ZXSC380

Issue 3 - September 2007

6

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© Zetex Semiconductors plc 2007

Application notes

Typical application circuit

Figure 2 shows a typical boost circuit for 18mA LED current from a single cell 1.5V supply. The

inductor value is 100 H. The operating frequency is typically 160kHz typical for 1.5V supply.

Figure 2 Typical application circuit

The LED current can be set by the choice of inductor values described below.

Inductor selection and current setting

Figure 3 shows the LED current vs the supply voltage while Figure 4 shows the switching

frequency vs the supply voltage for various inductor values. Figure 3 and 4 can be used as guides

to the inductor value selection. The inductor should have a low DC resistance for high efficiency.

The switching frequency is dependent on the programmed peak current, the input voltage, the

input to output voltage differential and the inductor value.

Figure 3 The LED current vs the supply voltage for various inductor values

GND

GND

L1

100uH

LED1

ZXSC380

1

2

3

LED Current vs Supply Voltage

for L = 22, 47, 68 & 100uH

0

2

4

6

8

10

12

14

16

18

20

11.1

1.2

1.3

1.4

1.5

Supply Voltage Vvcc (V)

L = 100uH

L = 68uH

L = 47uH

L = 22uH