TLC5921 Datasheet, PDF (9/14 Pages) Texas Instruments – LED DRIVER

PRINCIPLES OF OPERATION
TLC5921
LED DRIVER
SLLS390 – SEPTEMBER 1999
LOD function (LED open detection)
If any terminal voltage of constant current output (OUT0 TO 15) to be turned on is approximately below 0.3 V,
XDOWN output goes low during output on by knowing LED disconnection. This function is operational for
sixteen OUTn individually. To know which constant current output is disconnected, the level of XDOWN is
repeatedly checked 16 times from OUT0 to OUT15 turning one constant current output on. The power supply
voltage for LED should be set to that the constant current output is applied to above 0.4 V to prevent from
XDOWN low when LED is lighting on normally. Note that on-time should be minimum1µs after the constant
current output is turned on since XDOWN output is required approximately 1 µs.
As discussed earlier, XDOWN is used for both TSD and LOD function. Therefore, BLANK is used to know which
one of TSD or LOD worked when XDOWN went low at LED disconnection, that is, in this condition, when set
BLANK to high, all the constant current outputs are turned off and LOD disconnection detection is disabled, then,
if XDOWN was changed to high, LED disconnection must be occurred.
Table 1 is an example for XDOWN output status using four LEDs.
Table 1. XDOWN Output Example
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{LED NUMBER}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{LED STATUS}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _OUTn
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{DETECTION RESULT}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ XDOWN
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ^{LED NUMBER}
LED STATUS
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _OUTn
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{DETECTION RESULT}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _XDOWN
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{LED NUMBER}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{LED STATUS}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _OUTn
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ DETECTION RESULT
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ^XDOWN2
1
GOOD
ON
GOOD
1
GOOD
ON
GOOD
1
GOOD
OFF
GOOD
2
3
NG
GOOD
ON
ON
NG
GOOD
LOW (by case 2, 4)
2
3
NG
GOOD
ON
OFF
NG
GOOD
LOW (by case 2)
2
3
NG
GOOD
OFF
OFF
GOOD
GOOD
HIGH–IMPEDANCE
4
NG
ON
NG
4
NG
OFF
GOOD
4
NG
OFF
GOOD
noise reduction : output slope
When output current is 80 mA, the time to change constant current output to turn-on and turn-off is approximately
150 ns and 250 ns respectively. This allows to reduce concurrent switching noise occurred when multiple
outputs turn or off at the same time.
thermal pad
The thermal pad should be connected to GND to eliminate the noise influence since it is connected to the bottom
side of IC chip. Also, desired thermal effect will be obtained by connecting this pad to the PCB pattern with better
thermal conductivity.
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