ACE735E

36V Input Standoff Voltage, 1.5A Step-Down Converter

VER 1.1

5

FUNCTIONAL DESCRIPTIONS

Loop Operation

The ACE735E is a wide input range, high-efficiency, DC-to-DC step-down switching regulator, capable

of delivering up to 1.5A of output current, integrated with a 300mΩ high side MOSFET. 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.

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. ACE735E 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.

APPLICATION INFORMATION

Setting Output Voltages

Output voltages are set by external resistors. The FB threshold is 0.8V.

Inductor Selection

The peak-to-peak ripple is limited to 30% of the maximum output current. This places the peak current

far enough from the minimum overcurrent trip level to ensure reliable operation while providing enough

current ripples for the current mode converter to operate stably. In this case, for 1.5A maximum output

current, the maximum inductor ripple current is 500 mA. The inductor size is estimated as following

equation:

The inductor values is calculated to be L = 13μH. Chose 10μH or 15μH

The inductor values is calculated to be L = 9.2μH. Chose 10μH

Output Capacitor Selection

For most applications a no

minal 22μF or larger capacitor is suitable. The ACE735E internal

compensation is designed for a fixed corner frequency that is equal to

=5V, L=15μH, C

OUT

=22μF.

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:

[1 / (2π x f

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