LTC1736IG Linear Technology, LTC1736IG Datasheet - Page 17

• Manufacturer Part Number: LTC1736IG
• Description: IC REG SW SYNC STEPDWN HE 24SSOP
• Manufacturer: Linear Technology

Specifications of LTC1736IG

• Applications: Converter, Intel Pentium® II, III
• Voltage - Input: 3.5 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.93 ~ 2 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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
increasing the controller’s current limit I
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
be driven directly from logic as shown in Figure 3. Releas-
ing the RUN/SS pin allows an internal 1.2 A current
source to charge up the external soft-start capacitor C
If RUN/SS has been pulled all the way to ground there is
a delay before starting of approximately:
When the voltage on RUN/SS reaches 1.5V the LTC1736
begins operating with a current limit at approximately
25mV/R
1.5V to 3.0V, the internal current limit is increased from
25mV/R
ramps up slowly, taking an additional 1.25s/ 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
diode and C
The RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
SS
3.3V OR 5V
t
DELAY
to charge up slowly for the soft-start function. This
SENSE
SENSE
(a)
SS
D1
1 2
1 5
. As the voltage on RUN/SS increases from
Figure 3. RUN/SS Pin Interfacing
.
to 75mV/R
.
can be deleted if soft-start is not needed.
RUN/SS
V
A
U
C
SS
C
SS
SENSE
U
1 25
.
. The output current limit
s
/
Q
W
F C
< 25 A). This pin can
TH(MAX)
SS
(b)
IN
RUN/SS
by gradually
1736 F03
C
U
SS
. This pin
SS.
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
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
voltage falls to less than 70% of its nominal output voltage
after C
output is in a severe overcurrent and/or short-circuit
condition and C
lasts for a long enough period as determined by the size of
C
voltage is recycled.
This built-in latchoff can be overridden by providing a
current > 5 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 A current from V
Figure 4a, current latchoff is always defeated. A diode
connecting this pull-up resistor to INTV
eliminates any extra supply current during controller shut-
down while eliminating the INTV
ing controller start-up. If the voltage on C
4.1V, the overcurrent latch is not armed and the function
is disabled.
SS
, the controller will be shut down until the RUN/SS pin
3.3V OR 5V
Figure 4. RUN/SS Pin Interfacing with Latchoff Defeated
SS
reaches 4.1V , the assumption is made that the
SS
D1
is used as a short-circuit timer. If the output
(a)
V
SS
IN
R
SS
begins discharging. If the condition
RUN/SS
C
SS
CC
loading from prevent-
SS
INTV
is used initially to
CC
SS
LTC1736
CC
, as in Figure 4b,
R
does not exceed
D1
SS
(b)
RUN/SS
17
IN
1736 F04
C
SS
as in