EC9223

Multi-Channel TFT LCD Supply

E-CMOS Corp. (www.ecmos.com.tw)

5E26N-Rev. F001

9/13

Compensation

This current mode boost converter has a current sense loop and a voltage feedback loop. The current sense

loop does not need any compensation. The voltage feedback loop is compensated by an external series R-C

the phase margin which relates to loop stability.

Output Capacitor Selection

The output voltage ripple due to converter switching is determined by the output capacitor total capacitance,

compensation R and C in order to provide adequate phase margin and loop bandwidth.

The second ripple component can be reduced by selecting low-ESR ceramic capacitors and using several

smaller capacitors in parallel instead of just one large capacitor.

Inductor Selection

To prevent magnetic saturation of the inductor core the inductor has to be rated for a maximum current larger

recommended that the inductor be rated at least for 1.8A. Selection of the inductor requires trade-off between

the physical size (footprint x height) and its electrical properties (current rating, inductance, resistance). Within a

given footprint and height, an inductor with larger inductance typically comes with lower current rating and often

larger series resistance. Larger inductance typically requires more turns on the winding, a smaller core gap or a

core material with a larger relative permeability. An inductor with a larger physical size has better electrical

properties than a smaller inductor.

It is desirable to reduce the ripple current ∆ILPP in order to reduce voltage noise on the input and output

capacitors. In practice, the inductor is often much larger than the capacitors and it is easier and cheaper to

increase the size of the capacitors. The ripple current ∆ILPP is then chosen the largest possible while at the

same time not degrading the maximum input and output current that the converter can operate with before

reaching the current limit of the chip or the rated current of the inductor.