ACE701

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

VER 1.5

6

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.

An inductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers large

output current (for example: output current is great than 50mA), large inductor value is recommended in

order to improve efficiency. If DC-DC must output very large current at low input supply voltage, small

inductor value is recommended.

resistor，then the energy can be calculated by following expression:

Δη≈ R

2

Consider all above，inductor value of 47uH、ESR<0.5

Ω is recommended in most applications. Large

value is recommended in high efficiency applications and smaller value is recommended

(2)Capacitor Selection

Ignore ESR of capacitor，the ripple of output voltage is:

So large value capacitor is needed to reduce ripple. But too large capacitor value will slow down system

reaction and cost will improve. So 100uF capacitor is recommended. Larger capacitor value will be used

in large output current system. If output current is small (<10mA), small value is needed.

Consider ESR of capacitor，ripple will increase：

When current is large, ripple caused by ESR will be main factor. It may be greater than 100mV。The ESR

will affects efficiency and increase energy loss. So low-ESR capacitor (for example: tantalum capacitor) is

recommend or connect two or more filter capacitors in parallel.