TLC5922 Datasheet, PDF(9/16 Page) Texas Instruments

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TLC5922 Datasheet, PDF (9/16 Pages) Texas Instruments – LED DRIVER

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TLC5922

SLVS486 â SEPTEMBER 2003

PRINCIPLES OF OPERATION (continued)

Error Information Output

The open-drain output, XDOWN, is used to report both of the TLC5922 error flags. (Note: the XDOWN output

must be pulled to VCC with a pullup resistor)

During normal operating conditions, an internal FET connected to the XDOWN pin is turned off. The voltage on

XDOWN is pulled up to VCC through the pullup resistor. If TEF or LOD is detected (see following sections), the

internal FET is turned on, and XDOWN is pulled to GND.

Since XDOWN is an open-drain output, multiple ICs can be ORâed together and pulled up to VCC with a single

pullup resistor. This reduces the number of signals needed to report a system error.

TEF: Thermal Error Flag

The TLC5922 provides a temperature error flag (TEF) circuit to indicate an over-temperature condition of the IC.

If the junction temperature exceeds the threshold temperature (160oC typ), the TEF circuit trips and pulls

XDOWN to ground.

LOD: LED Open Detection

The TLC5922 provides an LED open detection circuit (LOD). This circuit reports an error if any one of the 16

LEDs is open, or is disconnected from the circuit. The LOD circuit trips when two conditions are met

simultaneouly: When OUTn is programmed to be on, and when the voltage at OUTn is less than 0.3V (Note: the

voltage at each OUTn is sampled 1ÂµS after being turned on).

An LOD failure is reported in two ways. First, the LOD circuit only monitors OUTn when the OUTn is turned on.

Each OUTn is individually monitored. The state of all 16 outputs are ORâed together and reported at the XDOWN

pin.

Second, the LOD circuit may also be monitored from the SOUT pin. When the MODE pin is low and the XLAT

pin goes high, the 16 bits of ON/OFF data get transferred from the EN_REG registers to the EN_LATCHn

latches. At the same time, the LOD status of each output is transferred to the EN_REG registers. These 16 bits

of LOD data are then clocked out of the SOUT pin as the new ON/OFF data is clocked into the SIN pin. This

reporting scheme allows the processor to determine the state of each LED.

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