ACE738D

2A, 24V Synchronous Step-Down Converter

VER 1.1

6

Functional Description

The ACE738D is a synchronous, buck voltage converter.

Current-Mode Control

The ACE738D utilizes current-mode control to regulate the FB voltage. Voltage at the FB pin is regulated

at 0.923V so that by connecting an appropriate resistor divider between VOUT and GND, designed output

voltage can be achieved.

PFM Mode

The ACE738D operates in PFM mode at light load. In PFM mode, switch frequency is continuously

controlled in proportion to the load current, i.e. switch frequency is decreased when load current drops to

boost power efficiency at light load by reducing switch-loss, while switch frequency is increased when

load current rises, minimizing both load current and output voltage ripples.

Internal Soft-Start

Soft-start makes output voltage rising smoothly by following an internal SS voltage until SS voltage is

higher than the internal reference voltage. It can prevent the overshoot of output voltage during startup.

Power Switch

N-Channel MOSFET switches are integrated on the ACE738D to down convert the input voltage to the

regulated output voltage. Since the top MOSFET needs a gate voltage greater than the input voltage, a

boost capacitor connected between BST and SW pins is required to drive the gate of the top switch. The

boost capacitor is charged by the internal 3.3V rail when SW is low.

Vin Under-Voltage Protection

A resistive divider can be connected between Vin and ground, with the central tap connected to EN, so

that when Vin drops to the pre-set value, EN drops below 1.2V to trigger input under voltage lockout

protection.

Output Current Run-Away Protection

At start-up, due to the high voltage at input and low voltage at output, current inertia of the output

inductance can be easily built up, resulting in a large start-up output current. A valley current limit is

designed in the ACE738D so that only when output current drops below the valley current limit can the

bottom power switch be turned off. By such control mechanism, the output current at start-up is well

controlled.

Over Current Protection and Hiccup

ACE738D has a cycle-by-cycle current limit. When the inductor current triggers current limit, ACE738D

enters hiccup mode and periodically restart the chip. ACE738D will

exit

hiccup

mode while not

triggering current limit.