TLC5902 Datasheet, PDF (14/22 Pages) Texas Instruments – LED DRIVER

TLC5902
LED DRIVER
SLLS382 – DECEMBER 1999
PRINCIPLES OF OPERATION
frequency division ratio setting for GSCLK(clock for gray scale) – panel brightness adjustment
By using the upper 3 bits of the brightness control latch, GSCLK can be divided into 1/1 to 1/8. If GSCLK is set
to 8 times the speed (256 × 8 = 2048) of the frequency during the horizontal scanning time, the brightness can
be adjusted to 8 steps by selecting the frequency division ratio. Thus, the total panel brightness can be adjusted
at once and applied to the brightness of day or night. When BC_ENA is pulled low, GSCLK is not divided. When
BC_ENA is pulled high, the brightness can be adjusted as shown in the following table.
CODE
MSB 000 LSB†
001
010
011
100
101
110
111
† _{BC_ENA is low.}
FREQUENCY DIVISION RATIO
1/1
1/2
1/3
1/4
1/5
1/6
1/7
1/8
RELATIVE BRIGHTNESS RATIO
(%)
12.5
25.0
37.5
50.0
62.5
75.0
87.5
100
constant output current operation
The constant current output turns on (sink constant current) if all the gray-scale data latched into the gray-scale
latch is not zero on the falling edge of GCLK, after the next rising edge of GCLK when BLANK goes from high
to low. After that, the number of the falling edge is counted by the 8-bit gray scale counter. Output counted that
corresponds to gray-scale data is turned off (stop to sink constant current). If the shift register for gray scale is
updated during LATCH high, data on the gray-scale data latch is also updated, affecting the number of gray
scale of constant current output. Accordingly, during the on state of constant current output, LATCH is kept to
low and the gray-scale data latch is held. When unconnected constant current output terminals exist, operation
is complete after writing zero (data for LED turn off) to the corresponding gray-scale data latch. If this action is
not completed, the supply current (I_PVCC) in the constant current driver portion increases.
protection
This device incorporates WDT and TSD functions. In WDT or TSD functions, the current output is stopped (Logic
portion is still operating). By monitoring the DOWN terminal, these failures are detected immediately. Since
DOWN output is configured as an open collector, outputs of multiple ICs are brought together.
WDT
When the scan signal is stopped during a fixed period in dynamic scanning operation, the constant current
output is turned off, preventing the LED from burning damage. The time detected can be set using the external
capacitor (C1). The typical value is approximately 3 ms without a capacitor, 33 ms with a 1000 pF capacitor,
and 300 ms with a 0.01 µF capacitor. Once the scan signal is applied again, the abnormal status is released
and normal operation is resumed. During static operation, the WDT function is disabled, connecting WD_CAP
to GND. The scan signal should be applied to the WDT terminal even though the WDT function is not used.
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