TLC5905 Datasheet, PDF (16/27 Pages) Texas Instruments – LED DRIVER

TLC5905
LED DRIVER
SLLS401 – NOVEMBER 1999
PRINCIPLES OF OPERATION
OVM (output voltage monitor) function (continued)
OVM comparator
The OVM comparator compares the voltage on the constant current output terminal during turnon with
comparison voltage set by the OVM latch. When the voltage on the constant current output terminal is lower,
XDOWN2 goes low. As shown in Figure 5, the comparator is provided in every output portion, and the
comparison result corresponding to the output to be turned on appears in the XDOWN2 terminal. Since the
XDOWN2 terminal is an open-collector output, outputs of multiple ICs are brought together.
The output terminal for comparison result is XDOWN2 only. The voltage on all the constant current output can
be checked to monitor XDOWN2, turning output on in turn. The voltage on the constant current output, when
turned on, can also be measured, resulting in a change to the comparison voltage set by the OVM latch. Using
this function, the sensing (LOD function) LED disconnection (output voltage is below 0.3 V) and short circuit
(output voltage is extremely high) can be detected and specifies which LED has encountered this failure. Also,
by monitoring the output voltage and controlling the voltage across anode of the LED to minimize the voltage
on the constant current output (approximately 0.7 V at I_O = 80 mA), the rising temperature of the chip can be
minimized. Furthermore, by setting BLANK to low during LED on, the previous comparison result can be held.
Thus, synchronizing timing to check XDOWN2 from the system to the LED lighting timing is not required. Note
that the gray scale data being turned on should be a minimum of 5 µs since the XDOWN2 output is required
approximately 5 µs after the constant current output is turned on. The comparison result is also required
approximately 5 µs after latch data is changed.
OUT0
–
+
Internal OUT0
Turn ON Signal
OUT1
–
+
Internal OUT1
Turn ON Signal
XDOWN2
OUT14
Internal OUT014
Turn ON Signal
OUT15
Internal OUT015
Turn ON Signal
–
+
–
+
Comparison Voltage
D
Q
LATCH
BLANK
When BLANK is high, hold the data
When BLANK is low, data is out.
Figure 5. OVM Functional Diagram
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