ACE740E

4A, 18V High Efficiency Synchronous Step-Down Converter

VER 1.1

5

Functional Descriptions

Loop Operation

The ACE740E is a wide input range, high-efficiency, DC-to-DC step-down switching regulator, capable of

delivering up to 4A of output current, integrated

with a 90/85mΩ synchronous MOSFET pair, eliminating

the need for external diode. It uses a PWM current-mode control scheme.

An error amplifier integrates

error between the FB signal and the internal reference voltage. The output of the integrator is then

compared to the sum of a current-sense signal and the slope compensation ramp. This operation

generates a PWM signal that modulates the duty cycle of the power MOSFETs to achieve regulation for

output voltage.

Current Limit

There is a cycle-by-cycle current limit on the high-side MOSFET of 5A(typ). When the current flowing out

of SW exceeds this limit, the high-side MOSFET turns off and the synchronous rectifier turns on. Unlike

the traditional method of current limiting by limiting the voltage at the compensation pin, which usually has

large variation due to duty cycle variance, this type of peak current limiting scheme provides a relatively

more accurate limit for output current, thereby lowering the requirements for system design.

Light Load Operation

Traditionally, a fixed constant frequency PWM DC-DC regulator always switches even when the output

load is small. When energy is shuffling back and forth through the power MOSFETs, power is lost due to

the finite RDSONs of the MOSFETs and parasitic capacitances. At light load, this loss is prominent and

efficiency is therefore very low. ACE740E employs a proprietary control scheme that improves efficiency

in this situation by enabling the device into a power save mode during light load, thereby extending the

range of high efficiency operation.