ACE721G

5V 1A, 1.5MHz

Synchronous Buck Converter

VER 1.1

8

Soft-start

ACE721G Series has a 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.

UVLO and Thermal Shutdown

If IN drops below 2.5V, the UVLO circuit inhibits switching. Once IN rises above 2.5V, the UVLO clears,

and the soft-start sequence activates. Thermal-overload protection limits total power dissipation in the

device. When the junction temperature exceeds TJ= +160°C, a thermal sensor forces the device into

shutdown, allowing the die to cool. The thermal sensor turns the device on again after the junction

temperature cools by 15°C, resulting in a pulsed output during continuous overload conditions. Following

a thermal-shutdown condition, the soft-start sequence begins.

Design Procedure

Setting Output Voltages

Output voltages are set by external resistors. The FB_ threshold is 0.6V.

Input Capacitor Selection

The input capacitor in a DC-to-DC converter reduces current peaks drawn from the battery or other input

power source and reduces switching noise in the controller. The impedance of the input capacitor at the

switching frequency should be less than that of the input source so high-frequency switching currents do

not pass through the input source. The output capacitor keeps output ripple small and ensures

control-loop stability. The output capacitor must also have low impedance at the switching frequency.

Ceramic, polymer, and tantalum capacitors are suitable, with ceramic exhibiting the lowest ESR and

high-frequency impedance. Output ripple with a ceramic output capacitor is approximately as follows:

π x f

If the capacitor has significant ESR, the output ripple component due to capacitor ESR is as follows: