AP5100

1.2A Step-Down Converter with 1.4MHz Switching

Frequency

AP5100

9 of 12

MARCH 2010

Document number: DS32130 Rev. 1 - 2

www.diodes.com

Setting the Output Voltage

The output voltage can be adjusted from 0.81V to 15V

using an external resistor divider. Table 1 shows a list of

resistor selection for common output voltages. Resistor

R1 is selected based on a design tradeoff between

efficiency and output voltage accuracy. For high values

of R1 there is less current consumption in the feedback

network. However the trade off is output voltage

accuracy due to the bias current in the error amplifier. R2

can be determined by the following equation:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

1

0.81

OUT

V

2

R

1

R

1.8

80.6 (1%)

64.9 (1%)

2.5

49.9 (1%)

23.7 (1%)

3.3

49.9 (1%)

16.2 (1%)

5

49.9 (1%)

9.53 (1%)

Table 1. Resistor Selection for Common

Output Voltages

Inductor

Calculating the inductor value is a critical factor in

designing a buck converter. For most designs, the

following equation can be used to calculate the inductor

value;

SW

f

L

ΔI

IN

V

)

OUT

V

IN

(V

OUT

V

L

×

×

−

×

=

Where

L

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% of the

maximum load current. The maximum inductor peak

current is calculated from:

2

L

ΔI

LOAD

I

L(MAX)

I

+

=

Peak current determines the required saturation current

rating, which influences the size of the inductor.

Saturating

the

inductor

decreases

the

converter

efficiency while increasing the temperatures of the

inductor, the MOSFET and the diode. Hence choosing

an inductor with appropriate saturation current rating is

important.

A 1µH to 10µH inductor with a DC current rating of at

least 25% percent higher than the maximum load current

is recommended for most applications.

For highest efficiency, the inductor’s DC resistance

should be less than 200m

Ω. Use a larger inductance

for improved efficiency under light load conditions.

Input Capacitor

The input capacitor reduces the surge current drawn

from the input supply and the switching noise from the

device. The input capacitor has to sustain the ripple

current produced during the on time on the upper

MOSFET. It must hence have a low ESR to minimize the

losses.

Due to large dI/dt through the input capacitors,

electrolytic or ceramics should be used. If a tantalum

must be used, it must be surge protected. Otherwise,

capacitor failure could occur. For most applications, a

4.7µF ceramic capacitor is sufficient.

Output Capacitor

The output capacitor keeps the output voltage ripple

small, ensures feedback loop stability and reduces the

overshoot of the output voltage. The output capacitor is a

basic component for the fast response of the power

supply. In fact, during load transient, for the first few

microseconds it supplies the current to the load. The

converter recognizes the load transient and sets the duty

cycle to maximum, but the current slope is limited by the

inductor value.

Maximum capacitance required can be calculated from

the following equation: