LTC3731 Datasheet, PDF (18/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
APPLICATIO S I FOR ATIO
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) ON/OFF, 2)
soft-start and 3) a defeatable short-circuit latch off timer.
Soft-start reduces the input power sources’ surge currents
by gradually increasing the controller’s current limit (pro-
portional to an internal buffered and clamped V_ITH). The
latchoff timer prevents very short, extreme load transients
from tripping the overcurrent latch. A small pull-up cur-
rent (>5µA) supplied to the RUN/SS pin will prevent the
overcurrent latch from operating. A maximum pull-up
current of 200µA is allowed into the RUN/SS pin even
though the voltage at the pin may exceed the absolute
maximum rating for the pin. This is a result of the limited
current and the internal protection circuit on the pin. The
following explanation describes how this function operates.
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 < 100 µA). The RUN/SS
pin can be driven directly from logic as shown in Figure7.
Diode, D1, in Figure 7 reduces the start delay but allows
C_SS to ramp up slowly providing the soft-start function.
3.3V OR 5V RUN/SS PIN
5V
SHDN
SHDN
C_SS
V_{CC RUN/SS PIN}
D1 ^RSS
C_SS
3731 F07
Figure 7. RUN/SS Pin Interfacing
18
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
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
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.
3731fa