SC620EVB datasheet(13/18 Pages) SEMTECH

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SC620

Applications Information (continued)

Backlighting with Series Connected LEDs connected

to a Boosted Output Voltage

This example uses the SC620 to drive 32 LEDs in a 4-in-

series by 8-in-parallel arrangement. Other arrangements

of series and parallel combinations are possible.

To prevent the boost voltage from illuminating the LEDs

while the current sinks are off, the boost voltage must

follow the SC620 in the start-up sequence. The boost

voltage must also power-off before the current sinks turn

off in the shut-down sequence. Protection diodes may be

necessary to protect the current sinks from destructive

voltage levels produced by the boost voltage supply.

Schottky diodes are shown in the schematic for the

purpose of voltage clamping. These diodes prevent

damage to the current sinks in the event that the sink

turns off while the boost circuit is on.

The operating voltage limit of (V

— 1.5V) at the sink pins

must be observed to achieve the specified accuracy of the

device.

SC620

SCL

SDA

GND

ILED1

ILED2

ILED3

ILED4

ILED6

ILED7

ILED8

ILED5

1μF

SCL

SDA

LED Backlighting

R2 = (3/7)*(R1)

Limit SC620 sink pins to < [VIN – 1.5]

DC/DC Boost Voltage

Boost

limiting

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