APW7322 datasheet(14/22 Pages) ANPEC

14 / 22 page

Rev. A.6 - Aug., 2013

APW7322/A

www.anpec.com.tw

Application Information

Input Capacitor Selection

Because buck converters have a pulsating input current,

a low ESR input capacitor is required. This results in the

best input voltage filtering, minimizing the interference

with other circuits caused by high input voltage spikes.

Also, the input capacitor must be sufficiently large to sta-

bilize the input voltage during heavy load transients. For

good input voltage filtering, usually a 22

µF input capaci-

tor is sufficient. It can be increased without any limit for

better input voltage filtering. Ceramic capacitors show

better performance because of the low ESR value, and

they are less sensitive against voltage transients and

spikes compared to tantalum capacitors. Place the input

capacitor as close as possible to the input and GND pin

of the device for better performance.

Inductor Selection

For high efficiencies, the inductor should have a low dc

resistance to minimize conduction losses. Especially at

high-switching frequencies the core material has a higher

impact on efficiency. When using small chip inductors,

the efficiency is reduced mainly due to higher inductor

core losses. This needs to be considered when select-

ing the appropriate inductor. The inductor value deter-

mines the inductor ripple current. The larger the inductor

value, the smaller the inductor ripple current and the lower

the conduction losses of the converter. Conversely, larger

inductor values cause a slower load transient response.

A reasonable starting point for setting ripple current,

∆I

is 40% of maximum output current. The recommended

inductor value can be calculated as below:

OUT

∆

⋅









−

≥

L(MAX) = IOUT(MAX) + 1/2 x ∆IL

To avoid saturation of the inductor, the inductor should be

rated at least for the maximum output current of the con-

verter plus the inductor ripple current.

Output Voltage Setting

In the adjustable version, the output voltage is set by a

resistive divider. The external resistive divider is con-

nected to the output, allowing remote voltage sensing as

shown in “Typical Application Circuits”. A suggestion of

maximum value of R2 is 300k

Ω to keep the minimum

current that provides enough noise rejection ability through

the resistor divider. The output voltage can be calculated

as below:







 +

⋅

REF

OUT

Output Capacitor Selection

The current-mode control scheme of the APW7322 al-

lows the use of tiny ceramic capacitors. The higher ca-

pacitor value provides the good load transients response.

Ceramic capacitors with low ESR values have the lowest

output voltage ripple and are recommended. If required,

tantalum capacitors may be used as well. The output

ripple is the sum of the voltages across the ESR and the

ideal output capacitor.









⋅









−

⋅

≅

∆

OUT

ESR

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage char-

acteristics of all the ceramics for a given value and size.

OUT

N-FET

OUT

ESR

P-FET

≤ 300KΩ

APW7322

GND

VOUT

≤1MΩ

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