Data Sheet

8 www.fairchildsemi.com
FAN5331 Rev. 1.0.1
FAN5331 High Efficiency Serial LED Driver and OLED Supply with 20V Integrated Switch
The feedback loop tightly regulates the current in the branch
connected to FB pin, while the current in the other branch
depends on the sum of the LED’s forward voltages, V
OUT
and
the ballast resistor. The input and the output ripple is less than
3mV
RMS
, for load currents up to 40mA.
A Zener diode (V
Z
= 22V) connected between V
OUT
and GND
can prevent the FAN5331 from being damaged by over-voltage,
if the load is accidently disconnected during operation.
2. Dual Boost Converter
A negative voltage can be provided by adding an external
charge pump (C1, C2, D2, and D3).
Figure 7. Dual (±) Boost Converter
While the feedback loop tightly regulates V
OUT
, the negative out-
put voltage (-V
OUT
) can supply a light load with a negative volt-
age. Nevertheless, the negative voltage depends on the
changes of the load current in both -V
OUT
and +V
OUT
, as shown
in the graph below.
2.5 3.0 3.54.04.55.05.56.0
19.7
19.8
19.9
20.0
20.1
20.2
LED Current (mA)
Input Voltage (V)
LED Current vs Input Voltage
(String Connected to FB Pin)
SHDN
V
IN
IN
V
OUT
-V
OUT
R1
R2
GND
V
FB
SW
F
AN5331
C
F
C
IN
10µH
1
3
2
4
5
4.7µF
120pF
C
OUT
L
BAT54
BAT54S
4.7µF
2.7V to 5.5V
ON
OFF
C2
C1
D1
D3
D2
4.7µF
0.1µF
I
OUT
= 10mA
I
OUT
= 50mA
01020304050
-18
-16
-14
-12
-10
Negative Output Voltage vs Load Current
Load Current On Positive Output Side (mA)
Negative Output Voltage (V)
-15V/Unloaded
-15V/10mA Load