Datasheet

LTM4602HV
12
4602hvf
APPLICATIO S I FOR ATIO
WUU
U
After the controller has been started and given adequate
time to charge up the output capacitor, C
SS
is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8µA current then
begins discharging C
SS
. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
The over-current protection timer requires the soft-start
timing capacitor C
SS
be made large enough to guarantee
that the output is in regulation by the time C
SS
has reached
the 4V threshold. In general, this will depend upon the size
of the output capacitance, output voltage and load current
characteristic. A minimum external soft-start capacitor
can be estimated from:
CpFCVFV
SS EXT OUT OUT S_
–
•([/])+>1000 10
3
Generally 0.1µF is more than suffi cient.
Since the load current is already limited by the current mode
control and current foldback circuitry during a short circuit,
overcurrent latchoff operation is NOT always needed or
desired, especially if the output has large capacitance or
the load draws high current during start-up. The latchoff
feature can be overridden by a pull-up current greater than
5µA but less than 80µA to the RUN/SS pin. The additional
current prevents the discharge of C
SS
during a fault and
also shortens the soft-start period. Using a resistor from
RUN/SS pin to V
IN
is a simple solution to defeat latchoff. Any
pull-up network must be able to maintain RUN/SS above
Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up
Resistor to V
IN
Figure 3. RUN/SS Pin Voltage During Startup and
Short-Circuit Protection
4V maximum latchoff threshold and overcome the 4µA
maximum discharge current. Figure 3 shows a conceptual
drawing of V
RUN
during startup and short circuit.
V
RUN/SS
3.5V
t
t
75%V
O
SWITCHING
STARTS
SOFT-START
CLAMPING
OF I
L
RELEASED
SHORT-CIRCUIT
LATCHOFF
OUTPUT
OVERLOAD
HAPPENS
SHORT-CIRCUIT
LATCH ARMED
4V
3V
1.5V
4602HV F03
V
O
V
IN
V
IN
500k
RUN/SS
LTM4602HV
PGND SGND
4602HV F04
V
IN
4.5V TO 5.5V
10.8V TO 13.8V
24V TO 28V
R
RUN/SS
50k
150k
500k
RECOMMENDED VALUES FOR R
RUN/SS