When the supply voltage drops below LED string total forward voltage plus headroom voltage at required current, the TPS92633 device operates in low-dropout conditions to deliver current output as close as possible to target value. The actual current output is less than preset value due to insufficient headroom voltage for power transistor. As a result, the voltage across the sense resistor fails to reach the regulation target. The headroom voltage is the summation of V_(DROPOUT) and V_{(CS_REG)}.

If the TPS92633 is designed to operate in low-dropout condition, the open-circuit diagnostics and single LED short-circuit detection must be disabled by pulling the DIAGEN pin voltage lower than V_IL(DIAGEN). Otherwise, the TPS92633 detects an open-circuit fault or single LED short-circuit fault and reports a fault on the FAULT pin. The DIAGEN pin is used to avoid false diagnostics due to low supply voltage.