APL5331 datasheet(11/20 Pages) ANPEC

11 / 20 page

 ANPEC Electronics Corp.

Rev. A.8 - Oct., 2003

APL5331

www.anpec.com.tw

Application Information

Power Inputs

Input power sequence are not required for VIN and

VCNTL. However, do not apply a voltage to VOUT

when there is not voltage VCNTL. This is due to the

internal parasitic diodes between VOUT to VIN and

VOUT to VCNTL which will be forward bias. The

APL5331 can source few current or sinks current up

to 3A for load when the input Voltage at VIN is not

present.

Reference Voltage

A reference voltage is applied at the VREF pin by a

resistor divider between VIN and GND pins. Normally

the bias current of the VREF pin flows out of the IC

and is about 150nA(typ.), creating voltage offset at

the resistor divider and affecting the output voltage

accuracy. The recommended resistor is <5k

Ω to

maintain the accuracy of the output voltage. An ex-

ternal bypass capacitor is also connected to VREF.

The capacitor and the resistor divider form a low-

pass filter to reduce the inherent reference noise from

VIN. A ceramic capacitor can be use and is selected

to be greater than 0.1

µF. Connected the capacitor

as close to VREF as possible for optimal effect. More

capacitance and large resistor divider will increase

the soft-start interval. Do not place any additional load-

ing on this reference input pin.

Output Capacitor

The APL5331 requires a proper output capacitor to

maintain stability over full temperature and current

ranges, and improve transient response. The output

capacitor selection is dependent upon the ESR

(equivalent series resistance) and capacitance of the

output capacitor over full temperature range. The fol-

lowing chart shows the stable region of the output

capacitor for APL5331. The stable region is above

the curve, indicating minimum required ESR

and

capacitance to maintain stability. However, the out

put capacitor should have an ESR less than 1

Ω.

Ultra-low-ESR capacitors, such as ceramic chip

capacitors, may promote under-damped transient

response, but proper ceramic chip capacitors placed

near loads can be used as decoupling capacitors. A

low-ESR solid tantalum and aluminum electrolytic ca-

pacitor (ESR<1

Ω) works extremely well and provides

good transient response and stability over temperature.

The output capacitors are also used to reduce the

slew rate of load current and help the APL5331 to

minimize variations of the output voltage, improving

transient response. For this purpose, the low-ESR

capacitors are recommended and depend on the step-

ping and slew rate of load current.

Input Capacitor

The input capacitors of VCNTL and VIN pins are not

required for stability but for supplying surge currents

during large load transients, This will prevent the in-

put rail from drooping and improve the performance

of the APL5331. Because of parasitic inductors from

voltage sources or other bulk capacitors to the VCNTL

and VIN pins will limit the slew rate of the surge cur-

rents during large load transients, resulting in voltage

drop at VIN and VCNTL pins.

100

1000

Capacitance(

µF)

Stable Region

100

1000

Capacitance(

µF)

Stable Region

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