ACE708_12 Datasheet, PDF (3/8 Pages) ACE Technology Co., LTD. – 300mA Iout, 0.85V startup, 1.2MHz, Synchronous Boost Converter

ACE708
300mA Iout, 0.85V startup, 1.2MHz, Synchronous Boost Converter
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.
FB=G
FB=OUT
FB to resistor divider
V_OUT=5.0V
V_OUT=3.3V
V_OUT=0.6V(1+R1/R2)
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.
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