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APW7079

Low-Supply-Current Synchronous Step-up DC-DC Converter

Anpec Electronics 

APW7079

Application Information

Input Capacitor Selection

The input capacitor is chosen based on the voltage rating

and the RMS current rating. For reliable operation, it is

recommended to select the capacitor voltage rating at

least 1.3 times higher than the maximum input voltage.

The maximum RMS current rating of the input capacitor is

calculated as the following equation:

IRMS =

1 ⋅ VIN ⋅ TON

3L

where

TON = main switch max. on-time (4µs typical)

VIN = input voltage

L = inductor value in µH

The capacitors should be placed close to the inductor

and the GND.

Output Capacitor Selection

An output capacitor is required to filter the output and sup-

ply the load transient current. The output ripple is the sum

of the voltages across the ESR and the ideal output

capacitor. The peak-to-peak voltage of the ESR is calcu-

lated as the following equations:

∆VESR = IPEAK x ESR

IPEAK

=

VOUT ⋅ IOUT

VIN ⋅ η

+

VIN ⋅ TON

2⋅L

≤ ILIM

Where

IPEAK = peak current of inductor in amp

η = efficiency (0.85 typical)

The peak-to-peak voltage of the ideal output capacitor is

calculated as the following equation:

∆VCOUT = IOUT × TON

COUT

For the applications using tantalum capacitors, the ∆VCOUT

is much smaller than the VESR and can be ignored.

Therefore, the AC peak-to-peak output voltage (∆VOUT) is

shown as below:

∆VOUT = IPEAK x ESR

Since the output ripple is the product of the peak inductor

current and the output capacitor ESR, using low-ESR tan-

talum capacitors for the best performance or connecting

two or more filter capacitors in parallel is recommended.

Inductor Selection

The inductor value determines the inductor ripple current

and affects the load transient response. It is recom-

mended to select the boost inductor in order to keep the

maximum peak inductor current below the current limit

threshold of the power switch. For example, the current

limit threshold of the APW7079’s switch is 600mA. For

choosing an inductor which has peak current passed,

firstly, it is necessary to consider the output load (I ),

OUT

input (VIN), and output voltage (VOUT). Secondly, the de-

sired current ripple in the inductor also needed to be

taken into account. The current was calculated in “Output

Capacitor Selection”. Since the output ripple is the prod-

uct of the peak inductor current and the output capacitor

ESR, the larger inductor value reduces the inductor cur-

rent ripple and output voltage ripple but typically offers a

larger physical size.

The inductor value also slightly affects the maximum out-

put current. The maximum output current can be calcu-

lated as below:

where

IOUT (MAX )

=

VIN

VOUT



ILIM



−

TOFF



VOUT − VIN

2×L

 ⋅ η

TOFF = main switch min. off-time (0.9µs typical)

Therefore, to consider the balance of the efficiency and

component size, an inductor value of 22µH to 47µH is

recommended in most applications.

VIN IIN

CIN

ILX

LX

N-FET

ISWP

ISWN P-FET

IOUT

VOUT

ESR

COUT

Copyright © ANPEC Electronics Corp.

9

Rev. A.3 - Oct., 2008

www.anpec.com.tw

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