AP65352

Document number: DS38482 Rev. 2 - 3

9 of 15

www.diodes.com

December 2017

© Diodes Incorporated

AP65352

Application Information

EN

FB

VREG5

SS

EP

PGND

SW

VIN

BST

8.25K

Ω

22.1K

Ω

1.5µH

EN

VOUT

VIN

VOUT

AP65352

1

2

3

4

5

6

7

8

8.2nF

1µF

10µF

22µF

0.1µF

R2

R1

C5

C4

L1

C6

10µF

C1

C2

22µF

C8

C9

C7

Figure 3 Typical Application of AP65352 evaluation board

PWM Operation and Adaptive On-time Control

The AP65352 is a synchronous step-down converter with internal power MOSFETs. Adaptive constant on time (COT) control is employed to

provide fast transient response and easy loop stabilization. At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is

turned off after internal one-shot timer expires. This one-shot is set by the converter input voltage (VIN), and the output voltage (VOUT) to maintain a

pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The output voltage variation is sensed by

FB voltage. The one-shot timer is reset and the high-side MOSFET is turned on again when FB voltage falls below the 0.76V.

AP65352 uses an adaptive on-time control scheme and does not have a dedicated in-board oscillator. It runs with a pseudo-constant frequency of

650kHz by using the input voltage and output voltage to set the on-time one-shot timer. The on-time is inversely proportional to the input voltage

and proportional to the output voltage. It can be calculated using the following equation:

f

V





IN

V

OUT

ON

T

VOUT is the output voltage

VIN is the input voltage

f is the switching frequency

After an ON-time period, the AP65352 goes into the OFF-time period. The OFF-time period length depends on VFB in most case. It will end when

the FB voltage decreases below 0.76V, then the ON-time period is triggered. If the OFF-time period is less than the minimum OFF time, the

minimum OFF time will be applied, which is about 260ns typical.

Power Save Mode

The AP65352 is designed with Power Save Mode (PSM) at light load conditions for high efficiency. The AP65352 automatically reduces the

switching frequency and changes the Ton time to Tmin-on time during a light load condition to get high efficiency and low output ripple. As the

output current decreases form heavy load condition, the inductor current decreases as well, eventually comes close to zero current, which is the

boundary between CCM and DCM. The low side MOSFET is turned off when the inductor current reaches zero level. The load is provided only by

output capacitor, when FB voltage is lower than 0.76V, the next ON cycle begins. The on-time is the minimum on time that benefits for decreasing

VOUT ripple at light load condition. When the output current increases from light to heavy load the switching frequency increases to keep output

voltage. The transition point to light load operation can be calculated using the following equation:

ON

T

V







2L

OUT

IN

V

LOAD

I

TON is on-time

Enable

Above the ‘EN high-level input voltage’, 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 AP65352

, EN must be pulled above the ‘EN high-level

input voltage

.’ To disable the AP65352, EN must be pulled below ‘EN low-level input voltage.’

In Figure 3, EN is a high voltage input that can be safely connected to VIN (up to 18V) for automatic start-up.