Diodes ZXSC380 - Manual (Page 6)

ZXSC380
Issue 3 - September 2007 6 www.zetex.com
© 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
V
CC
V
OUT
GND
V
IN
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
1 1.1 1.2 1.3 1.4 1.5
Supply Voltage Vvcc (V)
LE
D Cu
rr
e
n
t I
L
ed
(
m
A
)
L = 100uH L = 68uH L = 47uH L = 22uH