APL5332KA datasheet(9/17 Pages) ANPEC

9 / 17 page

 ANPEC Electronics Corp.

Rev. A.3 - Oct., 2003

APL5332

www.anpec.com.tw

Application Information

Internal Parasitic Diode

Do not apply a voltage to VOUT when the voltage ap-

plied at VIN is not present. The reason is the internal

parasitic diodes from VOUT to VIN will conduct due to

the forward-voltage applied at VOUT.

Output Voltage Selection

The APL5332 allows operation in either fixed voltage

or adjustable mode. Connecting FB to VIN selects

fixed output voltage which is either 1.225V or 1.45V

by setting the BS pin to be logic “High” or “Low”. The

output voltage may also be adjusted by connecting a

resistor-divider from VOUT to FB to GND (See the

Typical Application Circuit). Selecting R2 in the 100

Ω

to 5k

Ω range ignors the voltage offset caused by the

internal pull-up current of FB. Calculate R1 with the

following equation:

R1 = R2 [(VOUT / VREF) - 1]

where VREF = 0.8V.

The output voltage selection table is :

Output Capacitor

The APL5332 requires a proper output capacitor to

maintain stability and improve transient response. The

output capacitor selection is dependent upon the ESR

(equivalent series resistance) and capacitance over

temperature and current ranges. The following chart

shows a stable region to select output capacitor for

APL5332. This region above the curve indicates mini-

mum required ESR and capacitance to maintain

stability. However, the output capacitor should have

an ESR less than1

Ω.

1.225V

1.45V

Adjustable

Ultra-low-ESR capacitors, such as ceramic chip

capacitors, may promote unstable or under-damped

transient response, but proper ceramic chip capaci-

tors placed near loads can be used as decoupling

capacitors. A low-ESR solid tantalum and aluminum

electrolytic capacitor (ESR<1

Ω) works extremely well

and provides good transient response and stability over

temperature.

100

1000

Capacitance(

µF)

Stable Region

100

1000

Capacitance(

µF)

Stable Region

The output capacitors are also used to reduce the slew

rate of load current and help the APL5332 to minimize

variations of the output voltage, improving transient

response. For this purpose, the low-ESR capacitors

are recommended.

Input Capacitor

The VIN input capacitor is not required for stability but

for supplying surge currents during large load transients,

preventing the input rail from dropping and improving

performance of APL5332. The parasitic inductors from

the voltage sources or other bulk capacitors to the

VIN pins will limit the slew rate of the surge currents

during large load transients, resulting in voltage drop

at VIN pin.

An aluminum electrolytic capacitor (>47

µF) is recom-

mended for VIN pin, and It is not necessary to use

low-ESR capacitors.

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