APL5331 datasheet(14/23 Pages) ANPEC

14 / 23 page

Rev. B.4 - Mar., 2008

APL5331

www.anpec.com.tw

Application Information (Cont.)

100

1000

Capacitance(

µF)

Stable Region

100

1000

Capacitance(

µF)

Stable Region

For DDR SDRAM VTT Applications

Reference Voltage

A reference voltage is applied at the VREF pin by a

resistor divider between VIN and GND pins. Normally,

the bias current flowing out of the VREF pin 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

accuracy of the output voltage. An external bypass

capacitor (>0.1

µF) is also connected to VREF. The

capacitor and the resistor divider form a low-pass filter

to reduce the inherent reference noise from VIN.

Connect the capacitor as close to VREF as possible

for optimal effect. Do not place any additional loading

on this reference input pin.

Output Capacitor

The APL5331 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 of the

output capacitor over full temperature range. The

following 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 output 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

capacitor (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

depending on the step size and slew rate of a load

current step are recommended.

Input Capacitor

The input capacitors for VCNTL and VIN pins are not

required for stability but for supplying surge currents

during large load transients. The input capacitors

prevent the input rail from dipping to improve the

performance of the APL5331. Reducing the parasitic

inductance and resistance of current paths from

power sources to the APL5331 also reduces voltage

dips on VCNTL and VIN pins.

A capacitor of 1

µF (ceramic chip capacitor) or greater

(aluminum electrolytic capacitor) is recommended for

VCNTL pin. For VIN pin, an aluminum electrolytic

capacitor (>50

µF) is recommended. It is not neces-

sary to use low-ESR capacitors.

Layout and Thermal Consideration

The input capacitors for VIN and VCNTL pins are normally

placed near each pin for good performances. Ceramic

decoupling capacitors for loads must be placed as close

to the loads to reduce the parasitic inductors of traces.

It is also recommended that the APL5331 and output

capacitors are placed near the load for good load

Output Capacitor (Cont.)

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