LTC3738 Datasheet, PDF (22/32 Pages) Linear Technology – 3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning

LTC3738
APPLICATIO S I FOR ATIO
given adequate time to charge up the output capacitor and
provide full load current, the SS capacitor is used for a
short-circuit timer. If the output voltage falls to less than
62.5% of its nominal value, the 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 SS
capacitor, the controller will be shut down until the RUN
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 SS
capacitor will continue charging and will provide addi-
tional 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 SS pin from V_CC as
shown in Figure 6. When V_CC is 5V, a 200k resistance will
prevent the discharge of the SS capacitor during an
overcurrent condition but also shortens the soft-start
period, so a larger SS capacitor value will 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_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.
Current Foldback
In certain applications, it may be desirable to defeat the
internal current foldback function. A negative impedance
is experienced when powering a switching regulator.
That is, the input current is higher at a lower V_IN and
decreases as V_IN is increased. Current foldback is de-
signed to accommodate a normal, resistive load having
increasing current draw with increasing voltage. The EAIN
pin should be artificially held 62.5% above its nominal
operating level of 0.6V, or 0.375V in order to prevent the
IC from “folding back” the peak current level. A suggested
circuit is shown in Figure 7.
The emitter of Q1 will hold up the EAIN pin to a voltage in
the absence of V_OUT that will prevent the internal sensing
circuitry from reducing the peak output current. Remov-
ing the function in this manner eliminates the external
MOSFET’s protective feature under short-circuit condi-
tions. This technique will also prevent the short-circuit
latchoff function from turning off the part during a short-
circuit event and the output current will only be limited to
N • 75mV/R_SENSE.
V_CC
V_CC
V_CC
SS PIN
R_SS
C_SS
3738 F06
Figure 6. Defeating Overcurrent Latchoff
CALCULATE FOR
0.375V TO 0.55V
LTC3738
Q1
EAIN
3738 F07
Figure 7. Foldback Current Elimination
3738f
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