AP65455

Document number: DS37994 Rev. 3 - 4

9 of 15

www.diodes.com

October 2018

© Diodes Incorporated

AP65455

PART OBSOLETE - NO ALTERNATE PART

Application Information

EN

FB

VREG5

SS

EP

PGND

SW

VIN

BS

8.25K

Ω

22.1K

Ω

1.5µH

EN

VOUT

VIN

VOUT

AP65455

1

2

3

4

11

6, 7

8

9, 10

8.2nF

1µF

10µF

22µF

0.1µF

R3

R1

C4

C6

L1

C1

10µF

C2

C8

22µF

C5

C9

C10

100K

Ω

R4

PG

100K

Ω

5

VIN

R2

JP1

Figure 3 Typical Application of AP65455 evaluation board

PWM Operation and Adaptive On-time Control

The AP65251 is a synchronous step-down converter with internal power MOSFETs. Adaptive constant on time (aCOT) 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 for a fixed one shot

timer, ON-time period. This one shot is calculated by the converter

’s input voltage (VIN) and the output voltage (VOUT) cycle-by-cycle based to

maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The high-side MOSFET turned off after

the fixed on time expire and turn on the low-side MOSFET. Once the output voltage dropped below the output regulation, the low-side turned off.

The one-shot timer then reset and the high-side MOSFET is turned on again.

AP65455 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:

S

f

V





IN

OUT

ON

V

T

VOUT is the output voltage

VIN is the input voltage

fS is the switching frequency

After an ON-time period, the AP65455 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 AP65455 is designed with Power Save Mode (PSM) at light load conditions for high efficiency. The AP65455 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 its valley 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 AP65455, EN must be pulled above

the ‘EN high-level

input voltage

.’ To disable the AP65455, 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) directly or through a 100K

Ω pull-up to VIN for automatic

start-up.