TLC5904

LED DRIVER

SLLS391 – NOVEMBER 1999

18

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

protection

This device incorporates WDT and TSD functions. If WDT or TSD functions, constant current output is stopped

and XDOWN1 goes low. Therefore, by monitoring the XDOWN1 terminal, these failures can be detected

immediately. Since the XDOWN1 output is configured as an open collector, outputs of multiple ICs are brought

together.

WDT (watchdog timer)

The constant current output is forced to turn off and XDOWN1 goes low when the fixed period elapsed after the

signal applied to WDTRG has not been changed. Therefore, by connecting a scan signal (signal to control line

displayed) to WDTRG, the stop of the scan signal can be detected and the constant current output is turned

off. This prevents the LED from burning and damage caused by continuous LED turnon at the dynamic scanning

operation. The detection time can be set using an external capacitor, Cext. The typical value is approximately

10 ms without a capacitor, 160 ms with a 1000 pF capacitor and 1500 ms with a 0.01

µF capacitor. During static

operation, the WDT function is disabled connecting WDCAP to GND (high or low level should be applied to

WDTRG). Note that normal operations will resume changing the WDTRG level when WDT functions.

WDT operational time: T (ms)

≅ 10 + 0.15 × Cext (pF)

0

0.001

0.01

Cext – External Capacitor –

µF

1500

160

10

TLC5904

WDTRG

WDCAP

Scan Signal

Cext

Figure 6. WDT Operational Time and Usage Example

TSD (thermal shutdown)

When the junction temperature exceeds the limit, TSD starts to function and turns the constant current output

off, and XDOWN1 goes low. When TSD is used, TSENA should be pulled high. When TSD is not used, TSENA

should be pulled low. To recover from the constant current output off-state to normal operations, the power

supply should be turned off or TSENA should be pulled low once.