Data Sheet

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POWER DISSIPATION CALCULATION
P =V xI +
D CC CC
V xI
OUT MAX
x
DC
n
63
xd
PWM
xN
(
(
)
)
(7)
OPERATING MODES
SETTING DOT CORRECTION
I
OUTn
+ I
max
DCn
63
(8)
TLC5940
SLVS515C – DECEMBER 2004 – REVISED OCTOBER 2007
The device power dissipation must be below the power dissipation rating of the device package to ensure correct
operation. Equation 7 calculates the power dissipation of device:
where:
V
CC
: device supply voltage
I
CC
: device supply current
V
OUT
: TLC5940 OUTn voltage when driving LED current
I
MAX
: LED current adjusted by R
(IREF)
Resistor
DC
n
: maximum dot correction value for OUTn
N: number of OUTn driving LED at the same time
d
PWM
: duty cycle defined by BLANK pin or GS PWM value
The TLC5940 has operating modes depending on the signals DCPRG and VPRG. Table 4 shows the available
operating modes. The TPS5940 GS operating mode (see Figure 11 ) and shift register values are not defined
after power up. One solution to solve this is to set dot correction data after TLS5940 power-up and switch back
to GS PWM mode. The other solution is to overflow the input shift register with 193 bits of dummy data and latch
it while TLS540 is in GS PWM mode. The values in the input shift register, DC register and GS register are
unknown just after power on. The DC and GS register values should be properly stored through the serial
interface before starting the operation.
Table 4. TLC5940 Operating Modes Truth Table
SIGNAL
INPUT SHIFT REGISTER MODE DC VALUE
DCPRG VPRG
L EEPROM
GND 192 bit Grayscale PWM Mode
H DC Register
L EEPROM
V
CC
96 bit Dot Correction Data Input Mode
H DC Register
L EEPROM
V
(VPRG)
X EEPROM Programming Mode
H Write dc register value to EEPROM. (Default
data: 3Fh)
The TLC5940 has the capability to fine adjust the output current of each channel OUT0 to OUT15 independently.
This is also called dot correction. This feature is used to adjust the brightness deviations of LEDs connected to
the output channels OUT0 to OUT15. Each of the 16 channels can be programmed with a 6-bit word. The
channel output can be adjusted in 64 steps from 0% to 100% of the maximum output current I
max
. Dot correction
for all channels must be entered at the same time. Equation 8 determines the output current for each output n:
where:
I
max
= the maximum programmable output current for each output.
DCn = the programmed dot correction value for output n (DCn = 0 to 63).
n = 0 to 15
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