17
LTC1736
APPLICATIO S I FOR ATIO
W
U
U
U
Soft-Start/Run Function
The RUN/SS pin is a multipurpose pin that provides a soft-
start function and a means to shut down the LTC1736.
Soft-start reduces surge currents from V
IN
by gradually
increasing the controller’s current limit I
TH(MAX)
. This pin
can also be used for power supply sequencing.
Pulling the RUN/SS pin below 1.5V puts the LTC1736 into
a low quiescent current shutdown (I
Q
< 25
µ
A). This pin can
be driven directly from logic as shown in Figure 3. Releas-
ing the RUN/SS pin allows an internal 1.2
µ
A current
source to charge up the external soft-start capacitor C
SS.
If RUN/SS has been pulled all the way to ground there is
a delay before starting of approximately:
t
V
A
C
s
F C
DELAY
SS
SS
=
µ
=
µ
(
)
1 5
1 2
1 25
.
.
.
/
When the voltage on RUN/SS reaches 1.5V the LTC1736
begins operating with a current limit at approximately
25mV/R
SENSE
. As the voltage on RUN/SS increases from
1.5V to 3.0V, the internal current limit is increased from
25mV/R
SENSE
to 75mV/R
SENSE
. The output current limit
ramps up slowly, taking an additional 1.25s/
µ
F to reach
full current. The output current thus ramps up slowly
reducing the starting surge current required from the input
power supply.
Diode D1 in Figure 3 reduces the start delay while allowing
C
SS
to charge up slowly for the soft-start function. This
diode and C
SS
can be deleted if soft-start is not needed.
The RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to shut off the
controller and latchoff when an overcurrent condition is
detected. The RUN/SS capacitor C
SS
is used initially to
turn on and limit the inrush current of the controller. After
the controller has been started and given adequate time to
charge up the output capacitor and provide full load
current, C
SS
is used as a short-circuit timer. If the output
voltage falls to less than 70% of its nominal output voltage
after C
SS
reaches 4.1V, the assumption is made that the
output is in a severe overcurrent and/or short-circuit
condition and C
SS
begins discharging. If the condition
lasts for a long enough period as determined by the size of
C
SS
, the controller will be shut down until the RUN/SS pin
voltage is recycled.
This built-in latchoff can be overridden by providing a
current > 5
µ
A at a compliance of 5V to the RUN/SS pin as
shown in Figure 4. This current shortens the soft-start
period but also prevents net discharge of the RUN/SS
capacitor during a severe overcurrent and/or short-circuit
condition. When deriving the 5
µ
A current from V
IN
as in
Figure 4a, current latchoff is always defeated. A diode
connecting this pull-up resistor to INTV
CC
, as in Figure 4b,
eliminates any extra supply current during controller shut-
down while eliminating the INTV
CC
loading from prevent-
ing controller start-up. If the voltage on C
SS
does not exceed
4.1V, the overcurrent latch is not armed and the function
is disabled.
Figure 3. RUN/SS Pin Interfacing
3.3V OR 5V
RUN/SS
D1
RUN/SS
(a)
(b)
C
SS
C
SS
1736 F03
3.3V OR 5V
RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(a)
(b)
D1
C
SS
R
SS
C
SS
R
SS
1736 F04
Figure 4. RUN/SS Pin Interfacing with Latchoff Defeated
