4 & 6 CHANNEL CHARGE PUMPS FOR WHITE LED BIAS

2007-4-10

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FSP3302/FSP3303

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As shown in the Functional Block Diagram, the FSP3302/FSP3303 incorporate proprietary charge pump technology

that includes automatic 1x/1.5x mode-switching to maximize efficiency. Accurate matched current sources are

controlled by a 32-level logarithmic DAC in order to provide an accurate, efficient, and programmable bias supply for

white LEDs.

CHARGE PUMP

The FSP3302/FSP3303 charge pump operates at 1.2MHz, and alternately charges and discharges the flying

capacitors at C1P, C1N, C2P and C2N to generate sufficient output voltage at OUT to allow accurate biasing of white

LEDs. These devices feature an automatic 1x/1.5x mode-switching charge pump architecture to ensure the highest

possible efficiency for a charge pump solution. These devices utilize a proprietary regulation technique that

minimizes the output impedance of the charge pump, allowing operation that is superior to competing products down

to lower input voltages.

EN/PROG PIN

EN/PROG is a multi-function pin, providing both an enable/disable function as well as permitting the user a simple

means to digitally program the LED current

ENABLE/DISABLE

is initialized to -31dB upon the next high to low transition of EN/PROG.

To properly reset the DAC upon enabling the IC, the input voltage should be applied first while EN/PROG is driven

low. A subsequent low to high transition on EN/PROG pin turns the IC on.

PROGRAMMING LED CURRENT

The interface control block decodes the digital serial input signal at EN/PROG and produces a programming code

for the 32-level logarithmic DAC. The logarithmic DAC sets the output currents to one of 32 different levels, in 1dB

steps, with the highest level of 0dB corresponding to the full-scale current of 44mA. Logarithmic current scaling

results in linear increments in LED brightness. When enabled (on an EN/PROG rising edge), the DAC initializes to

its -31dB level. Thereafter, each high to low transition of the EN/PROG pin that occurs within 75µs increments the

1x /1.5x

CHARGE

PUMP

MATCHED

CURRENT

SOURCES

REFERENCE

INTERFACE

CONTROL

OSCILLATOR

32-LEVEL

LOGARITHMIC

DAC

THERMAL

SHUTDOWN

CHARGE

FEEDBACK

DISCHARGE

CHARGE

IN

EN/

PROG

C1P

C1N

C2P

C2N

OUT

D1

D2

D3

D4

D5

G

D6