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S P 7 6 1 6
H i g h A c c u r a c y 4 - C h a n n e l L o w S i d e L E D D r i v e r f o r
C C F L R e p l a c e m e n t
THEORY OF OPERATION
I NTRODUCTION
The SP7616 is a four-channel constant current
source LED driver with programmable output
current level. The design consists of a
regulator reference voltage source, current
amplifier, and output driver. The precision
reference voltage ensures good performance
over voltage and temperature. The four
outputs are tightly coupled allowing for
I LED =
mA
I LED ≈ ( 1 V ? V MOD )
mA
excellent LED current matching.
S ETTING LED CURRENT
The current in the LED strings is set by
adjusting the R SET resistor connected between
the I SET pin and ground. The LED current is
set using the following the following equation.
K
R SET
Where:
K = Output Current Multiplication Ratio
I LED is the desired LED current
PWM D IMMING
The LED dimming control is done through the
PWM pin. The acceptable frequency range of
this signal is 100Hz to 5kHz. The acceptable
duty cycle range of the signal is 10% to 90%
at 1KHZ. When the PWM pin is driven low,
only the LED current sources are disabled
while the rest of the chip is still enabled.
A NALOG DIMMING CONTROL
Besides digital PWM control, the LED current
can be controlled continuously (from high to
low LED current) by raising the voltage at the
bottom of R SET , V MOD , from 0.0V to 1V
maximum. This configuration is shown in
figure 1 below.
Figure 1
The R SET value may be determined from the
previous equation for V MOD = 0V. The
equation below shows the effect of using a
V MOD in the circuit on the LED current.
K
R SET
Where:
1V = Typical I SET Voltage
K = Output Current Multiplication Ratio
I LED is the desired LED current
V MOD is the adjustment voltage
Note that this method of current control is not
as precise as adjusting R SET . The voltage at
the I SET pin is adjusted slightly during
manufacturing to ensure that K, the output
current multiplication ratio, is as accurate as
possible. When the SP7616 was originally
released at the end of 2006, the multiplication
ratio and I SET voltage were specified
separately. However, market feedback for
better accuracy when using R SET alone
prompted the change to the way the part is
specified today.
S HUTDOWN USING I SET PIN
In normal operation, the voltage at I SET pin is
around 1V. To ensure fast turn on at low duty
cycle and high PWM frequency, only the
output drivers are switching in PWM mode.
However, the whole chip can be shutdown by
pulling the voltage at I SET above 3V minimum.
? 2009 Exar Corporation
7/11
Rev. 2.0.4
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