LTC3714 Linear, LTC3714 Datasheet - Page 19

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
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
30 A). Releasing the pin allows an internal 1.2 A internal
current source to charge up the external timing capacitor
C
is a delay before starting of about:
When the voltage on RUN/SS reaches 1.5V, the LTC3714
begins operating with a clamp on I
0.9V. As the RUN/SS voltage rises to 3V, the clamp on I
is raised until its full 2.4V range is available. This takes an
additional 1.3s/ F, 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
slowly for the soft-start function.
After the controller has been started and given adequate
time to charge up the output capacitor, C
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 1.7 A cur-
rent then begins discharging C
persists until the RUN/SS pin drops to 3.5V, then the con-
SS
t
. If RUN/SS has been pulled all the way to ground, there
DELAY
1 2
1 5
.
.
V
A
U
C
SS
U
1 3
. /
s F C
SS
. If the fault condition
3.3V OR 5V
Figure 7. RUN/SS Pin Interfacing with Latchoff Defeated
W
TH
SS
of approximately
SS
SS
D1
to charge up
(7a)
is used as a
U
V
IN
R
SS
*
RUN/SS
*OPTIONAL TO OVERRIDE
Q
OVERCURRENT LATCHOFF
TH
C
SS
troller 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
antee that the output is in regulation by the time C
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:
Generally 0.1 F 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
operation can prove annoying during troubleshooting.
The feature can be overridden by adding a pull-up current
of > 5 A to the RUN/SS pin. The additional current pre-
vents the discharge of C
the soft-start period. Using a resistor to V
Figure 7a is simple, but slightly increases shutdown
current. Connecting a resistor to INTV
Figure 7b eliminates the additional shutdown current, but
requires a diode to isolate C
be able to pull RUN/SS above the 4.5V maximum thresh-
old that arms the latchoff circuit and overcome the 4 A
maximum discharge current.
C
SS
> C
INTV
OUT
CC
R
D2*
SS
(7b)
*
V
RUN/SS
OUT
3714 F07
C
R
SS
SENSE
SS
SS
be made large enough to guar-
during a fault and also shortens
SS
(10
. Any pull-up network must
– 4
[F/Vs])
LTC3714
CC
IN
as shown in
as shown in
19
SS
has
3714f