TLC5941-Q1 Datasheet, PDF (21/27 Pages) Texas Instruments – 16-CHANNEL LED DRIVER WITH DOT CORRECTION AND GRAYSCALE PWM CONTROL

TLC5941-Q1
www.ti.com ........................................................................................................................................................................................... SLDS165 – DECEMBER 2008
GRAYSCALE PWM OPERATION
The grayscale PWM cycle starts with the falling edge of BLANK. The first GSCLK pulse after BLANK goes low
increases the grayscale counter by one and switches on all OUTn with grayscale value not zero. Each following
rising edge of GSCLK increases the grayscale counter by one. The TLC5941 compares the grayscale value of
each output OUTn with the grayscale counter value. All OUTn with grayscale values equal to the counter values
are switched off. A BLANK=H signal after 4096 GSCLK pulses resets the grayscale counter to zero and
completes the grayscale PWM cycle (see Figure 21). When the counter reaches a count of FFFh, the counter
stops counting and all outputs turn off. Pulling BLANK high before the counter reaches FFFh immediately resets
the counter to zero.
BLANK
GS PWM
Cycle n
GS PWM
Cycle n+1
GSCLK
OUT0
(Current)
OUT1
(Current)
t_pd1
t_pd1 + t_d
t_wh1
t_wl1
1
2
t_wl1
t_pd3
3
t_pd3
nxt _d
t_h4
4096
t_wh3
t_pd3+ n x t _d
t_su4
1
t_pd1 + 15 x t_d
OUT15
(Current)
t_pd2
XERR
Figure 21. Grayscale PWM Cycle Timing Chart
Output On Time
The amount of time that each output is turned on is a function of the grayscale clock frequency and the
programmed grayscale PWM value. The on-time of each output can be calculated using Equation 10.
T _ on_n
=
GSn
f_{(GSCLK )}
+ t _{on _ err}
(10)
Where
• T_on_n is the time that OUTn turns on and sinks current
• GSn is OUTn's programmed grayscale PWM value between 0 and 4095
• t_{on_err} is the Output on time error defined in the Switching Characteristics Table
When using Equation 10 with very high GSCLK frequencies and very low grayscale PWM values, the resulting
T_on time may be negative. If T_on is negative, the output does not turn on. For example, using f(GSCLK) = 30
MHz, GSn = 1, and the typical t_{on_err} = 50 nS, Equation 10 calculates that OUTn turns on for –16.6 nS. This
output may not turn on under these conditions. Increasing the PWM value or reducing the GSCLK clock
frequency ensures turn-on.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): TLC5941-Q1
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