ACE724

18V (30V spike) 2A Synchronous PFM/PWM Buck Converter

VER 1.3

6

Functional Descriptions

Loop Operation

The ACE724 is a wide input range, high-efficiency, DC-to-DC step-down switching regulator, capable of

delivering up to 2A of output current, integrated with a 110m

synchronous MOSFET, 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 3A(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 current mode 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. ACE724 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.

Faster Transient Response

Normally, people use 3.3nF and 5.6Kohm RC for compensation to keep the loop stable. However, if one

needs to have faster load transient response, 3.3nF and 8.5Kohm is recommended.