TLC5947 Datasheet, PDF (18/27 Pages) Texas Instruments – 24-Channel, 12-Bit PWM LED Driver with

TLC5947
SBVS114A – JULY 2008 – REVISED SEPTEMBER 2008 ................................................................................................................................................. www.ti.com
NOISE REDUCTION
Large surge currents may flow through the IC and the board on which the device is mounted if all 24 LED
channels turn on simultaneously at the start of each grayscale cycle. These large current surges could introduce
detrimental noise and electromagnetic interference (EMI) into other circuits. The TLC5947 turns on the LED
channels in a series delay, to provide a current soft-start feature. The output current sinks are grouped into four
groups of six channels each. The first group is OUT0, 4, 8, 12, 16, 20; the second group is OUT1, 5, 9, 13, 17,
21; the third group is OUT2, 6, 10, 14, 18, 22; and the fourth group is OUT3, 7, 11, 15, 19, 23. Each group turns
on sequentially with a small delay between groups; see Figure 9. Both turn-on and turn-off are delayed.
POWER DISSIPATION CALCULATION
The device power dissipation must be below the power dissipation rate of the device package (illustrated in
Figure 11) to ensure correct operation. Equation 3 calculates the power dissipation of the device:
P_D = (V_CC ´ I_CC) + (V_OUT ´ I_OLC ´ N ´ d_PWM)
(3)
Where:
• V_CC = device supply voltage
• I_CC = device supply current
• V_OUT = OUTn voltage when driving LED current
• I_OLC = LED current adjusted by R_IREF resistor
• N = number of OUTn driving LED at the same time
• d_PWM = duty ratio defined by GS value
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