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AN017
6
Figure 4 illustrates the typical results for a critically damped step load response between
500ma and 1.0A.
Figure 4 (Typical Step Response)
Current Sharing and Synchronization
There are several advantages to using a multiple switcher approach compared to a single
larger switcher. The inductor size is considerably reduced. Three 2A inductors store less
energy (LI
2
/2) than one 6A inductor so are far smaller. In addition, synchronizing three
converters 120° out of phase with each other reduces input and output ripple currents. This
reduces the ripple rating, size and cost of filter capacitors. If the SYNC pin is not used in the
application, tie it to ground. To synchronize switching to an external clock, apply a logic-
level signal to the SYNC pin. The recommended clock source is a square-wave with 20% to
80% duty cycle. The clock source rise and fall times must be faster than 1uS. The
Synchronization range is from 250KHz to 2.4MHz. The RT pin resistor must be set to a
frequency which is 20% below the lowest synchronized frequency. Reference the PGOOD
pin description. It is important to note that slope compensation is set by the RT value: When
the sync frequency is much higher than the one set by RT, the slope compensation will be
significantly reduced which may require a larger inductor value to prevent subharmonic
oscillation.
PGOOD Pin
The PGOOD pin is an open collector output driven by a comparator with a 0.7V reference
and the FB pin as its input. PGOOD is a low until the FB pin is 86% of its final voltage. For
PGOOD to be valid VIN must be greater than 3.6V and the RUN/SS pin must be high. A
high level on the PGOOD pin also indicates the regulator is ready for switching frequency
synchronization at the SYNC pin.
RUN/SS
The RUN/SS pin provides for shutting the regulator down and soft-start control. Less than
0.2V on the RUN/SS pin shuts down the regulator. If the RUN/SS pin is greater than 2.5V,
the regulator runs in normal mode. Soft-start control is achieved by adding a RC network on
the RUN/SS pin. The voltage ramp on the RUN/SS pin limits the current during startup to
minimize startup surge current and output overshoot. Tie RUN/SS to VIN if shutdown and
soft-start are not required.