ACE703

White LED PFM Step-Up DC-DC Converter

VER 1.2

6

Detailed description

The ACE703 series are boost structure, voltage-type pulse-frequency modulation (PFM) step-up DC-DC

converter. It can deliver constant current or constant voltage. In constant current application, only five

external components are necessary to supply output current from 0 to 500mA:an inductor, an output filter

capacitor, a shottky diode, a compensation capacitor and a resistor which adjusts the output current. In

constant voltage application, to deliver adjustable and low noise output voltage from 2.5V to 6.0V, only six

external components are necessary as well: an inductor, a output filter capacitor, a shottky diode, a

compensation capacitor and two resistors for output voltage detect. The ACE703 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, a voltage limit protection circuit and a driver

transistor.

● How to determine Rc in constant current application condition

For example, if we need Iout=100mA,and we choose the chip of VIFB=200mV,thus

Rc=200mV/100mA=2Ω

● How to determine R1,R2 in constant voltage application

need Vout =3.3V,and we choose the ACE703 of VIFB=400mV and select R2=4KΩ,thus R1=29KΩ.

In constant voltage application, we suggest using the high VIFB type ACE703,because there is voltage

ripple in existence on feedback voltage. When VIFB is high, the infection of the ripple is less than that of

low VIFB type, so the output is more stable.

The PFM control circuit is the core of the ACE703 IC. This block controls power switch on/off 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 ACE703 provides fixed

frequency and pulse width wave.

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 bandwidth.

Though at very low load condition, the quiescent current of chip do affect 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,

ACE703 integrates low on-resistor N-channel MOSFET and well designed driver circuits. The switch

energy loss is limited at very low level.