ACE701D

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

VER 1.3

5

Detailed Description

The ACE701D 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 ACE701D 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 ACE701D 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,

ACE701D 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)

The above expression is got under conditions of continuous current mode, neglect Schottky diode

’s

voltage, ESR of both inductor and capacitor. The actual value is greater that it. If inductor

’s value is less

than Lmin，the efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will not be stable.

Secondly, consider the ripple of the output voltage,

ΔI=D•Vin / Lf

Im ax=Vin / (1-D)