L3380

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

9 of 9

www.unisonic.com.tw

QW-R502-099,A

EXTERNAL COMPONENTS(Cont.)

Since the pulse current flows through the transistor, the exact Rb value should be finely tuned by the

experiment. Generally, a small Rb value can increase the output current capability, but the efficiency will

decrease due to more energy is used to drive the transistor.

Moreover, a speed up capacitor, C2, should be connected in parallel with R1 to reduce switching loss and

improve efficiency. C2 can be calculated by the equation below:

1

2

2

1

0.7

OSC

C

Rf

π

≤

××

×

It is due to the variation in the characteristics of the transistor used. The calculated value should be used as

the initial test value and the optimized value should be obtained by the experiment.

*Enhancement MOS FET

For enhancement N-channel MOSFET, since enhancement MOSFET is a voltage driven, it is a more efficient

switch than a BJT transistor. However, the MOSFET requires a higher voltage to turn on as compared with BJT

transistors. An enhancement N-channel MOSFET can be selected by the following guidelines:

-

Input capacitance less than 700pF.

-

Low gate threshold voltage.

-

Low on-resistance.

-

The allowable maximum current of drain should be larger than peak current of inductor.

UTC assumes no responsibility for equipment failures that result from using products at values that

exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or

other parameters) listed in products specifications of any and all UTC products described or contained

herein. UTC products are not designed for use in life support appliances, devices or systems where

malfunction of these products can be reasonably expected to result in personal injury. Reproduction in

whole or in part is prohibited without the prior written consent of the copyright owner. The information

presented in this document does not form part of any quotation or contract, is believed to be accurate

and reliable and may be changed without notice.