Selection of Current Sense Resistor, R_SNS

The recommended range of the overcurrent protection threshold voltage, V_(SCP), extends from 30 mV to 300 mV. Values near the low threshold of 30 mV can be affected by the system noise. Values near the upper threshold of 300 mV can cause high power dissipation in the current sense resistor. To minimize both the concerns, 40 mV is selected as the short-circuit protection threshold voltage. Use the following equation to calculate the current sense resistor, R_SNS.

The next smaller available sense resistor 1 mΩ, 1% is chosen.

To improve signal to noise ratio or for better short-circuit protection accuracy, higher short-circuit protection threshold voltage, V_(SCP) can be selected.

Programming the Short-Circuit Protection Threshold – R_SCP Selection

The R_SCP sets the short-circuit protection threshold. Use the following equation to calculate the value.

To set 30-A as short-circuit protection threshold, R_SCP value is calculated to be 15 kΩ.

Choose the closest available standard value: 15 kΩ, 1%.

Refer to Equation 9 in Application Limitations section for update in equation in final revision of IC.

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 Short-Circuit Protection Delay – C_TMR Selection

For the design example under discussion, overcurrent transients are allowed for 1-ms duration. This short-circuit protection delay, t_SC can be set by selecting appropriate capacitor C_TMR from TMR pin to ground. Use the following equation to calculate the value of C_TMR to set 1 ms for t_SC.

Choose closest available standard value: 82 nF, 10%.

Selection of MOSFETs, 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 35 V as the maximum application voltage, MOSFETs with V_DS voltage rating of 40 V is designed for this application.

The maximum V_GS TPS1210-Q1 can drive is 13 V, so a MOSFET with 15-V minimum V_GS rating must be selected.

To reduce the MOSFET conduction losses, lowest possible R_DS(ON) is preferred.

Based on the design requirements, BUK7S1R0-40H is selected and the ratings are:

• 40-V V_DS(MAX) and 20-V V_GS(MAX)
• R_DS(ON) is 0.88-mΩ typical at 10-V V_GS
• Maximum MOSFET Q_g(total) is 137 nC

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 two BUK7S1R0-40H MOSFETs.

Choose closest available standard value: 330 nF, 10 %.

Setting the Undervoltage Lockout

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

Equation 14.

For minimizing the input current drawn from the power supply, TI recommends to use higher values of resistance for R₁ and R₂. 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(R₁₂) must be chosen to be 20 times greater than the leakage current of UVLO pin.

From the device electrical specifications, V_(UVLOR) = 1.21 V. From the design requirements, VIN_UVLO is 6.5 V. To solve the equation, first choose the value of R₁ = 470 kΩ and use Equation 14 to solve for R₂ = 107.5 kΩ. Choose the closest standard 1% resistor values: R₁ = 470 kΩ, and R₂ = 105 kΩ.