LTC3731HGTR Linear Technology, LTC3731HGTR Datasheet - Page 18

• Manufacturer Part Number: LTC3731HGTR
• Manufacturer: Linear Technology

Specifications of LTC3731HGTR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIO S I FOR ATIO
LTC3731H
An internal 1.5µA current source charges up the C
capacitor. When the voltage on RUN/SS reaches 1.5V, the
controller is permitted to start operating. As the voltage on
RUN/SS increases from 1.5V to 3.5V, the internal current
limit is increased from 20mV/R
The output current limit ramps up slowly, taking an
additional 1s/µF to reach full current. The output current
thus ramps up slowly, eliminating the starting surge
current required from the input power supply. If RUN/SS
has been pulled all the way to ground, there is a delay
before starting of approximately:
By pulling the RUN/SS controller pin below 0.4V the
IC is put into low current shutdown (I
RUN/SS pin can be driven directly from logic as shown in
Figure 7. Diode, D1, in Figure 7 reduces the start delay
but allows C
function. The RUN/SS pin has an internal 6V zener clamp
(see the Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor is used initially to turn on and limit the
inrush current of all three output stages. After the control-
lers have been started and been given adequate time to
charge up the output capacitor and provide full load
current, the RUN/SS capacitor is used for a short-circuit
18
SHDN
t
t
IRAMP
DELAY
3.3V OR 5V
=
=
SS
1 5
3
1 5
Figure 7. RUN/SS Pin Interfacing
.
V
1 5
.
to ramp up slowly, providing the soft-start
µ
.
−
RUN/SS PIN
V
A
µ
1 5
U
C
.
A
SS
V
C
C
SS
=
SS
U
(
1
SHDN
s F C
=
/
µ
(
1
s F C
SENSE
)
/
µ
5V
W
SS
)
Q
to 75mV/R
SS
D1
< 100 µA). The
V
CC
R
SS
RUN/SS PIN
U
SENSE
3731H F07
C
SS
SS
.
timer. If the output voltage falls to less than 70% of its
nominal value, the RUN/SS capacitor begins discharging
on the assumption that the output is in an overcurrent
condition. If the condition lasts for a long enough period,
as determined by the size of the RUN/SS capacitor, the
discharge current, and the circuit trip point, the controller
will be shut down until the RUN/SS pin voltage is recycled.
If the overload occurs during start-up, the time can be
approximated by:
If the overload occurs after start-up, the voltage on the
RUN/SS capacitor will continue charging and will provide
additional time before latching off:
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin from V
as shown in Figure 7. When V
will prevent the discharge of the RUN/SS capacitor
during an overcurrent condition but also shortens the
soft-start period, so a larger RUN/SS capacitor value may
be required.
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem with
noise pick-up or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
foldback current limiting still remains active, thereby
protecting the power supply system from failure. A deci-
sion can be made after the design is complete whether to
rely solely on foldback current limiting or to enable the
latchoff feature by removing the pull-up resistor.
The value of the soft-start capacitor C
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of
C
SS
t
t
C
LO1
LO2
SS
= 0.1µF will be sufficient for most applications.
> (C
>> (C
>> (C
OUT
SS
SS
)(V
• 0.6V)/(1.5µA) = 4 • 10
• 3V)/(1.5µA) = 2 • 10
OUT
) (10
–4
) (R
CC
is 5V, a 200k resistance
SENSE
6
SS
)
(C
5
may need to be
(C
SS
SS
)
)
3731hfa
CC