TLC5904

LED DRIVER

SLLS391 – NOVEMBER 1999

16

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

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 only XDOWN2. 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 be also measured changing the comparison voltage set by the OVM latch. Using this function,

sensing (LOD function) an LED disconnection (output voltage is below 0.3 V) and short circuit (output voltage

is extremely high) can be detected and specifies which LED 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

Furthermore, by setting BLANK to low during LED on, the comparison result immediately before 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 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 the changed latch data.

D

+

–

+

–

+

–

+

–

LATCH

Q

XDOWN2

OUT0

Internal OUT0

Turn ON Signal

OUT1

OUT14

OUT15

Internal OUT1

Turn ON Signal

Internal OUT014

Turn ON Signal

Internal OUT015

Turn ON Signal

Comparison Voltage

BLANK

When BLANK is high, hold the data

When BLANK is low, data is out.

Figure 5. OVM functional diagram