3 Discrete Fault Detection Circuit Details

The discrete fault detection circuit utilizes the ATL431LI-Q1 device as a comparator to monitor the TPS9261x-Q1 output voltage. The ATL431LI-Q1 is a high bandwidth low I_Q programmable shunt regulator that can act as a comparator in open loop mode. Additionally, the device has an internal reference voltage of 2.5 V and works well across temperature providing an accurate output voltage measurement. For more information on the ALT431LI-Q1 performance across temperature, see Using the TL431 for Undervoltage and Overvoltage Detection application note. The ATL431LI-Q1 output (V_comp) is used to differentiate between OL, SB, and SG or OT conditions. Figure 3-1 illustrates the initial schematic with the fault detection circuit added on to the TPS9261x-Q1.

Figure 3-1 Discrete Fault Detection Schematic

A resistor divider must be used to properly divide the voltage going into the ATL431LI-Q1 from the output of the TPS9261x-Q1. This is to properly compare the OUT voltage to the reference voltage. In addition, this resistor ladder creates the trip point (where the comparator output changes). A trip point of 8.5 V was used for the tests in Section 3.1 to Section 3.4. To verify the resistor ladder behaves as expected, the total series resistance (R_total) of the resistive divider R8 and R11 must be approximately 8.5 kΩ. Due to the I_(retry), a R_total that is too large causes the output and input voltages to be close to equal and therefore the ATL431LI-Q1 comparator does not trigger. R_sup and R_bias are used in accordance to Using the TL431 for Undervoltage and Overvoltage Detection application note to verify that the BJT has an appropriate voltage drop and that the ATL431LI-Q1 is not sinking excess current. R_gnd is utilized to pullup the SB, OL output to V_sup. Additionally, a BJT is used to verify that the ATL431LI-Q1 output swings from Vsup (3.3 V) to 0 V as described in the previously-referenced application note. Table 3-1 documents the logic truth table associated with the fault detection circuit. Each unique outcome is highlighted to show the logic for a microcontroller to differentiate between each fault scenario.