SP6685 Datasheet, PDF (12/14 Pages) Sipex Corporation – Charge Pump LED Driver For Camera Flash

SP6685
700mA Buck/Boost Charge Pump LED Driver
RESISTOR SELECTION
The sense resistor R_SENSE is determined by the
value needed in the Torch mode for the
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:
R_SET = (1.26V/ V_FB)*11.2KΩ (Flash Mode)
For an example of 200mA Torch mode and
600mA Flash mode, the values R_SENSE =
0.25Ω, VFB = 150mV (Flash Mode), and R_SET
= 94KΩ 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
range of typical resistor values and sizes are
shown in table 2.
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
68KΩ
75KΩ
82KΩ
91KΩ
100KΩ
110KΩ
120KΩ
130KΩ
140KΩ
150KΩ
0.22Ω
0.27Ω
0.33Ω
0.39Ω
0.47Ω
Tolerance
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
Table 2: Resistor Value and Sizes
Size
0402
0402
0402
0402
0402
0402
0402
0402
0402
0402
0603
0603
0603
0603
0603
EVALUATION BOARD CIRCUIT LAYOUT
© 2009 Exar Corporation
12/14
Rev. 2.0.0