ACE721E

1A, 1.5MHz Step-Down Converter

VER 1.3

6

FUNCTION DESCRIPTION

The ACE721E high efficiency switching regulator is a small, simple, DC-to-DC step-down converter

capable of delivering up to 1A of output current. The device operates in pulse-width modulation (PWM) at

1.5MHz from a 2.6V to 5.5V input voltage and provides an output voltage from 0.6V to VIN, making the

ACE721E ideal for on-board post-regulation applications. An internal synchronous rectifier improves

efficiency and eliminates the typical Schottky free-wheeling diode. Using the on resistance of the internal

high-side MOSFET to sense switching currents eliminates current-sense resistors, further improving

efficiency and cost.

Loop Operation

ACE721E uses a PWM current-mode control scheme. An open-loop comparator compares the

integrated voltage-feedback signal against the sum of the amplified current-sense signal and the slope

compensation ramp. At each rising edge of the internal clock, the internal high-side MOSFET turns on

until the PWM comparator terminates the on cycle. During this on -time, current ramps up through the

inductor, sourcing current to the output and storing energy in the inductor. The current mode feedback

system regulates the peak inductor current as a function of the output voltage error signal. During the off

cycle, the internal high –side P-channel MOSFET turns off, and the internal low-side N-channel MOSFET

turns on. The inductor releases the stored energy as its current ramps down while still providing current to

the output.

Current Sense

An internal current-sense amplifier senses the current through the high-side MOSFET during on time

and produces a proportional current signal, which is used to sum with the slope compensation signal. The

summed signal then is compared with the error amplifier output by the PWM comparator to terminate the

on cycle.

Current Limit

There is a cycle-by-cycle current limit on the high-side MOSFET. When the current flowing out of SW

exceeds this limit, the high-side MOSFET turns off and the synchronous rectifier turns on. ACE721E

utilizes a frequency fold-back mode to prevent overheating during short-circuit output conditions. The

device enters frequency fold-back mode when the FB voltage drops below 200mV, limiting the current to

condition.

Soft-start

ACE721E has an internal soft-start circuitry to reduce supply inrush current during startup conditions.

When the device exits under-voltage lockout (UVLO), shutdown mode, or restarts following a

thermal-overload event, the l soft-start circuitry slowly ramps up current available at SW.