Selection of MOSFET, Q₁

For selecting the MOSFET Q₁, important electrical parameters are the maximum continuous drain current I_D, the maximum drain-to-source voltage V_DS(MAX), the maximum drain-to-source voltage V_GS(MAX), and the drain-to-source ON resistance R_DSON.

The maximum continuous drain current, I_D, rating must exceed the maximum continuous load current.

The maximum drain-to-source voltage, V_DS(MAX), must be high enough to withstand the highest voltage seen in the application. Considering 35 V as the maximum application voltage due to load dump, MOSFETs with V_DS voltage rating of 40 V is chosen for this application.

The maximum V_GS TPS12000-Q1 can drive is 11 V, so a MOSFET with 15-V minimum V_GS rating must be selected.

To reduce the MOSFET conduction losses, an appropriate R_DS(ON) is preferred.

Based on the design requirements, BUK7J1R4-40H is selected and its ratings are:

• 40-V V_DS(MAX) and ±20-V V_GS(MAX)

• R_DS(ON) is 1.06-mΩ typical at 10-V V_GS

• MOSFET Q_g(total) is 73 nC typical

TI recommends to make sure that the short-circuit conditions such max V_IN and I_SC are within SOA of selected FET (Q₁) for at-least > t_SC timing.

Selection of Bootstrap Capacitor, C_BST

The internal charge pump charges the external bootstrap capacitor (connected between BST and SRC pins) with approximately 345 μA. Use the following equation to calculate the minimum required value of the bootstrap capacitor for driving BUK7J1R4-40H MOSFET

Choose closest available standard value: 100 nF, 10 %.

Programming the Short-Circuit Protection Threshold – R_ISCP Selection

The R_ISCP sets the short-circuit protection threshold, whose value can be calculated using below equation:

Refer to Equation 9 in Section 8.1.1 section for update in equation in final revision of IC.

To set 100 A as short-circuit protection threshold, R_ISCP value is calculated to be 45 kΩ.

Choose the closest available standard value: 45.2 kΩ, 1%.

In case where large di/dt is involved, the system and layout parasitic inductances can generate large differential signal voltages between CS+ and CS- pins. This action can trigger false short-circuit protection and nuisance trips in the system. To overcome such scenario, TI suggests to add placeholder for RC filter components across sense resistor (R_SNS) and tweak the values during test in the real system. The RC filter components should not be used in current sense designs by MOSFET VDS sensing to avoid impact on the short-circuit protection response.

Programming the Fault timer Period – C_TMR Selection

For the design example under discussion, overcurrent transients are allowed for 50-μs duration. This blanking interval, t_SC (or circuit breaker interval, T_CB) can be set by selecting appropriate capacitor C_TMR from TMR pin to ground. The value of C_TMR to set 50 μs for t_SC can be calculated using following equation:

Choose closest available standard value: 3.3 nF, 10 %.

Setting the Undervoltage Lockout and Overvoltage Set Point

The undervoltage lockout (UVLO) and overvoltage set point are adjusted using an external voltage divider network of R₁, R₂ and R₃ connected between VS, EN/UVLO, OV and GND pins of the device. The values required for setting the undervoltage and overvoltage are calculated by solving Equation 13 and Equation 14.

Equation 13.

Equation 14.

For minimizing the input current drawn from the power supply, TI recommends to use higher values of resistance for R₁, R₂ and R₃. However, leakage currents due to external active components connected to the resistor string can add error to these calculations. So, the resistor string current, I(R₁₂₃) must be chosen to be 20 times greater than the leakage current of UVLO and OV pins.

From the device electrical specifications, V_(OVR) = 1.24 V and V_(UVLOR) = 1.24 V. From the design requirements, VIN_OVP is 36 V and VIN_UVLO is 6.5 V. To solve the equation, first choose the value of R₁ = 470 kΩ and use Equation 13 to solve for (R₂ + R₃) = 108.3 kΩ. Use Equation 14 and value of (R₂ + R₃) to solve for R₃ = 19.6 kΩ and finally R₂ = 88.7 kΩ. Choose the closest standard 1 % resistor values: R₁ = 470 kΩ, R₂ = 88.7 kΩ, and R₃ = 19.6 kΩ.