AP66300

Document number: DS43722 Rev. 2 - 2

13 of 25

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August 2022

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AP66300

Application Information

Theory of Operation

The AP66300 is a 3A current mode control, synchronous buck regulator with integrated power MOSFETs. Current mode control assures

excellent line regulation, load regulation, and a wide loop bandwidth for fast response to load transients. Figure 1 and Figure 4 depict the typical

application schematic and functional block diagram of the AP66300. The buck controller drives the internal N-FETs. The buck regulator can

operate from an unregulated DC source, such as a battery, with a voltage ranging from 3.8V to 60V. The converter output can be regulated as

low as 0.8V to as high as 50V.

The feedback loop is compensated internally.

See “Loop Compensation Design” for more details.

Internal VCC Regulator

An internal low dropout regulator produces the 4.8V supply from VIN that powers the drivers and the internal bias circuity. The VCC can supply

enough c

urrent for the AP66300’s circuitry and must be bypassed to PGND with a minimum of 1µF ceramic capacitor. Good bypassing is

necessary to supply the high transient currents required by the power MOSFET gate drivers. To improve efficiency, the internal 5V regulator can

also draw current from the BIAS pin when its voltage is at 4.5V or higher. If BIAS is connected to an external supply far away, be sure to bypass

with a local ceramic capacitor. If the BIAS pin voltage is below 4.2V, the internal 5V regulator will source current from VIN. Application with high

input voltage or high switching frequency where the internal 4.8V regulator pulls current from VIN will increase the die temperature.

Enable, Soft-Start, Tracking, Sequencing, and Disable

The enable (EN) input allows the user to control turning on or off the regulator. Once the voltage on the EN pin is above its threshold, the internal

5V LDO power up and soft-start begins.

The regulator does not allow the output to sink current during the soft-start period. The default time is 1.7ms if SS/TR pin is tied to VCC. The

soft-start time can be extended by connecting an external capacitor between SS/TR and GND. The capacitor along with an internal ISS of 1µA,

sets the soft-start interval of the converter, tSS, according to equation below:

CSS (nF) = 1.25*tSS (ms)

Ratiometric tracking is achieved in Figure 37 by using the same value for the soft-start capacitor on each power rail.

IC#1

SS/

TR

IC#2

SS/

TR

CSS

5V

3.3V

Ratiometric Configuration

Performance

T

5V

3.3V

Figure 37. Ratiometic Configuration

By connecting a feedback network from the higher output voltage as shown in Figure 38 below, coincidental track is implemented. The ratio of

R3 and R4 should match the ratio feedback divider of IC#2.