AP65552

Document number: DS37306 Rev. 4 - 4

8 of 13

www.diodes.com

October 2018

© Diodes Incorporated

AP65552

PART OBSOLETE

– NO ALTERNATE PART

Application Information

EN

FB

VREG5

SS

EP

GND

SW

VIN

BS

8.25K

Ω

22.1K

Ω

1.5µH

EN

VOUT

VIN

VOUT

AP65552

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

100K

Ω

R3

Figure 3. Typical Application of AP65552

PWM Operation and Adaptive On-time Control

The AP65552 is synchronous step-down converters 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 an 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.

AP65552 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 AP65552 goes into the OFF-time period. The OFF-time period length depends on VFB in most cases. 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.

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 AP65552

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

input voltage’ and to disable the AP65552, EN must be pulled below ‘EN low-level input voltage’.

In Figure 3, EN has a positive voltage through a 100K

Ω pull-up to VIN. No supply input is required for EN.

Soft-Start

The soft-start time of the AP65552 is programmable by selecting a different CSS value. When the EN pin becomes high, the CSS is charged by a

6μA current source, generating a ramp signal fed into non-inverting input of the error comparator. Reference voltage VREF or the internal soft-start

voltage SS, (whichever is smaller), dominates the behavior of the non-inverting inputs of the error amplifier. Accordingly, the output voltage will

follow the SS signal and ramp up smoothly to its target level. The capacitor value required for a given soft-start ramp time can be expressed as:

SS

FB

SS

C

SS

t

I

V





Where CSS is the required capacitor between SS pin and GND, tSS is the desired soft-start time and VFB is the feedback voltage.