APL5913 datasheet(13/19 Pages) ANPEC

13 / 19 page

Rev. A.4 - May., 2005

APL5913

www.anpec.com.tw

Application Information (Cont.)

Output Capacitor (Cont.)

The APL5913 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) all can 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 APL5913 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 APL5913 requires proper input capacitors to sup-

ply 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 (>100mF, ESR

<300mW) is recommended as the input capacitor. It

is not necessary to use low-ESR capacitors. More

capacitance 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

amplifier to provide proper frequency response for the

linear regulator. The ESR is the equivalent series

resistance of the output capacitor. The C

OUT is ideal

capacitance in the output capacitor. The V

OUT is the

setting of the output voltage.

VERR

VFB

VOUT

VREF

EAMP

APL5913

C OUT

ESR

Figure 1

The feedback network selection depends on the values

of the ESR and C

OUT, which 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:

(1)

..........

0.8

(V)

OUT(V)

(V)

)

⋅

Ω

(2)

......

)

OUT(V)

(nF)

)

OUT(V)

Ω

⋅

≤

⋅

(3)

.........

37.5

ESR

2157

OUT(V)

)

⋅

−

Ω

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