APL5912-KAC-TR データシートの表示（PDF） - Anpec Electronics

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APL5912-KAC-TR

0.8V Reference Ultra Low Dropout Linear Regulator

Anpec Electronics 

APL5912

Application Information (Cont.)

Output Capacitor (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,

the low-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 more input

capacitance.

Ultra-low-ESR capacitors, such as ceramic chip

capacitors, are very good for the input capacitors. An

aluminum electrolytic capacitor (>100µF, ESR<

300mΩ) 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

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.

APL5912

V ERR

EAMP

VR E F

VOUT

R1

FB

VFB

R2

Figure 1

VOUT

ESR

C1

C OUT

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 400Ω ~ 2.4kΩ

- Calculate the R2 as the following:

R2(kΩ) = R1(kΩ) ⋅

0.8(V)

.......... (1)

VOUT(V) - 0.8(V)

- Calculate the C1 as the following:

10 ⋅ VOUT(V) ≤ C1(nF) ≤ 40 ⋅ VOUT(V) ...... (2)

R1(kΩ)

R1(kΩ)

• Condition 2 : Middle ESR

- Calculate the R1 as the following:

R1(kΩ) = 1500 − 37.5⋅ VOUT(V) + 30 ......... (3)

ESR(mΩ)

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

the calculated R1.

- Calculate the C1 as the following:

[ ] C1(pF) = ESR(mΩ) + 50 ⋅ COUT(uF) ................... (4)

R1(kΩ)

Where R1=R1(selected)

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

Copyright © ANPEC Electronics Corp.

13

Rev. A.6 - Jun., 2005

www.anpec.com.tw

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