8.1.3 V_GS Considerations

The V_GS Status Comparator accomplishes two purposes:

1. As the gate of the external MOSFET is charged, the V_GS voltage transitions from cut-off, through an active region, and into the ohmic region. The LM5060 provides two fault timer modes to monitor these transitions. The TIMER pin capacitor is initially charged with a constant 6 µA (typical) until either the MOSFET V_GS reaches the V_GATE–TH threshold (typically 5 V) indicating that the MOSFET channel is at least somewhat enhanced, or the voltage on the TIMER pin reaches the V_TMRH threshold (typically 2 V) indicating a fault condition. If the MOSFET V_GS reaches 5-V threshold before the TIMER pin reaches the typical 2 V timer fault threshold, the timer capacitor is then discharged to 300 mV, and then begins charging with 11-µA current source while the MOSFET transitions through the active region. The lower timer capacitor charge current during the initial start-up sequence allows more time before a fault is indicated. The turn-on time of the MOSFET will vary with input voltage, load capacitance, load resistance, as well as the MOSFET characteristics.
2. Figure 22 shows a start-up waveform with excessive gate leakage. The initial charge current on the timer capacitor is 6 µA (typical), while the simultaneous charge current to the gate is 24 µA (typical). Due to excessive gate leakage, the 24 µA is not able to charge the gate to the required typical 5 V V_GS threshold and the V_DS Fault Comparator will indicate a fault when the timer capacitor is charged to the V_TMRH fault threshold. When the timer capacitor voltage reaches theV_TMRH fault threshold (typically 2 V) the MOSFET gate is discharged at an 80 mA (typical) rate.

Figure 22. Voltages During Startup With V_GS Gate Leakage Condition