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S P 6 6 8 5
7 0 0 m A B u c k / B o o s t C h a r g e P u m p L E D D r i v e r
R ESISTOR S ELECTION
The sense resistor R SENSE is determined by the
value needed in the Torch mode for the
range of typical resistor values and sizes are
shown in table 2.
desired output current by the equation:
R SENSE = V FB /I OUT
Where V FB = 50mV (Torch Mode)
Once the R SENSE resistor has been selected for
Torch mode, the V FB voltage can be selected
for Flash mode using the following equation:
V FB = I OUT * R SENSE (Flash Mode)
Where I OUT is for Flash Mode.
Next, the R SET resistor can be selected for
Flash mode using the following equation:
Part Ref.
R SET
R SET
R SET
R SET
R SET
R SET
R SET
R SET
R SET
R SET
R SENSE
R SENSE
R SENSE
R SENSE
R SENSE
Value
68K ?
75K ?
82K ?
91K ?
100K ?
110K ?
120K ?
130K ?
140K ?
150K ?
0.22 ?
0.27 ?
0.33 ?
0.39 ?
0.47 ?
Tolerance
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
Size
0402
0402
0402
0402
0402
0402
0402
0402
0402
0402
0603
0603
0603
0603
0603
R SET = (1.26V/ V FB )*11.2K Ω (Flash Mode)
For an example of 200mA Torch mode and
600mA Flash mode, the values R SENSE =
0.25 Ω , VFB = 150mV (Flash Mode), and R SET
= 94K Ω are calculated. The power obtained in
the Flash mode would be:
P FLASH = V FB *I OUT = 150mV*600mA = 90mW.
The typical 0603 surface mount resistor is
rated 1/10 Watt continuous power and 1/5
Watt pulsed power, more than enough for this
application. For other applications, the P FLASH
power can be calculated and resistor size
selected. The R SENSE resistor is recommended
to be size 0603 for most applications. The
Table 2: Resistor Value and Sizes
E VALUATION BOARD CIRCUIT LAYOUT
? 2009 Exar Corporation
12/14
Rev. 2.0.0
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