LTM4600
APPLICATIONS INFORMATION
time to charge up the output capacitor, C is used as a
SS
V
RUN/SS
short-circuittimer.AftertheRUN/SSpinchargesabove4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8μA current then
4V
3.5V
3V
beginsdischargingC . Ifthefaultconditionpersistsuntil
SS
1.5V
SHORT-CIRCUIT
LATCH ARMED
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.
t
SOFT-START
CLAMPING
OF I RELEASED
SHORT-CIRCUIT
LATCHOFF
OUTPUT
OVERLOAD
HAPPENS
L
The over-current protection timer requires the soft-start
V
O
timing capacitor C be made large enough to guarantee
SS
75%V
O
that the output is in regulation by the time C has reached
SS
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:
t
SWITCHING
STARTS
4600 F03
Figure 3. RUN/SS Pin Voltage During Startup and
Short-Circuit Protection
C
SS EXT +1000pF >COUT • VOUT(10–3[F / VS])
V
V
IN
IN
Generally 0.1μF is more than sufficient.
R
LTM4600
RUN/SS
RUN/SS
Since the load current is already limited by the current
mode control and current foldback circuitry during a
shortcircuit, over-currentlatchoffoperationisNOTalways
needed or desired, especially if the output has a large
amount of capacitance or the load draws huge currents
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 currents prevents the
PGND SGND
4600 F04
RECOMMENDED VALUES FOR R
RUN/SS
V
R
RUN/SS
IN
4.5V TO 5.5V
10.8V TO 13.8V
16V TO 20V
50k
150k
330k
Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up
Resistor to VIN
discharge of C during a fault and also shortens the soft-
SS
start period. Using a resistor from RUN/SS pin to V is
IN
a simple solution to defeat latchoff. Any pull-up network
must be able to maintain RUN/SS above 4V maximum
latchoff threshold and overcome the 4μA maximum dis-
charge current. Figure 3 shows a conceptual drawing of
Enable
The RUN/SS pin can be driven from logic as shown in
Figure 5. This function allows the LTM4600 to be turned
on or off remotely. The ON signal can also control the
sequence of the output voltage.
V
during startup and short circuit.
RUN
RUN/SS
LTM4600
ON
PGND SGND
2N7002
4600 F05
Figure 5. Enable Circuit with External Logic
4600fc
12
Linear [ Linear ]
