AP65403

Document number: DS37126 Rev. 1 - 2

8 of 15

www.diodes.com

March 2014

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AP65403

Application Information

Theory of Operation

The AP65403 is a 4A current mode control, synchronous buck regulator with built in power MOSFETs. Current mode control assures excellent

line and load regulation and a wide loop bandwidth for fast response to load transients. The Figure 1 depicts the functional block diagram of

AP65403.

The operation of one switching cycle can be explained as follows. At the beginning of each cycle, HS (high-side) MOSFET is off. The error

amplifier (EA) output voltage is higher than the current sense amplifier output, and the current comparator’s output is low. The rising edge of the

750kHz oscillator clock signal sets the RS Flip-Flop. Its output turns on HS MOSFET. The current sense amplifier is reset for every switching

cycle.

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.

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 0.8V. This also increases the inductor current as it is proportional to the comp voltage.

If in one cycle the current in the power MOSFET does not reach the COMP set current value, the power MOSFET will be forced to turn off. When

the HS MOSFET turns off, the synchronous LS MOSFET turns on until the next clock cycle begins. There is a “dead time” between the HS turn

off and LS turn on that prevents the switches from “shooting through” from the input supply to ground.

The voltage loop is compensated through an internal transconductance amplifier and can be adjusted through the external compensation

components.

Enable

Above the ‘EN Rising Threshold’, the internal regulator is turned on and the quiescent current can be measured above this threshold. The enable

(EN) input allows the user to control turning on or off the regulator. To enable the AP65403, EN must be pulled above the ‘EN Lockout Threshold

Voltage’ and to disable the AP65403, EN must be pulled below ‘EN Lockout Threshold Voltage - EN Lockout Hysteresis’

(2.2V-0.22V =1.98V).

Automated No-Load and Light-Load Operation

The AP65403 operates in Light load high efficiency mode during light load operation. The advantage of this light load high efficiency mode is low

power loss at no-load and light-load conditions. The AP65403 automatically detects the output current and enters the light load high efficiency

mode. The output current reaches a critical level at which the transitions between the light-load and heavy current mode occurs. Once the output

current exceeds the critical level, the AP65403 transitions from light load high efficiency mode to continuous PWM mode.

External Soft Start

Soft start is traditionally implemented to prevent the excess inrush current. This in turn prevents the converter output voltage from overshooting

when it reaches regulation. The AP65403 has an internal current source with a soft start capacitor to ramp the reference voltage from 0V to

0.800V. The soft start current is 6µA. The soft start sequence is reset when there is a Thermal Shutdown, Under Voltage Lockout (UVLO) or

when the part is disabled using the EN pin.

External Soft Start can be calculated from the formula below:

DT

DV

*

C

SS

I

=

Where;

C = External Capacitor

DV=change in feedback voltage from 0V to maximum voltage

DT = Soft Start Time