GSCLK

BLANK

GSPWM

Cyclen

1

2

3

1

GSPWM

Cyclen+1

OUT0

OUT1

OUT15

XERR

nxt d

tpd1

tpd1 +td

tpd1 +15xtd

tpd2

tpd3

twh1

twl1

twl1

tpd3

4096

th4

twh3

tpd3+nxtd

tsu4

(Current)

(Current)

(Current)

)

GSCLK

(

n

f

GSn

on

_

T

err

_

on

t

+

=

TLC59401

www.ti.com

SBVS137 – DECEMBER 2009

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 a grayscale value not equal to zero. Each

following rising edge of GSCLK increases the grayscale counter by one. The TLC59401 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 high BLANK signal after 4096 GSCLK pulses resets the grayscale counter to

zero and completes the grayscale PWM cycle ,as Figure 21 shows. 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.

If there are any unconnected outputs (OUTn), including LEDs in a failed short or failed open condition, the GS

data corresponding to the unconnected output should be set to '0' before turning on the LEDs. Otherwise, the

VCC supply current (ICC) increases while the constant-current output is on.

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.

(10)

where:

T_onn is the time that OUTn turns on and sinks current

GSn is the OUTn programmed grayscale PWM value between 0 and 4095

ton_err is the output on-time error defined in the Switching Characteristics Table

Copyright © 2009, Texas Instruments Incorporated

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