC1736 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 LTC1736 (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
L SC
=
+
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
VID controlled output voltage decreases may cause the
overvoltage protection to be momentarily activated. This
will not cause permanent latchoff nor will it disrupt the
desired voltage change.
With soft-latch overvoltage protection, dynamic VID code
changes are allowed and the overvoltage protection tracks
the new VID code, always protecting the load (CPU). If
dynamic VID code changes are anticipated and the mini-
mum load current is light, it may be necessary to either
force continuous operation by pulling FCB low during the
transition to maximize current sinking capability or con-
nect PGOOD to FCB to automatically force continuous
operation during VID transitions.
