ACE709G

0.85V Startup Voltage, Synchronous Step-Up Converter

VER 1.1

8

FUNCTIONAL DECRIPTIONS

Loop Operation

The ACE709G is a wide input range, high-efficiency, DC-to-DC step-up switching regulator, capable of

delivering up to 3W of output power, integrated with

a 150mΩ high side MOSFET and 150 mΩ

synchronous rectifier. 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. ACE709G 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.

Short-Circuit Protection

Unlike most step-up converters, the ACE709G allows for short circuits on the output. In the event of a

short circuit, the device first turns off the NMOS when the sensed current reaches the current limit. After

VOUT drops below VIN the device then enters a linear charge period with the current limited same as with

the start-up period. In addition, the thermal shutdown circuits disable switching if the die temperature rises

above 165°C.

The ACE709G will continue to supply the output voltage even when the input voltage exceeds the

output voltage. Since the PMOS no longer acts as a low-impedance switch in this mode, power

dissipation increases within the IC to cause a sharp drop in efficiency. Limit the maximum output current

to maintain an acceptable junction temperature