AP5004_10 Datasheet, PDF (8/11 Pages) Diodes Incorporated – PWM CONTROL 2.5A STEP-DOWN CONVERTER

AP5004
PWM CONTROL 2.5A STEP-DOWN CONVERTER
Functional Description
PWM Control
The AP5004 consists of DC/DC converters that employ a pulse-width modulation (PWM) system. The PWM
controller is internally clocked by a fixed 300KHz oscillator.
When used as a converter, the AP5004’s pulse width varies in a range from 0% to 90%, according to the load
current. The ripple voltage produced by the switching can easily be removed through a filter because the
switching frequency remains constant. Therefore, these converters provide a low-ripple power over broad ranges
of input voltage and load current.
Enable Control
The Enable Control allows the output of the AP5004 to be turned ON or OFF. Connecting the Enable pin to GND
or any voltage source lower than 0.8V will turn the output OFF. In the OFF state, the current drawn through the
V_CC input is approximately 2µA. Please note that the slew rate from ON to OFF must be >0.013v/µs to ensure
proper operation.
BOOST Capacitor
This capacitor boosts the gate drive to the internal MOSFET above V_IN to fully turn it ON thus minimizing
conduction losses in the power switch to maintain high efficiency. The recommended value of the capacitor is
0.1µF. The capacitor must be connected from pin 3 to the switch output, pin 4.
Feedback
This is the input to a two-stage high gain amplifier, which drives the PWM controller. Two external resistors are
required to set the DC output voltage. For stable operation of the power supply, it is important to prevent coupling
of any inductor flux to the feedback input.
Current Limit
The current limit threshold is set by the internal circuit such that the minimum switching current is 3.5A. If the
switching current exceeds the threshold, the output voltage will drop and the switching frequency will be reduced
to 50KHz.
Inductor Selection
Most designs operate with inductors of 15µH to 33µH. The inductor value can be derived from the following
equation:
L = ^{(VIN − VOUT )VOUT}
ΔIL × fosc × VIN
Large value inductors will result in a lower ripple current where as small value inductors will result in high ripple
currents. The inductor ripple current should be set to approximately 15% of the maximum load current 2.5A,
∆IL=0.375A. The DC current rating of the inductor should be at least equal to the maximum load current plus half
the ripple current to prevent core saturation (2A+0.19A).
Input Capacitor Selection
The input current to the step-down converter is discontinuous, and therefore an input capacitor C2 is required to
supply the AC current to the step-down converter while maintaining the DC input voltage. A low ESR capacitor is
required to keep the noise at the IC to a minimum. Its RMS current rating should be greater than approximately
half of the DC load current.
AP5004
Document number: DS31444 Rev. 5 - 2
8 of 11
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August 2010
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