AP3591

Document number: DS36906 Rev. 1 - 2

11 of 18

www.diodes.com

March 2014

© Diodes Incorporated

AP3591

A Product Line of

Diodes Incorporated

Application Information

1. Functional Description

The AP3591 is a synchronous step-down controller. Adaptive constant on time (COT) control is employed to provide fast transition response and

period, in which high side MOSFET is turned on and low side MOSFET is turned off. The ON-time period length is calculated using the following

equation:

f





IN

OUT

ON

V

V

t

directly proportional to the output voltage and inversely proportional to the input voltage. The implementation results in a nearly constant switching

frequency without the need of a clock generator.

minimum OFF time will be applied, which is about 400ns typical.

2. Mode Selection Operation

AP3591 has two operation modes: Continuous Conduction Mode (CCM) and Diode Emulation Mode (DEM). When the EN/DEM pin voltage is

higher than 2.9V, AP3591 will operate in DEM mode for high efficiency; when the EN/DEM pin is floating, AP3591 will operate in forced CCM

mode to a certain frequency during a light load condition.

2.1 Diode Emulation Mode

If the DEM mode is selected, the AP3591 automatically reduces the switching frequency under a light load condition to get high efficiency. When

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

current, which is the boundary between CCM and DEM. The low side MOSFET will turn off whenever the inductor current reaches zero level. The

load is provided only by the output capacitor. When FB voltage is lower than 0.75V, the next ON cycle is beginning. The ON-time is kept the same

load. The transition load point is calculated using the following equation:

ON

t







2L

V

V

I

OUT

IN

LOAD

2.2 Continuous Conduction Mode

inductor current decreases to reverse. The benefit of CCM is to keep the switching frequency fairly constant to avoid a certain frequency during a

light load condition.

3. Power On Reset and Soft-start

below 3.6V, the VDD under voltage lockout (UVLO) circuitry inhibits switching by keeping UGATE and LGATE low. A built-in soft-start is used to

Figure 1 shows a typical start-up interval for AP3591 when the EN/DEM pin has been released from a grounded (system shutdown) state.