Selection of Current Sense Resistor, R_SNS

The recommended range of the I²t based overcurrent protection threshold voltage, V_{(SNS_OCP)}, extends from 6mV to 200mV. Values near the low threshold of 6mV can be affected by the system noise. Values near the upper threshold of 200mV can cause high power dissipation in the current sense resistor. To minimize both the concerns, 20mV is selected as the I²t protection start threshold voltage. The current sense resistor, R_SNS can be calculated using following equation:

For 40A (I_OC) of I²t protection start threshold, R_SNS is calculated to be 0.5mΩ,

Two of 1mΩ, 1% sense resistor can be used in parallel.

Selection of IMON Scaling Resistor, R_SET

R_SET is the resistor connected between VS or input supply and CS1+ pins. This resistor scales the I²t based overcurrent protection threshold voltage and coordinates with R_IOC, charging current on C_I2t and R_IMON to determine the I²t profile and current monitoring output.

The maximum current on I²t pin can be calculated based on short-circuit protection (I_SC) threshold based on following equation:

where scaling factor, K can be calculate based on below equation:

R_SET needs to adjusted so that I_{I2t_MAX} is always less than 100µA. The recommended range of R_SET is 100Ω–500Ω.

R_SET is selected as 300Ω, 1% for this design example to get I_{2t_MAX} current < 100µA.

Choosing the Current Monitoring Resistor, R_IMON

Voltage at IMON pin V_(IMON) is proportional to the output load current. This can be connected to an ADC of the downstream system for monitoring the operating condition and health of the system. The R_IMON must be selected based on the maximum load current and the input voltage range of the ADC used. R_IMON is set using following equation:

Where V_SNS = I_{OC_MAX} × R_SNS and V_{(OS_SET)} is the input referred offset (±150µV) of the current sense amplifier. For I_{OC_MAX} = 120A and considering the operating range of ADC to be 0V to 3.3V (for example, V_(IMON) = 3.3V), R_IMON is calculated to be 18.33kΩ.

Selecting R_IMON value less than shown in Equation 21 ensures that ADC limits are not exceeded for maximum value of load current. Choose the closest available standard value: 18.2kΩ, 1%

Selection of Main path MOSFETs, Q1 and Q2

Q1 and Q2 For selecting the MOSFET Q1 and Q2, important electrical parameters are the maximum continuous drain current ID, 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_DS(ON). The maximum continuous drain current, ID, 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 VDS voltage rating of 40V is chosen for this application.

The maximum VGS TPS1214-Q1 can drive is 12V, so a MOSFET with 15V minimum VGS 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 10V VGS
• MOSFET Qg(total) is 73nC typical

TI recommends to make sure that the short-circuit conditions such V_{BATT_MAX} and I_SC are within SOA of selected FETs (Q1 and Q2) for > t_SC (5μs max) timing.

Selection of Bootstrap Capacitor, C_BST

The internal charge pump charges the external bootstrap capacitor (connected between BST and SRC pins) with approximately 600μA. 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 I²T Profile, R_IOC and C_I2t Selection

The R_IOC sets the I²T protection start threshold, whose value can be calculated using following equation:

where scaling factor, K can be calculate based on below equation:

To set 40A as I²T protection start threshold, R_IOC value is calculated to be 23kΩ.

Choose the closest available standard value: 23kΩ, 1%.

The time to turn OFF the gate drive at maximum overcurrent limit (I_{OC_MAX}) can be determined using below equation:

To set 3000A²s as I²T factor, t_{OC_MIN} value is calculated to be 208ms.

Use Equation 26 to calculate the required C_I2t value:

To set 3000A²s as I²T factor with 40A as I²T start threshold and 120A as maximum overcurrent, C_I2t is calculated to be ~880nF.

Choose the closest available standard value: 1µF, 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 130A as short-circuit protection threshold, R_ISCP value is calculated to be 2.53kΩ for two FETs in parallel. Choose the closest available standard value: 2.55kΩ, 1%.

Programming the Fault Timer Period, C_TMR Selection

For the design example under discussion, the auto-retry time, t_RETRY can be set by selecting appropriate capacitor C_TMR from TMR pin to ground. The value of C_TMR to set 1ms for t_RETRY can be calculated using following equation:

To set 1000ms as auto-retry time, C_TMR value is calculated to be 39.06nF.

Choose closest available standard value: 47nF, 10%.

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 ID, 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_DS(ON).

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

• 40V V_DS(MAX) and ±20V V_GS(MAX)
• R_DS(ON) is 17mΩ typical at 10V VGS
• MOSFET Q_g(total) is 11nC typical
• MOSET V_GS(th) is 1.3V min
• MOSFET C_ISS is 582pF typical

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) = 1.2V. From the design requirements, V_INUVLO is 6.5V. To solve the equation, first choose the value of R3 = 470kΩ and use Equation 29 to solve for R4 = 107.5kΩ.

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