LTC3728_15 Datasheet, PDF (22/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
Fault Conditions: Overcurrent Latchoff
The RUN/SS pins also provide the ability to latch off the
controller(s) when an overcurrent condition is detected. The
RUN/SS capacitor, C_SS, is used initially to turn on and limit
the inrush current. After the controller has been started and
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 regulator’s output voltage falls
to less than 70% of its nominal value after C_SS reaches
4.1V, C_SS 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
C_SS and the specified discharge current, 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(4.1 – 1.5 + 4.1 – 3.5)]/(1.2μA)
= 2.7 • 10⁶ (C_SS)
If the overload occurs after start-up, the voltage on C_SS
will begin discharging from the Zener clamp voltage:
t_LO2 ≈ [C_SS (6 – 3.5)]/(1.2μA) = 2.1 • 10⁶ (C_SS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor to the RUN/SS pin, as shown
in Figure 7. This resistance shortens the soft-start period
and prevents the discharge of the RUN/SS capacitor during
an over current condition. Tying this pull-up resistor to
V_IN (as in Figure 7) defeats overcurrent latchoff.
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design there may be a problem
with noise pickup or poor layout, causing the protection
circuit to latch off. Defeating this feature will easily allow
V_IN
3.3V OR 5V
RUN/SS
D1
R_SS*
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3728 F07
Figure 7. RUN/SS Pin Interfacing
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. After the design is complete, a decision can be
made whether to enable the latchoff feature.
The value of the soft-start capacitor C_SS may need to be
scaled with output voltage, 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.
Fault Conditions: Current Limit and Current Foldback
The LTC3728 current comparator has a maximum sense
voltage of 75mV, resulting in a maximum MOSFET cur-
rent of 75mV/R_SENSE. The maximum value of current
limit generally occurs with the largest V_IN at the highest
ambient temperature—conditions that cause the highest
power dissipation in the top MOSFET.
The LTC3728 includes current foldback to help further
limit load current when the output is shorted to ground.
The foldback circuit is active even when the overload
shutdown latch previously described is overridden. If the
output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
75mV to 25mV. Under short-circuit conditions with very
low duty cycles, the LTC3728 will begin cycle skipping in
order to limit the short-circuit current. In this situation,
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short-circuit ripple
current is determined by the minimum on-time t_ON(MIN)
of the LTC3728 (less than 200ns), the input voltage and
inductor value:
ΔI_L(SC) = t_ON(MIN) (V_IN/L)
The resulting short-circuit current is:
I_SC
=
25mV
R_SENSE
+
1
2

I_L(SC)
3728fg
22