Datasheet
LTM4600HV
12
4600hvfc
APPLICATIO S I FOR ATIO
WUU
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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, shuting 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
shortcircuit, over-current latchoff operation is NOT always
needed or desired, especially the output has large amount
of capacitance or the load draw huge 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. Us-
ing a resistor from RUN/SS pin to V
IN
is a simple solution
V
IN
V
IN
R
RUN/SS
RUN/SS
4600hv F04
LTM4600HV
PGND SGND
V
IN
4.5V TO 5.5V
10.8V TO 13.8V
24V TO 28V
R
RUN/SS
50k
150k
500k
RECOMMENDED VALUES FOR RUN/SS
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
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
4600hv F03
V
O
to defeat latchoff. Any pull-up network must be able to
maintain RUN/SS above 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.