AP1513

Document number: DS31053 Rev. 5 - 2

6 of 9

www.diodes.com

December 2017

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AP1513

Functional Description

PWM Control

The AP1513 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 AP1513 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:

R

I

R

I

OCSET

OCSET

DS(ON)

PEAK







2

)

I

(





I

I

OUT(MAX)

PEAK

where,

IN

OUT

OUT

IN

V

V

L

fs

V

V

I











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:

IN

OUT

OUT

IN

V

V

fs

V

V

L











I

Where

IL 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 2

A, ∆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.