L3380

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

8 of 9

www.unisonic.com.tw

QW-R502-099,A

EXTERNAL COMPONENTS

1. Diode (D1)

The diode is the largest source of loss in DC-DC converters. The most important parameters which affect the

efficiency are the forward voltage drop

D

just by having a voltage across the device while a current flowing through it. The reverse recovery time generates a

loss when the diode is reverse biased, and the current appears to actually flow backwards through the diode due to

the minority carriers being swept from the P-N junction. A Schottky diode with the following characteristics is

recommended:

*Low forward voltage:

0.3

D

UV

<

*Fast reverse recovery time/switching speed:

50nS

≤

*Rated current:

PK

I

>

*Reverse voltage:

OD

UU

≥+

2. Inductor (L1)

Low inductance values supply higher output current, but also increase the ripple and reduce efficiency. Choose a low

DC-resistance inductor to minimize loss. It is necessary to choose an inductor with saturation current greater than

the peak current that the inductor will encounter in the application. Saturation occurs when the inductor’s magnetic

flux density reaches the maximum level the core can support and inductance falls.

3. Capacitor (C1,C3)

The input capacitor C1 improves the efficiency by reducing the power impedance and stabilizing the input current.

Select a C1 value according to the impedance of the power supply used. Small Equivalent Series Resistance(ESR)

Tantalum or ceramic capacitor with an appropriate value should be suitable

The output capacitor is used for smoothing the output voltage and sustaining the output voltage when the switch is

on. Select an appropriate capacitor depending on the ripple voltage that increases in case of a higher output voltage

or a higher load current. The capacitor value should be 10uF minimum. Small ESR should be used to reduce output

ripple voltage. However, the best ESR may depend on L, capacitance, wiring and applications(output load).

Therefore, fully evaluate ESR under an actual condition to determine the best value.

4. External transistor (Q1 R1 C2)

An enhancement N-channel MOSFET or a bipolar NPN transistor can be used as the external switch transistor.

*Bipolar NPN transistor

current using a bipolar transistor. 1KΩ is recommended for R1. R1 is selected from the following calculation.

PK

b

FE

I

I

h

=

0.7

0.4

1

||

bEXTH

Vout

R

II

−

=−