To provide single-LED short-circuit diagnostics, the TPS929121-Q1 uses internal ADC to compare the output channel voltage with respect to pre-set threshold V_(ADCSHORTTH).

Setting the register CONF_SSSTART to 1 starts the diagnostics immediately. The CONF_SSSTART returns to 0 in the next clock cycle. Once the diagnostics starts, the on-demand diagnostics ready flag FLAG_ODREADY are cleared to 0. Once the diagnostics finished, the FLAG_ODREADY are set to 1.

In off-state single-LED short-circuit diagnostics, once the master controller initiates single-LED short-circuit diagnostics by setting the register CONF_SSSTART, the device sequentially turns on all outputs starting from OUT0 with DC current I_(ODIOUT) programed by register CONF_ODIOUT and pulse width T_(ODPW) programmable by CONF_ODPW. At the end of pulse, the device initiates an AD conversion. As long as the completion of ADC conversion, the result are compared with pre-set threshold V_(ADCSHORTTH) and start the diagnostics for the next channel. After all channels have been checked, the TPS929121-Q1 also checks if the supply voltage is over V_{(ADCLOWSUPTH)} to make sure the device is not in low-dropout conditions. If the supply voltage is truly lower than V_{(ADCLOWSUPTH)}, the single-LED short-circuit fault cannot be detected and reported. If the supply voltage is high enough, and any one channel output voltage is less than pre-set threshold V_(ADCSHORTTH), the TPS92910-Q1 pulls the ERR pin down with pulsed current sink for 50 μs to report fault and set the flag register including FLAG_ODDIAGCHx, FLAG_OUT and FLAG_ERR to 1. The master controller must write 1 to CLR_FAULT register to clear the fault flags after fault removal is verified by another on-demand off-state single-LED short-circuit diagnostic.

The configurable DC current I_(ODIOUT) and pulse width T_(ODPW) can be used to minimize the optical impact during on-demand diagnostics. TI recommends using the normal current setting and short pulse-width to avoid visible pulse; however, the parasitic capacitance impact at each output must taken care of to leave enough charging time and avoid false alarm. Low DC current setting also reduces LED forward voltage, which also affects the integrity of the detection. Thus the threshold set by CONF_ADCSHORTTH must be selected carefully. Setting CONF_ODIOUT to 0xFh uses the channel current setting by register CONF_IOUTx as on-demand pulse current.

The V(ADCSHORTTH) can be calculated with Equation 7.

Equation 7.

where

• a = 0.2878 V.
• k = 0.1583 V/LSB.
• CONF_ADCSHORTTH is decimal number from 0 to 255.

Figure 8-5 Single-LED Short-Circuit Off-state Timing Sequence