LTC3731H_15 Datasheet, PDF (18/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
Applications Information
An internal 1.5µA current source charges up the C_SS
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_SENSE to 75mV/R_SENSE. 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:
( ) ^t_DELAY
=
1.5V
1.5µA
C _SS
=
1s/µF
C _SS
( ) ^t_IRAMP
=
3V − 1.5V
1.5µA
C _SS
=
1s/µF
C _SS
By pulling the RUN/SS controller pin below 0.4V the
IC is put into low current shutdown (I_Q < 100µA). The
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_SS to ramp up slowly, providing the soft-start
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 con-
trollers 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
3.3V OR 5V RUN/SS PIN
5V
SHDN
SHDN
C_SS
V_{CC RUN/SS PIN}
D1 ^RSS
C_SS
3731H F07
Figure 7. RUN/SS Pin Interfacing
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:
t_LO1 >> (C_SS • 0.6V)/(1.5µA) = 4 • 10⁵ (C_SS)
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:
t_LO2 >> (C_SS • 3V)/(1.5µA) = 2 • 10⁶ (C_SS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin from V_CC
as shown in Figure 7. When V_CC is 5V, a 200k resistance
will prevent the discharge of the RUN/SS capacitor dur-
ing 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 pro-
tecting the power supply system from failure. A decision
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_SS may need to be
scaled with output current, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C_SS > (C_OUT)(V_OUT) (10^–4) (R_SENSE)
The minimum recommended soft-start capacitor of
C_SS = 0.1µF will be sufficient for most applications.
3731Hfb
18