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AP1520SL-13

PWM CONTROL 2A STEP-DOWN CONVERTER

Diodes Incorporated. 

AP1520

PWM CONTROL 2A STEP-DOWN CONVERTER

Functional Description

PWM Control

The AP1520 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 AP1520 assures that the

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

supply voltage drops below 3.3V. Normal operation resumes

once VCC rises above 3.5V.

Current Limit Protection

The current limit threshold is set by external resistor ROCSET

connected from VCC supply to OCSET pin. The internal sink

current IOCSET (90μA typical) across this resistor sets the voltage

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:

IPEAK × RDS(ON) = IOCSET × R OCSET

where,

IPEAK

>

IOUT(MAX) +

(ΔI)

2

ΔI = VIN − VOUT × VOUT

fs ×L

VIN

IPEAK is the output peak current; RDS (ON) is the MOSFET ON

resistance; fS is the PWM frequency (300KHz typical). Also, the

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 or ambient

temperature over 100°C, higher ROCSET is recommended.

Inductor Selection

For most designs, the operation range with inductors is from

22µH to 33µH. The inductor value can be derived from the

following equation:

L = VIN − VOUT × VOUT

fs × ΔI

VIN

Where ΔIL is inductor Ripple Current. Large value inductors lower

ripple current and small value inductors result in high ripple

current. Choose inductor ripple current approximately 15% of the

maximum load current 2A, ∆IL=0.30A. 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

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.

VRIPPLE = ΔIL ×ESR

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

If you need low TC & TJ or large PD (Power Dissipation), The dual

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.

1. Connect FB circuits as closely as possible and keep away

from inductor flux for pure VFB.

2. Connect input capacitor to Vcc and Vss pin as closely as

possible to get good power filter effect.

3. Connect ROCSET to Vcc and OCSET pin as closely as

possible.

4. Connect ground side of the input capacitor & Schottky &

output capacitor as closely as possible and use ground

plane for best performance.

AP1520 Rev. 5

DS31174

7 of 9

www.diodes.com

JULY 2009

© Diodes Incorporated

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