Selection of MOSFET, Q₁ and Q₂

For selecting the MOSFET Q₁ and Q₂, 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 35V as the maximum application voltage due to load dump, MOSFETs with V_DS voltage rating of 40V is chosen for this application.

The maximum V_GS TPS12130-Q1 can drive is 11V, so a MOSFET with 15V 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, two of BUK7J1R4-40H are selected and its ratings are:

• 40V V_DS(MAX) and ±20V V_GS(MAX)
• R_DS(ON) is 1.06mΩ typical at 10- V_GS
• MOSFET Q_g(total) is 73nC typical

TI recommends to make sure that the short-circuit conditions such max V_IN and I_SC are within SOA of selected FETs (Q₁ and Q₂) for > 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 345μA. Use the following equation to calculate the minimum required value of the bootstrap capacitor for driving two parallel BUK7J1R4-40H MOSFETs

Choose closest available standard value: 150nF, 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 following equation:

To set 100A as short-circuit protection threshold, R_ISCP value is calculated to be 15.5kΩ for two FETs in parallel.

Choose the closest available standard value: 15.6kΩ, 1%.

Programming the Fault timer Period – C_TMR Selection

For the design example under discussion, overcurrent transients are allowed for 50μs 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 50μs for t_SC can be calculated using following equation:

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

TMR pin can be left floating for fast response of t_SC < 10 μs.

Programming the Load Wakeup Threshold – R_BYPASS and Q₃ Selection

During normal operation, the resistor R_BYPASS along with bypass FET R_DSON is used to set load wakeup current threshold.

For selecting the MOSFET Q₃, 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.

Based on the design requirements, BUK6D23-40E is selected and its ratings are:

• 40-V V_DS(MAX) and ±20-V V_GS(MAX)

• R_DS(ON) is 17-mΩ typical at 10-V V_GS

• MOSFET Q_g(total) is 11 nC typical

• MOSET V_GS(th) is 1.3 V min

• MOSFET C_ISS is 582 pF typical

The recommended range of the short-circuit threshold voltage which is same as load wakeup threshold, V_(SCP/LWU), extends from 30 mV to 500 mV. Values near the low threshold of 30 mV can be affected by the system noise. Values near the upper threshold of 500 mV would result in high short-circuit current threshold. To minimize both the concerns, 50 mV is selected as the short-circuit or load wakeup threshold voltage.

The V_(SCP/LWU) value can also be calculated based on selected R_ISCP resistor by following equation:

R_BYPASS resistor value can be selected using below equation:

To set 50 mA as load wakeup threshold, R_BYPASS value is calculated to be ~ 1 Ω.

The average power rating of the bypass resistor can be calculated by following equation:

The average power dissipation of R_BYPASS is calculated to be ~ 0.0025 W

The peak power dissipation in the bypass resistor is given by following equation:

The peak power dissipation of R_BYPASS is calculated to be ~ 256 W

The peak power dissipation time for power-up with short into LPM can be calculated based on following equation:

where,

V_{(G2_GOOD)} is internal threshold with 7 V (typical) value,

I_(G2) is 165 μA (typical),

V_GS(th) is gate to source voltage and C_ISS is effective input capacitance of selected bypass FET.

Based on Equation 28, T_PULSE is calculated to be ~ 32 μs.

One 1-Ω, 1.5-W, 1% CRCW25121R00JNEGHP resistor is used to support both average and peak power dissipation for > T_PULSE time calculated in Equation 28.

TI suggests the designer to share the entire power dissipation profile of bypass resistor with the resistor manufacturer and get their recommendation.

The peak short-circuit current in bypass path can be calculated based on following equation:

I_{PEAK_BYPASS} is calculated to be 16-A based on R_BYPASS selected in Equation 25.

Setting the Undervoltage Lockout Set Point, R₃ and R₄

The undervoltage lockout (UVLO) can be adjusted using an external voltage divider network of R₃ and R₄ connected between VS, EN/UVLO and GND pins of the device. The values required for setting the undervoltage and overvoltage are calculated by solving below equation:

For minimizing the input current drawn from the power supply, TI recommends to use higher values of resistance for R₃ and R₄. 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_(R34) must be chosen to be 20 times greater than the leakage current of UVLO pin.

From the device electrical specifications, V_(UVLOR) = 1.24 V. From the design requirements, V_INUVLO is 6.5 V. To solve the equation, first choose the value of R₃ = 470 kΩ and use Equation 21 to solve for R₄ = 107.5 kΩ.

Choose the closest standard 1% resistor values: R₃ = 470 kΩ, and R₄ = 107 kΩ.