AP5002

PWM CONTROL 2A STEP-DOWN CONVERTER

AP5002

Rev. 6

9 of 10

JUNE 2008

www.diodes.com

Functional Description

PWM Control

The AP5002 consists of DC/DC converters that employ a

pulse-width modulation (PWM) system.

In converters of the AP5002, the pulse width varies in a range up

to 100%, 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.

The current limit threshold is set by the internal circuit.

Setting the Output Voltage

Application circuit item shows the basic application circuit with

adjustable output version. The external resistor sets the output

voltage according to the following equation:

R1 (K

Ω)

R2 (K

Ω)

5

6.4

1.2

3.3

3.8

1.2

2.5

2.6

1.2

1.8

3.3

2.6

1.5

3.3

3.7

1.2

3.3

6.5

1

3.3

13

Inductor Selection

For most designs, the operates with inductors of 10µH to

22µH. The inductor value can be derived from the following

equation:

()

osc

L

ON

OUT

IN

f

I

T

V

V

L

×

Δ

×

−

=

2

Where is inductor Ripple Current. Large value inductors lower

ripple current and small value inductors result in high ripple

currents. Choose inductor ripple current approximately 15% of the

maximum load current 2A,

∆IL=0.3A. 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+0.15A).

Input Capacitor Selection

EL CAP

This capacitor should be located close to the IC using short leads

and the voltage rating should be approximately 1.5 times the

maximum input voltage. The RMS current rating requirement for

the input capacitor of a buck regulator is approximately 1⁄2 the

DC load current. A low ESR input capacitor sized for maximum

RMS current must be used. A 470µF low ESR capacitor for most

applications is sufficient.

MLCC CAP

A 22µF MLCC capacitor for most applications is sufficient.

Output Capacitor Selection

EL CAP

The output capacitor is required to filter the output voltage and

provides

regulator

loop

stability.

The

important

capacitor

parameters are the 100KHz Equivalent Series Resistance (ESR),

the RMS ripples current rating, voltage rating and capacitance

value. For the output capacitor, the ESR value is the most

important parameter. The output ripple can be calculated from

the following formula.

ESR

I

V

L

RIPPLE

×

Δ

=

The bulk capacitor’s ESR will determine the output ripple voltage

and the initial voltage drop after a high slew-rate transient.

An aluminum electrolytic capacitor's ESR value is related to the

capacitance and its voltage rating. In most case, higher voltage

electrolytic capacitors have lower ESR values. Most of the time,

capacitors with much higher voltage ratings may be needed to

provide the low ESR values required for low output ripple voltage.

MLCC CAP

A 22µF MLCC capacitor for most applications is sufficient.

PCB Layout Guide

SW pins(5& 6) and VSS pins(7& 8)on the SOP-8L package are

internally connected to die pad, The PCB layout should allow for

maximum possible copper area at the SW pins.

1. Connect C3 to VCC and VSS pin as closely as possible to get

good power filter effect.

2. Connect R4 to VCC and OCSET pin as closely as possible.

3. Connect ground side of the C2 & D1 as closely as possible.

Typical PC Board Layout

Top Side Layout Guide

Bottom Side Layout Guide

Use vias to conduct the heat into the backside of

PCB layer. The PCB heat sink copper area should be

solder-painted without masked. This approaches a