ACE703

White LED PFM Step-Up DC-DC Converter

VER 1.2

8

Cf

R

D

Vout

Vout

r

load

=

Δ

=

Vout

R

ax

r

r

ESR

•

+

=

Im

'

1

2

1 +

=

R

R

V

Vout

IFB

2

)

1

(

R

V

Vout

IFB

×

−

Vout

Vout

I

C

L

Vout

L

−

+

≈

Δ

2

2

(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.

(3) Diode Selection

Rectifier diode will affects efficiency greatly，Though a common diode (such as 1N4148) will work well for

light load, it will reduce about 5%~10% efficiency for heavy load, For optimum performance, a Schottky

diode (such as 1N5817.1N5819.1N5822) is recommended.

(4) Input Capacitor

If supply voltage is stable, the DC-DC circuit can output low ripple, low noise and stable voltage without

input capacitor. If voltage source is far away from DC-DC circuit, input capacitor value greater than 10uF

is recommended.

(5) Resistor R1.R2 for output voltage detect

In constant current application,R1 and R2 can be selected by following expression:

For example, when VIFB=400mV,Vout=3.3V.

If R2=4KΩ,then R1=

=29KΩ;if R2=40KΩ,then R1=290 KΩ.

(6) Selection of L.C and compensation capacitor C1

When the load current is large（eg: Iout=100mA）,the output voltage ripple will increase:

As we can see, When output current is large, the maximum current of the inductor IL will increase, thus

the ripple of the output voltage will increase. To decrease the ripple, there are two ways: