FAN5640S7X Datasheet, PDF (8/11 Pages) Fairchild Semiconductor – FAN5640 Dual High-Side Constant Current Source for High-Voltage Keypad LED Illumination

Application Information
Setting the Output Current Level
An internally generated reference current is mirrored on
the MOSFETs connected to the outputs IOUT1 and
IOUT2 (pins 1 and 6, respectively). The current mirror
ratio is 275 (typical). The voltage on pin 3 (RSET) is
0.475V in steady state; therefore, the programmed
current through each of the outputs is:
I_OUT

275 
0.475
R_EXT
EQ. 1
where R_EXT is the external resistor connected from pin 3
to ground. Increasing this external programming resistor
reduces the output current. For the maximum rated
25mA rating of each output, the minimum value of the
external resistor is:
R_EXT

275 
0.475
I_OUT

275 
0.475
0.025

5.225k
EQ. 2
The LED output current accuracy is ±10% for 25mA
current (see the Electrical Characteristics table). In the
worst-case scenario, the calculated value of I_OUT can
lead to an error of ±10% in the LED current. Since the
tolerance of R_EXT also affects the LED current accuracy,
a precision resistor should be chosen to have the least
effect on the overall accuracy of the LED current
(see Figure 12).
Floating vs. Tied Outputs
Unused outputs can be left floating. The current through is
zero, regardless of the current programmed at pin 3.
However, ESD protection is enhanced if the unused
output pin is tied to VIN (pin 5).
If the two output pins are tied together, they can deliver
a combined 50mA for the same programming resistor of
5.225kΩ.
External Capacitors
Because the FAN5640 is stable without capacitors on
the outputs, no capacitors are recommended. Typical
input decoupling usually present on incoming supply
rails should suffice in most applications. If necessary, a
small input capacitance may be placed between the
input pin and ground without adverse effects.
Dropout Limitations
As for any LDO regulator, there are limitations on how
close the input and output rails can be to maintain
regulation. The minimum difference is referred to as the
dropout. The relevant information is provided in the
Typical Performance curve Dropout Voltage vs. LED
Current (see Figure 8). The equation for the data is:
V_DO  0.35V  I_OUT  64
EQ. 3
This is equivalent to an R_DS of 64Ω with an additional
offset of 350mV. This equation is helpful in determining
the minimum dissipation in the device and the lowest
input voltage for a given application.
Multiple LED Displays
For portable applications, the FAN5640 can be powered
from the output of any typical boost regulator. Multiple
LED displays can be created with the FAN5640 powered
from the output of the FAN5333, as shown in Figure 20.
Note that the output voltage of the FAN5333 depends
upon the number of LEDs in its output string. Being
conscious of the minimum dropout requirements of the
FAN5640; if three series LEDs are required to be
present at its output, then the FAN5333 should have
four series LEDs in its output string.
L
6.8µH to
10µH
V _IN
C_IN
VIN SW
4.7µF
to
10µF
FB
ON
OFF
SSHHDDNN _GGNNDD
FAN5333
C_OUT
0.1µF
to
2.2µF
IOUT1 ₁
6 ^IOUT2
GND ₂
5 ^VIN
RSET ₃
4 ^EN
R_EXT ^FAN5640
© 2006 Fairchild Semiconductor Corporation
FAN5640 • Rev. 1.0.2
Figure 20. LED Display Example
8
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