ACE708

300mA Iout, 0.85V startup, 1MHz, Synchronous Boost Converter

VER 1.2

3

Detail Description

ACE708 is a low input voltage start up, current mode dc-dc step up converter. It’s operation can be best

understood by referring to the block diagram. Upon starting up, the low voltage startup circuitry drives SW

with on-off cycles, transferring energy from input to OUT by storing energy in the inductor during on-time

and releasing it to the output during off-time. When OUT reaches 2V, the startup circuit turns off and the

main controller takes over. The main control loop consists of a reference, a GM error amplifier, a PWM

controller, a current sense amplifier, an oscillator, a PWM logic control, and it is power stage including its

driver. The main control loop is a classic current mode control loop. The GM stage integrates the error

between FB and REF, and its output is used to compare with a triangular wave which the summing result

of the current sense amplifier output and a slope compensation voltage. The output of the comparator is

used to drive the power stage to reach regulation.

Application Information

Output Voltage selection

The output voltages can be set by connecting FB to OUT, to G or to the midpoint of a resistor divider

connected to OUT. See below table for details.

Inductor selection

With switching frequency up to 1MHz, small surface mount inductors can be used with values from

2.2uH to 4.7uH. For a given chosen inductor value and application conditions make sure the peak

inductor current does not exceed the maximum current rating of the selected vendor's inductor.

Input and output capacitor selection

The ACE708's bootstrap architecture allows the use of very small input capacitor. For applications that

only need to drive small output load current, the input capacitor is optional, because once output is started

up, the IC's is powered by OUT, a quiet power supply.

The output capacitor is used to stabilize the loop and provide ac current to the load. A low ESR ceramic

cap with values from 2.2uF to 22uF can be used. Smaller value capacitors are generally cheaper with

small footprints, while larger capacitor provides lower ripples and better transient load responses. Also,

when extreme low startup voltage is needed, larger output capacitors are needed for the part to startup

under heavy load condition.

FB=G

VOUT=5.0V

FB=OUT

VOUT=3.3V

FB to resistor divider

VOUT=0.6V(1+R1/R2)