ACE701

PFM STEP-UP DC/DC Converter with High Efficiency and Low Noise

VER 1.5

5

Detailed Description

The ACE701 series are boost structure, voltage-type pulse-frequency modulation(PFM) step-up DC-DC

converter. Only three external components are necessary: an inductor, an output filter capacitor and a

schottky diode. And the converter

’s low noise and low ripple output voltage can be adjusted from 2.5V to

5.0V, 0.1V step. By using the depletion technics, the quiescent current drawn from power source is lower

than 7uA. The high efficiency device consists of resistors for output voltage detection and trimming, a

start-up voltage circuit, an oscillator, a reference circuit, a PFM control circuit, a switch protection circuit

and a driver transistor.

The PFM control circuit is the core of the ACE701 IC. This block controls power switch on duty cycle to

stabilize output voltage by calculating results of other blocks which sense input voltage, output voltage,

output current and load conditions. In PFM modulation system, the frequency and pulse width is fixed.

The duty cycle is adjusted by skipping pulses, so that switch on-time is changed based on the conditions

such as input voltage, output current and load. The oscillate block inside ACE701 provides fixed

frequency and pulse width wave.

The reference circuit provides stable reference voltage to output stable output voltage. Because internal

trimming technology is used, The chip output change less than ±2%. At the same time，the problem of

temperature-drift coefficient of output voltage is considered in design,

so temperature-drift coefficient of

output voltage is less than 100ppm/℃。

High-gain differential error amplifier guarantees stable output voltage at difference input voltage

and load. In order to reduce ripple and noise, the error amplifier is designed with high band-with.

Though at very low load condition, the quiescent current of chip do effect efficiency certainly. The four

main energy loss of Boost structure DC-DC converter in full load are the ESR of inductor, the voltage of

Schottky diode, on resistor of internal N-channel MOSFET and its driver. In order to improve the efficiency,

ACE701 integrates low on-resistor N-channel MOSFET and well design driver circuits. The switch energy

loss is limited at very low level.

Selection of the External Components

Thus it can be seen, the inductor and schottky diode affect the conversion efficiency greatly. The

inductor and the capacitor also have great influence on the output voltage ripple of the converter. So it is

necessary to choose a suitable inductor, a capacitor and a right schottky diode, to obtain high efficiency,

low ripple and low noise.

Before discussion，we define D≡Vout-Vin / Vout

(1)Inductor Selection

Above all, we should define the minimum value of the inductor that can ensure the boost DC-DC to

operate in the continuous current-mode condition.

Lmin≧D(1-D)