AP5100

1.2A Step-Down Converter with 1.4MHz Switching

Frequency

AP5100

Document number: DS32130 Rev. 2 - 2

6 of 10

www.diodes.com

November 2010

© Diodes Incorporated

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Applications Information

OPERATION

The AP5100 is a current mode control, asynchronous buck

regulator. Current mode control assures excellent line and

load regulation and a wide loop bandwidth for fast

response to load transients. Figure. 4 depicts the

functional block diagram of AP5100.

The operation of one switching cycle can be explained as

follows. At the beginning of each cycle, HS (high-side)

MOSFET is off. The EA output voltage is higher than the

current

sense

amplifier

output,

and

the

current

comparator’s output is low. The rising edge of the 1.4MHz

oscillator clock signal sets the RS Flip-Flop. Its output

turns on HS MOSFET.

When the HS MOSFET is on, inductor current starts to

increase. The Current Sense Amplifier senses and

amplifies the inductor current. Since the current mode

control is subject to sub-harmonic oscillations that peak at

half the switching frequency, Ramp slope compensation is

utilized. This will help to stabilize the power supply. This

Ramp compensation is summed to the Current Sense

Amplifier output and compared to the Error Amplifier

output by the PWM Comparator. When the sum of the

Current

Sense

Amplifier

output

and

the

Slope

Compensation signal exceeds the EA output voltage, the

RS Flip-Flop is reset and HS MOSFET is turned off. The

external Schottky rectifier diode (D1) conducts the inductor

current.

For one whole cycle, if the sum of the Current Sense

Amplifier output and the Slope Compensation signal does

not exceed the EA output, then the falling edge of the

oscillator clock resets the Flip-Flop. The output of the

Error Amplifier increases when feedback voltage (VFB) is

lower than the reference voltage of 0.81V. This also

increases the inductor current as it is proportional to the

EA voltage.

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

Equation 1

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

SW

f

L

ΔI

IN

V

)

OUT

V

IN

(V

OUT

V

L

×

×

−

×

=

Equation 2

Where