LTM4600 LINER [Linear Technology], LTM4600 Datasheet - Page 12

• Manufacturer Part Number: LTM4600
• Description: 10A High Effi ciency DC/DC ?Module
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTM4600
After the controller has been started and given adequate
time to charge up the output capacitor, CSS 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 CSS. 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 CSS be made large enough to guarantee
that the output is in regulation by the time CSS has reached
the 4V threshold. In general, this will depends upon the
size of the output capacitance, output voltage and load
current characteristic. A minimum external soft-start
capacitor can be estimated from:
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 CSS
during a fault and also shortens the soft-start period. Us-
ing a resistor from RUN/SS pin to V
12
C
SS EXT
_
+
1000
U
pF
>
C
U
OUT
•
V
OUT
IN
W
is a simple solution
(
10
–
3
[ /
F V
U
S
])
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
short circuit.
1.5V
SWITCHING
STARTS
Figure 3. RUN/SS Pin Voltage During Startup and
Short-Circuit Protection
Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up
Resistor to V
OF I
SOFT-START
3V
CLAMPING
L
RELEASED
4V
V
IN
IN
500k
SHORT-CIRCUIT
LATCH ARMED
V
O
V
V
RUN/SS
RUN/SS
PGND SGND
IN
LTM4600
OVERLOAD
HAPPENS
OUTPUT
4600 F04
RUN
during startup and
SHORT-CIRCUIT
75%V
LATCHOFF
O
4600fa
3.5V
4600 F03
t
t