Datasheet

LTC3714
19
3714f
applicaTions inForMaTion
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3714 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 1.5V puts the
LTC3714 into a low quiescent current shutdown (I
Q
≤
30µA). Releasing the pin allows an internal 1.2µA internal
current source to charge up the external timing capacitor
C
SS
. If RUN/SS has been pulled all the way to ground,
there is a delay before starting of about:
t
DELAY
=
1.5V
1.2µA
C
SS
= 1.3s/µF
( )
C
SS
When the voltage on RUN/SS reaches 1.5V, the LTC3714
begins operating with a clamp on ITH of approximately
0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH
is raised until its full 2.4V range is available. This takes an
additional 1.3s/µF, during which the load current is folded
back until the output reaches 50% of its final value. The
pin can be driven from logic as shown in Figure 7. Diode
D1 reduces the start delay while allowing C
SS
to charge
up slowly for the soft-start function.
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 1.7µA 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 overcurrent protection timer requires that 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 soft-start capacitor
can be estimated from:
C
SS
> C
OUT
V
OUT
R
SENSE
(10
–4
[F/Vs])
Generally 0.1µF is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short-
circuit by the current foldback circuitry and latchoff op-
eration can prove annoying during troubleshooting. The
feature can be overridden by adding a pull-up current of
>5µA 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 to V
IN
as shown in
Figure 7a is simple, but slightly increases shutdown cur-
rent. Connecting a resistor to INTV
CC
as shown in Figure
7b eliminates the additional shutdown current, but requires
a diode to isolate C
SS
. Any pull-up network must be able
to pull RUN/SS above the 4.5V maximum threshold that
arms the latchoff circuit and overcome the 4µA maximum
discharge current.
Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(7a) (7b)
D2*
C
SS
R
SS
*
C
SS
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
R
SS
*
3714 F07