ACE705

High efficiency, Low supply current, step-up DC/DC Controller

VER 1.4

6

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.

ACE705 has a drive pin (EXT) for external transistor. So it is possible to load a large output current with

a power transistor and a low saturation voltage. At very light load condition, the switch current and

quiescent current of chip will effect efficiency certainly. So in very light load condition, the efficiency will

drop. Therefore, it is recommended that user use ACE705 in the condition of load current as large as

several tens of mA to several hundreds of mA

Selection of the External Components

Thus it can be seen, the inductor, schottky diode and external NMOSFET or NPN transistor.

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, an external NMOSFET or NPN transistor and a right schottky diode, to obtain high

efficiency and low ripple.

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)

If inductor value is too small, the current ripple through it will be great. Then the current through diode

and power switch will be great. Because the power switch on chip is not ideal switch, the energy of switch

will improve. The efficiency will fall.

Thirdly，in general, smaller inductor values supply more output current while larger values start up with

lower input voltage and acquire high efficiency.