Selection of Current Sense Resistor, R_SNS

The recommended range of the overcurrent protection threshold voltage, V_{(SNS_WRN)}, extends from 10 mV to 200 mV. Values near the low threshold of 10 mV can be affected by the system noise. Values near the upper threshold of 200 mV can cause high power dissipation in the current sense resistor. To minimize both the concerns, 25 mV is selected as the overcurrent protection threshold voltage. Use the following equation to calculate the current sense resistor, R_SNS.

Equation 14.

The next smaller available sense resistor 800 μΩ, 1% is chosen.

To improve signal to noise ratio or for better overcurrent protection accuracy, higher overcurrent protection threshold voltage, V_{(SNS_WRN)} can be selected.

Selection of Scaling Resistor, R_SET

R_SET is the resistor connected between VS and CS+ pins. This resistor scales the overcurrent protection threshold voltage and coordinates with R_IWRN and R_IMON to determine the overcurrent protection threshold and current monitoring output. The recommended range of R_SET is 50 Ω – 100 Ω.

R_SET is selected as 100 Ω, 1% for this design example.

Programming the Overcurrent Protection Threshold – R_IWRN Selection

The R_IWRN sets the overcurrent protection threshold, whose value can be calculated using Equation 15.

Equation 15.

To set 30 A as overcurrent protection threshold, R_IWRN value is calculated to be 49.5 kΩ.

Choose the closest available standard value: 49.9 kΩ, 1%

.

Programming the Short-Circuit Protection Threshold – R_ISCP Selection

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

To set 35 A as short-circuit protection threshold, R_ISCP value is calculated to be 1.19 kΩ.

Choose the closest available standard value: 1.2 kΩ, 1%.

In case where large di/dt is involved, the system and layout parasitic inductances can generate large differential signal voltages between ISCP and CS– pins. This action can trigger false short-circuit protection and nuisance trips in the system. To overcome such scenario, TI recommends to add filter capacitor of 1 nF across ISCP and CS– pins close to the device. Because nuisance trips are dependent on the system and layout parasitics, TI recommends to test the design in a real system and tweaked as necessary.

Programming the Fault timer Period – C_TMR Selection

For the design example under discussion, overcurrent transients are allowed for 1-ms duration. This blanking interval, t_OC 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_OC.

Choose closest available standard value: 68 nF, 10%.

Selection of MOSFETs, Q₁ and Q₂

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, MOSFETs with V_DS voltage rating of 40 V is suitable for this application.

The maximum V_GS TPS1211-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, BUK7S0R5-40HJ is selected and its ratings are:

• 40-V V_DS(MAX) and 20-V V_GS(MAX)

• R_DS(ON) is 0.47-mΩ typical at 10-V V_GS

• MOSFET Q_g(total) is 190 nC

Selection of Bootstrap Capacitor, C_BST

The internal charge pump charges the external bootstrap capacitor (connected between BST and SRC pins) with approximately 100 μA. Use the following equation to calculate the minimum required value of the bootstrap capacitor for driving two parallel BUK7S0R5-40HJ MOSFETs.

Equation 18.

Choose closest available standard value: 470 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 19.

Equation 19.

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) = 1.18 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 19 to solve for R₂ = 104.24 kΩ. Choose the closest standard 1% resistor values: R₁ = 470 kΩ, and R₂ = 105 kΩ.

Selection of Precharge Path Components, C_g and R_g

For charging the large capacitors on output, the output slew rate can be controlled by using a capacitor on the gate (G) of the precharge FET Q₃. The target inrush current to charge 1 mF of output capacitance to 12-V in 10 ms can be estimated by Equation 20. The required gate capacitance Cg to limit the inrush current to 1.2 A can be calculated by using , where I_(G) = 100 μA (typical) is the gate charging current of pin 'G'. By solving , we get C_g as 83.33 nF.

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

Equation 20.

A series resistor R_g must be used in conjunction with C_g to limit the discharge current from C_g during turn-off and to stabilize the gate 'G' during slew rate control. The recommended value for R_g is between 220 Ω to 470 Ω.

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 Equation 22.

Where V_SNS = I_OC × R_SNS and V_{(OS_SET)} is the input referred offset (± 200 µV) of the current sense amplifier.

The maximum voltage range for monitoring the current (V_(IMONmax)) is limited to minimum([V_(VS) – 0.5V], 5.5V) to ensure linear output. This puts a limitation on the maximum value of R_IMON resistor. The IMON pin has an internal clamp of 6.5 V (typical).

For I_OC = 30 A and considering the operating range of ADC to be 0 V to 3.3 V (for example, V_(IMON) = 3.3 V), R_IMON can be calculated as

Selecting R_IMON value less than shown in Equation 23 ensures that ADC limits are not exceeded for maximum value of load current. Choose the closest available standard value: 16.5 kΩ, 1%.