APW8811 datasheet(16/23 Pages) ANPEC

16 / 23 page

Rev. A.3 - Sep., 2012

APW8811

www.anpec.com.tw

Application Information

The output voltage of PWM1 can be adjusted from2V to

5.5V with a resistor-driver at FB1 between VOUT1 and

GND. Using 1% or better resistors for the resistive di-

vider is recommended. The FB1 pin is the inverter input

of the error amplifier, and the reference voltage is 2V.

Take the example, the output voltage of PWM1 is deter-

mined by:

Where F

SW is the switching frequency of the regulator.

Increasing the inductor value and frequency will re-

duce the ripple current and voltage. However, there is a

tradeoff between the inductor’s ripple current and the

regulator load transient response time.

A smaller inductor will give the regulator a faster load

transient response at the expense of higher ripple

current. Increasing the switching frequency (F

SW) also

re duces the ripple current and voltage , b u t i t wi l l

increase the switching loss of the MOSFETs and the

p ower dissipation of the converter. The maximum

ripple current occurs at the maximum input voltage. A

good starting point is to choose the ripple current to be

Where R

TOP1 is the resistor connected from VOUTI to VFB1

and R

GND1 is the resistor connected from FB1 to GND.

Similarly, the output voltage of PWM2 can be also ad-

justed from 2V to 5.5V.

Output Inductor Selection

The duty cycle of a buck converter is the function of the

input voltage and output voltage. Once an output voltage

is fixed, it can be written as:

OUT

RIPPLE

Output Capacitor Selection

The inductor value determines the inductor ripple current

and affects the load transient reponse. Higher inductor

value reduces the inductor’s ripple current and induces

lower output ripple voltage. The ripple current and can be

approxminated by:

ESR

RIPPLE

ESR

OUT

RIPPLE

OUT

∆

These two components constitute a large portion of the

total output voltage ripple. In some applications, multiple

capacitors have to be paralleled to achieve the desired

ESR value. If the output of the converter has to support

another load with high pulsating current, more capaci-

tors are needed in order to reduce the equivalent ESR

and suppress the voltage ripple to a tolerable level. A

small decoupling capacitor in parallel for bypassing

the noise is also recommended, and the voltage rating

of the output capacitors must also be considered.

To support a load transient that is faster than the

switching frequency, more capacitors have to be used

to reduce the voltage excursion during load step change.

Another aspect of the capacitor selection is that the

total AC current going through the capacitors has to be

less than the rated RMS current specified on the ca-

pacitors to prevent the capacitor from over-heating.









GND1

TOP1

OUTI

approximately 30% of the maximum output current.

Once the inductance value has been chosen, selecting

an inductor is capable of carrying the required peak cur-

rent without going into saturation. In some types of

inductors, especially core that is made of ferrite, the ripple

current will increase abruptly when it saturates. This will

result in a larger output ripple voltage.

Output Voltage Selection

Output voltage ripple and the transient voltage de-

viation are factors that have to be taken into consid-

eration when selecting an output capacitor. Higher

capacitor value and lower ESR reduce the output ripple

and the load transient drop. Therefore, selecting high

performance low ESR capacitors is intended for switch-

ing regulator applications. In addition to high frequency

noise related MOSFET turn-on and turn-off, the output

voltage ripple includes the capacitance voltage drop and

ESR voltage drop caused by the AC peak-to-peak current.

These two voltages can be represented by:

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