To achieve ASIL B compliance, the TFT bias block should ideally have these features, as shown in Figure 4:
- I2C (the data signal and the clock signal) and the fault pin to perform setting adjustments and diagnostics on each rail
- Undervoltage and overvoltage on each rail
- Internal resistors with fixed or adjustable voltage through I2C (external resistors mean more points of failure, so designers typically avoid them)
- Redundant reference
- Open enable, which provides additional redundancy. When the enable is open, then the chip will look at another pin to determine whether it is on or off.
There are various TFT bias fault scenarios to be aware of:
- VCOM voltage goes out of range
- VGON voltage is in an overvoltage situation
- Fail-safe operation with open enable pin
In the first two scenarios involving VCOM and VGON voltages, the fault pin will alert the MCU of the scenario. The MCU will then read the register to validate the situation and use I2C to adjust the voltages accordingly. In the last scenario, when the enable pin is open while FEN is still high, the output voltages will fall back to the default settings. The MCU can adjust the voltage via I2C.