Diodes Incorporated recently released an adaptive constant on-time (aCOT) synchronous DC-to-DC buck converter that offers high efficiencies and an improved transient response.

The AP65251 from Diodes Incorporated is a 2A adaptive constant on-time (aCOT) synchronous DC-to-DC buck converter that features a control algorithm aimed at improving light-load efficiencies. According to its datasheet, this buck converter, available only in a 6-pin TSOT26 package, is offered as a point-of-load (POL) power supply for applications such as gaming consoles, set top boxes, and home audio equipment.


What Exactly Is an aCOT Buck Converter?

Diodes Incorporated has characterized this aCOT converter as one that provides a seamless transition between the continuous conduction mode (CCM), which is the mode employed during high-load conditions, and the discontinuous mode (DCM), which is used during light-load conditions when the inductor's current is at or is less than 0A. And it's this seamless transition, between these two modes of operation, which is responsible for the converter's advertised better efficiencies and excellent transient load response.

By the way, if you need a quick refresher on the workings of buck regulators, this article on asynchronous vs. synchronous converters and this article on continuous and discontinuous modes may be of some help.


A Closer Look at Efficiency and Transient Response

Within the press release of this aCOT buck converter, Diodes Inc. has provided a product portfolio that includes other DC/DC buck converters, including non-aCOT types. So given this information, I decided to briefly compare the AP65251's efficiencies and transient load response with a similar converter... one that is non-aCOT. The device that I chose, because it has the same switching frequency (500kHz), the same rated output current (2A), and the same 6-pin package as the AP65251, is the AP65211A.

See the product portfolio table below.


The product portfolio table that is included in the AP65251 press release; the AP65251 and AP65211A have similar specifications. Table taken from the AP65251 Press Release (PDF).


Comparing the Efficiencies

The two plots below depict the efficiencies of the AP65251 and AP65211A under similar conditions. Some items to take note of:

  • Both devices have their VOUT set at 1.2V.
  • The VIN levels are the same, except for their VIN_MAX levels (16V vs. 18V).
  • The x-axis and y-axis ranges are NOT consistent.

As can be observed, the efficiencies of these two devices appear to be very similar; this is true for both light-load and high-load conditions.


Efficiency plots of the two buck regulators. Plots taken from the AP65251 datasheet (PDF) and the AP65211A datasheet (PDF).


Comparing the Transient Load Response

The two graphs below demonstrate the load transient response of the two buck regulators. Notice that the load transient—from 1A to 2A—is the same for both devices.

Notice that the AP65251 setting is at 50mV/division while the setting of the AP65211A is at 200mV/division. The aCOT converter (AP65251) exhibits much less output voltage deviation compared to the non-aCOT converter (AP65211A).


Load transient response plots of the two buck regulators, from their respective datasheets (AP65251 and AP65211A).


Based on these two comparisons, if you're looking for a converter that offers high efficiencies and an improved load transient response, when compared to non-aCOT converters, then the AP65251 may be a valid candidate. However, if transient response is not a critical factor in your POL application, then both aCOT and non-aCOT converters might be worth consideration.



Have you had a chance to use this new aCOT DC/DC converter? If so, leave a comment and tell us about your experiences.