AP5100

1.2A Step-Down Converter with 1.4MHz Switching

Frequency

AP5100

8 of 12

MARCH 2010

Document number: DS32130 Rev. 1 - 2

www.diodes.com

Application 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.

Figure 4.

Functional Block Diagram