FAN5331_05 Datasheet, PDF (8/10 Pages) Fairchild Semiconductor – High Efficiency Serial LED Driver and OLED Supply with 20V Integrated Switch

LED Current vs Input Voltage
(String Connected to FB Pin)
20.2
20.1
20.0
19.9
19.8
19.7
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Input Voltage (V)
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).
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.
Negative Output Voltage vs Load Current
-18
-15V/Unloaded
-16
-15V/10mA Load
-14
-12
-10
0
10
20
30
40
50
Load Current On Positive Output Side (mA)
VIN
2.7V to 5.5V
CIN
4.7µF
5
VIN
L
10µH
C1 ^BAT54S D2
0.1µF
D3
BAT54
D1
SW ¹
R1
C2
4.7µF
-VOUT
IOUT = 10mA
VOUT
COUT _{IOUT = 50mA}
4.7µF
CF
120pF
3
FB
R2
ON
4 _SHDN
2
GND
OFF
Figure 7. Dual (±) Boost Converter
8
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FAN5331 Rev. 1.0.1