LTC3707 Datasheet, PDF (20/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
By pulling both RUN/SS pins below 1V and/or pulling
the STBYMD pin below 0.2V, the LTC3707 is put into
low current shutdown (I_Q = 20μA). The RUN/SS pins
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.
Each RUN/SS pin has an internal 6V zener clamp (See
Functional Diagram).
V_IN
3.3V OR 5V
RUN/SS
D1
R_SS*
C_SS
INTV_CC
R_SS*
RUN/SS
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
(a)
3707 F07
(b)
Figure 7. RUN/SS Pin Interfacing
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 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 regulator’s
output voltage falls to less than 70% of its nominal value
after C_SS reaches 4.2V, C_SS begins discharging on the as-
sumption 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 volt-
age 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
20
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 7a, defeats overcurrent latchoff. Diode-
connecting this pull-up resistor to INTV_CC , as in Figure
7b, eliminates any extra supply current during controller
shutdown while eliminating the INTV_CC loading from
preventing controller start-up.
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
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 LTC3707 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 LTC3707 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 described above 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 LTC3707 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
3707fb