Diodes AP3421/A/B - Manual (Page 11)

Preliminary Datasheet
1.3MHz, Dual 1.0A Synchronous Step-down Converters AP3421/A/B
Feb. 2013 Rev. 1. 4 BCD Semiconductor Manufacturing Limited
11
Application Information
1. Operation
The DC-DC converters are current-mode buck
converters with a synchronous rectifier and internal
compensation. They are designed to be stable with a
1.5
µH to 6.8µH inductor value and 10µF to 22µF
output capacitor. Both output voltages are resistor
programmable. The switching frequency of the
converter is fixed and the switches turn on at
alternating 180° phase intervals.
The converter operates in 2 possible modes:
Continuous Mode (CM), and Pulse Skipping Mode
(PSM). CM is the default mode under normal loading.
Under light loads, PSM mode occurs, where
switching cycles are skipped if the current demand is
low.
2. Soft-start
The DC-DC converter contains a soft-start function
that brings the output voltages up in a slowly
increasing ramp with any resistive load from open
circuit to 1A (resistive) and any capacitor from 10
µF
to 22
µF. During soft-start, the peak inductor current
shall not exceed 750mA until the output voltage
reaches 25% of its final value. Current limit shall be
active but not trip during soft-start into a rated
resistive load. Overshoot voltage during soft-start is
limited to 1%.
3. Power Sequencing and Enable
When power is applied and if E/S input is asserted
(High) or is toggling, the DC-DC converters will
enter RUN mode after a short settling period. If the
E/S pin is a static low, the IC will enter a SLEEP
state where it draws very little input current.
When in RUN mode, if there is no fault condition, the
VO1 output (SW1) will be the first output to begin
soft-start. When the reference voltage for FB1
reaches approximately 90% of the final value, the
VO2 output (SW2) will begin soft-start.
4. POR
Under voltage comparators are provided to monitor
the output voltages and VMON which could be the
input voltage. If any of these voltages falls below its
POR threshold, a POR open drain output will turn on
which pulls the POR pin low. After the POR
condition is cleared, there is a delay of 20ms before
th
the POR output transistor is turned off; when off the
POR pin is high-Z and may be pulled up high with a
resistor. The POR function has built-in deglitching.
Once the POR is detected, the power supply outputs
will be discharged prior to restart using the
soft-start/sequencing routine.
5.1Over Current and Short Circuit
Protection
The DC-DC converters shall have over current and
short circuit protection. Under any load condition, at
any time, any value of load resistor (including 0
Ω)
can be applied to the DC-DC outputs instantaneously
and held in place indefinitely without the switch
current exceeding the peak current limit and without
the IC suffering any permanent damage or loss of
performance. The output voltage is allowed to drop
under over current or short circuit conditions.
Recovery to output voltage regulation is required
within 10ms of the instant the loading is reduced to
maximum allowable rated load; the output voltage
shall not exceed the dynamic load excursion limits
(±5% excursion) upon recovery. Over current shall
not be triggered by a sudden rate of load current
change provided the loading does not exceed the
output load rating of 1A.
6. E/S Function
Enable/Synchronization function. Pulling this pin
high statically enables the AP3421/A/B while pulling
the pin low statically for longer than 4
µs will shut
down the AP3421/A/B. Applying a pulse to this pin
will synchronize SW1 and SW2 switching frequency
to ½ the external clock frequency. The external
frequency lock-in ranges from 1.5MHz to 3.0MHz.