AP5101

1.5A Step-Down Converter with 1.4MHz Switching

Frequency

AP5101

Document number: DS32258 Rev. 1 - 2

9 of 15

www.diodes.com

July 2010

© Diodes Incorporated

Component Selection

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%)

5.0

49.9 (1%)

9.53 (1%)

Table 1. Resistor Selection for Common Output Voltage

Compensation Components

The AP5101 has an external COMP pin through which system stability and transient response can be controlled. COMP pin is

the output of the internal trans-conductance error amplifier. A series capacitor-resistor combination sets a pole-zero

combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by:

OUT

V

FB

V

VEA

A

CS

G

LOAD

R

VDC

A

×

×

×

=

The control loop transfer function incorporates two poles. One is due to the compensation capacitor (C3) and the output

resistor of error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at:

VEA

A

3

C

2

EA

G

P1

f

×

×

π

=

LOAD

R

2

C

2

1

P2

f

×

×

π

=

One zero is present due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at:

3

R

3

C

2

1

Z1

f

×

×

π

=