4 & 6 CHANNEL CHARGE PUMPS FOR WHITE LED BIAS
FSP3302/FSP3303
FUNCTIONAL BLOCK DIAGRAM
C1P
C1N
1x /1.5x
CHARGE
PUMP
C2P
C2N
IN
REFERENCE
THERMAL
SHUTDOWN
OUT
D1
DISCHARGE
OSCILLATOR
CHARGE
CHARGE
D
2
MATCHED
CURRENT
SOURCES
D3
D4
D5
D6
FEEDBACK
EN/
PROG
INTERFACE
CONTROL
32-LEVEL
LOGARITHMIC
DAC
G
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
If EN/PROG is driven low for more than 500µs (t
OFF
), a DISABLE condition is detected and the IC is disabled.
Conversely, if EN/PROG remains high for more than 500µs (t
FINISH
), an ENABLE condition is detected, and the DAC
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
4/10
2007-4-10
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
