ACE703

White LED PFM Step-Up DC-DC Converter

VER 1.2

9

Firstly, select the appropriate value of L.C. It require the value of L*C be not too large, and the value of

L/C be small. For example, select L=22uH,C=100uF;or L=10uH,C=47uF;etc.

If select L/C>１（such as L=100uH,C=47uF）,it is improper in large current（larger than 100mA）

application.

Secondly, add a compensation capacitor C1 to reduce the ripple. However, the value of the

compensation capacitor is different between constant current application and constant voltage mode.C1

and the resistor parallel connected with it will generate a pole, thus, it can compensate the syntonic

frequency made by L.C.

In constant voltage mode, if the resistor parallel connected with C1 is large,(eg: R1=290KΩ) then τ=R1‧

C1 is large, therefore, when C1≥0.1uF,the ripple can be decreased greatly.

But in constant current mode, the resistor parallel connected with C1 is just the equivalent resistor of the

white-LED, about 1Ω.Then,to maintain larger τ require larger compensation capacitor, normally between

10uF to 47uF.Many experiments have proved, when L=22uH,Cout=100uF,C1=22uF,the ripple of the

output voltage, the output current, and the feedback voltage is smallest, and the characteristic of the

constant current is excellent, even the efficiency is highest.

Test circuits

(1) Feedback voltage test circuit

(2) Start-up voltage test circuit（Iload=1mA）

ACE703

ACE703