9.2.4.4.3 Selecting the Disconnect FET

The TPS61178x provides a gate driver to control an external FET to disconnect the output from the input at shutdown or output short conditions, shown in Figure 25.

Figure 25. Load Disconnect FET Connection

The V_DS, I_DS and safe operation area (SOA) should be taken into consideration when selecting the FET:

• The drain-to-source voltage rating should be higher than the output max. voltage, V_{DS_DIS_MAX} = V_OUT,
• The drain-to-source RMS current rating is the maximum output current. I _{DS_DIS_RMS} = I_OUT,
• The SOA should be considered when the output short occurs, and there is heat caused by the short protection response time and surge current, SOA > Q_SHORT.
Equation 32.
where
• V_{DS_DIS_Max} is the maximum drain-source voltage
• I_{DS_DIS} is the drain-source RMS current
• I_SHORT is the short current
• T_SHORT is the response time before the short protection triggered
• Q_SHORT is the heat produced for the output short

For instance: V_OUT = 16 V, I_SHORT = 20 A , T_SHORT = 30 µs.

SOA ≥ 4.8 mJ, V_{DS_DIS_MAX} ≥ 16 V.

The CSD25404Q3 –20 V P-Channel NexFET™ Power FET is used for this design example.

An additional capacitor between the gate and source of the external FET is required to slow the turn-on speed.

Equation 33.

where

• T_{ON_PFET} is the on time of external FET
• V_{TH_PFET} is the gate threshold of external FET
• C_{GS_PFET} is the total gate capacitance of connected between gate and source external FET. (including the self-gate-source capacitance of the FET)
• _{IDIS_PFET} is the discharge current inside of TPS61178x, it is 55 µA typically

Given 1.5 V threshold, C_{GS_PFET} is 10 nF, the T_{ON_PFET} is around 300 µs.Please be aware that the maximum turn-on time should not exceed 3 ms, and the maximum capacitance C_{GS_PFET} should be < 100 nF. Otherwise, the TPS61178x could not startup normally if the disconnect FET could not be turn on within the 3 ms.

The gate resistor depends on the gate-source voltage of the external FET,

Equation 34.

Equation 35.

Given the 5-V V_GATE, the R_GATE = 100 kΩ