ACE708C

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

VER 1.2

3

Detail Description

ACE708C 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 ACE708C'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

FB=OUT

FB to resistor divider