AP1539

18V 4A 300KHz BUCK CONVERTER

AP1539 Rev. 2

9 of 12

FEBRUARY 2009

www.diodes.com

Functional Descriptions

PWM Control

The AP1539 is a DC/DC converter that employs pulse width

modulation (PWM) scheme. Its pulse width varies in the range of

0% to 99%, based on the output current loading. The output

ripple voltage caused by the PWM high frequency switching can

easily be reduced through an output filter. Therefore, this

converter provides a low ripple output supply over a broad range

of input voltage & output current loading

Under Voltage Lockout

The under voltage lockout circuit of the AP1539 assures that the

high-side MOSFET driver remains in the off state whenever the

supply voltage drops below 3.3V. Normal operation resumes

Current Limit Protection

at OCSET pin. When the PWM voltage is less than the voltage at

OCSET, an over-current condition is triggered.

The current limit threshold is given by the following equation:

R

I

R

I

OCSET

OCSET

DS(ON)

PEAK

×

=

×

2

I)

(

I

I

(MAX)

OUT

PEAK

Δ

+

>

where,

IN

OUT

S

OUT

IN

V

V

x

xL

F

V

V

I

−

=

Δ

inductor value will affect the ripple current

ΔI.

The above equation is recommended for input voltage range of

5V to 18V. For input voltage lower than 5V, higher than 18V or

ambient

temperature

over

100°C,

higher

is

recommended.

(V)

5V

12V

18V

0.8

6.8K

3.9K

4.7K

1.0

6.8K

3.9K

4.7K

1.2

6.8K

3.9K

4.7K

1.8

6.8K

3.9K

4.7K

2.5

6.8K

3.9K

4.7K

3.3

6.8K

3.9K

4.7K

5.0

N/A

3.9K

5.6K

Inductor Selection

For most designs, the operates with inductors of 22µH to

33µH. The inductor value can be derived from the following

equation:

(

)

osc

L

ON

OUT

IN

f

I

T

V

V

L

×

Δ

×

−

=

2

Where

ripple current and small value inductors result in high ripple

current. Choose inductor ripple current approximately 15% of the

maximum load current 4A,

inductor should be at least equal to the maximum load current

plus half the ripple current to prevent core saturation (4A+0.3A).

Input Capacitor Selection

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.

Output Capacitor Selection

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.

PCB Layout Guide

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

internally connected to die pad, The evaluation board should be

allowed for maximum copper area at output (SW) pins.

2. Connect C3 to Vcc and Vss pin as closely as possible to get

good power filter effect.

3. Connect R4 to Vcc and OCSET pin as closely as possible.

4. Connect ground side of the C2 & D1 & C4 as closely as

possible and use ground plane for best performance.