For limiting inrush current during turn ON of the FET with capacitive loads, use R₁, R₂, C₁ as shown in Figure 8-6. The R₁ and C₁ components slow down the voltage ramp rate at the gate of the FET. The FET source follows the gate voltage resulting in a controlled voltage ramp across the output capacitors.

Figure 8-6 Inrush Current limiting

Use the Equation 6 to calculate the inrush current during turn-ON of the FET.

Equation 2.

Equation 3.

Where,

C_LOAD is the load capacitance, VBATT is the input voltage and T_charge is the charge time, V_(BST-SRC) is the charge pump voltage (11 V),

Use a damping resistor R₂ (~ 10 Ω) in series with C₁. Equation 3 can be used to compute required C₁ value for a target inrush current. A 100 kΩ resistor for R₁ can be a good starting point for calculations.

Connecting PD pin of TPS12000-Q1 directly to the gate of the external FET ensures fast turn OFF without any impact of R₁ and C₁ components.

C₁ results in an additional loading on C_BST to charge during turn ON. Use Equation 4 to calculate the required C_BST value.

Equation 4.

Where, Q_g(total) is the total gate charge of the FET.