APL5912 datasheet(13/20 Pages) ANPEC

13 / 20 page

Rev. A.6 - Jun., 2005

APL5912

www.anpec.com.tw

Application Information (Cont.)

The APL5912 is designed with a programmable feedback

compensation adjusted by an external feedback network

for the use of wide ranges of ESR and capacitance in

all applications. Ultra-low-ESR capacitors (such as

ceramic chip capacitors), low-ESR bulk capacitors

(such as solid tantalum, POSCap, and Aluminum

electrolytic capacitors) can all be used as an output

capacitor. The value of the output capacitors can be

increased without limit.

During load transients, the output capacitors, depending on

the stepping amplitude and slew rate of load current,

are used to reduce the slew rate of the current seen

by the APL5912 and help the device to minimize the

variations of output voltage for good transient response.

For the applications with large stepping load current,

thelow-ESR bulk capacitors are normally recommended.

Decoupling ceramic capacitors must be placed at the

load and ground pins as close as possible and the

impedance of the layout must be minimized.

Input Capacitor

The APL5912 requires proper input capacitors to supply

current surge during stepping load transients to prevent

the input rail from dropping . Because the parasitic

inductor from the voltage sources or other bulk capacitors

to the VIN pin limit the slew rate of the surge currents.

More parasitic inductance needs m ore input

capacitance.

Ultra-low-ESR capacitors, such as ceramic chip

capacitors, are very good for the input capacitors. An

aluminum electrolytic capacitor (>100

µF, ESR<

300m

Ω) is recommended as the input capacitor. It is

not necessary to use low-ESR capacitors. More ca-

pacitance reduce the variations of the input voltage of

VIN pin.

Feedback Network

Output Capacitor (Cont.)

Figure 1 shows the feedback network between VOUT,

GND and FB pins. It works with the internal error am-

plifier to provide proper frequency response for the

linear regulator. The ESR is the equivalent series

resistance of the output capacitor. The COUT is ideal

capacitance in the output capacitor. The VOUT is the

setting of the output voltage.

The feedback network selection, depending on the val-

ues of the ESR and COUT, has been classified into

three conditions :

• Condition 1 : Large ESR (

≥18mΩ )

- Select the R1 in the range of 400

Ω ~ 2.4kΩ

- Calculate the R2 as the following:

- Calculate the C1 as the following:

• Condition 2 : Middle ESR

- Calculate the R1 as the following:

Select a proper R1(selected) to be a little larger than

the calculated R1.

- Calculate the C1 as the following:

Where R1=R1(selected)

Select a proper C1(selected) to be a little smaller than

V ERR

VFB

VOUT

VREF

EAMP

APL5912

C OUT

ESR

Figure 1

(1)

..........

0.8

(V)

OUT(V)

(V)

)

⋅

Ω

(2)

......

)

OUT(V)

(nF)

)

OUT(V)

Ω

⋅

≤

⋅

[

]

(4)

.........

..........

ESR

)

OUT(uF)

)

(pF)

Ω

⋅

(3)

.........

37.5

ESR

1500

OUT(V)

)

⋅

−

Ω

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