By factoring in a small variation in the junction to ambient thermal resistance (R_θJA), a single TPS1685x eFuse is rated at a maximum steady state DC current of 20A with a maximum junction temperature of less than 125°C. Therefore, Equation 19 can be used to calculate the number of devices (N) to be in parallel to support the maximum steady state DC load current (I_LOAD(max)), for which the solution must be designed.

According to Table 8-1, I_OUT(max) is 80A. Therefore, 4 TPS1685 eFuses are connected in parallel.

Setting up the primary and secondary devices in a parallel configuration

The MODE pin is used to configure one TPS1685x eFuse as the primary device in a parallel chain along with the other TPS1685x eFuses as the secondary devices. As a result, some of the TPS1685 pin functions can be changed to facilitate primary and secondary configuration as described in Multiple Devices, Parallel Connection.

Leaving the pin open configures the corresponding device as the primary one. For the secondary devices, this pin must be connected to GND.

Selecting the C_DVDT capacitor to control the output slew rate and start-up time

A capacitor (C_DVDT) must be added at the DVDT pin to GND to set the required value of slew rate. Equation 20 is used to compute the value of C_DVDT. The DVDT pins of all the eFuses in a parallel chain must be connected together.

To get slew rate of 2.2V/ms , as per above equation we get C_DVDT as 21.82nF. We can keep nearby standard value of 22nF.

Selecting the R_IREF resistor to set the reference voltage for overcurrent protection and active current sharing

In this parallel configuration, the IREF internal current source (I_IREF) of the primary eFuse interacts with the external IREF pin resistor (R_IREF) to generate the reference voltage (V_IREF) for the overcurrent protection and active current sharing blocks. When the voltage at the IMON pin (V_IMON) is used as an input to an ADC to monitor the system current or to implement the Platform Power Control (Intel PSYS) functionality inside the VR controller, V_IREF must be set to half of the maximum voltage range of the ISYS_IN input of the controller. This action provides the necessary headroom and dynamic range for the system to accurately monitor the load current up to the fast-trip threshold (2 × I_OCP). Equation 21 is used to calculate the value of R_IREF.

In this design example, V_IREF is set at 1V. With I_IREF = 25µA (typical), we can calculate the target R_IREF to be 40kΩ. The closest standard value of R_IREF is 40.2kΩ with 0.1% tolerance and power rating of 100mW. For improved noise immunity, place a 100pF ceramic capacitor from the IREF pin to GND.

Selecting the R_IMON resistor to set the overcurrent (circuit-breaker) and fast-trip thresholds during steady-state

TPS1685x eFuse responds to the output overcurrent conditions during steady-state by turning off the output after a user-adjustable transient fault blanking interval. This eFuse continuously senses the total system current (I_OUT) and produces a proportional analog current output (I_IMON) on the IMON pin. This generates a voltage (V_IMON) across the IMON pin resistor (R_IMON) in response to the load current, which is defined as Equation 22.

G_IMON is the current monitor gain (I_IMON : I_OUT), whose typical value is 18.2µA/A. The overcurrent condition is detected by comparing the V_IMON against the V_IREF as a threshold. The circuit-breaker threshold during steady-state (I_OCP) can be calculated using Equation 23.

In this design example, I_OCP(TOTAL) is considered as 85A, and R_IMON can be calculated to be 646.4Ω with G_IMON as 18.2µA/A and V_IREF as 1V. The nearest value of R_IMON is 642Ω with 0.1% tolerance and power rating of 100mW. For noise reduction, place a 22pF ceramic capacitor across the IMON pin and GND.

Selecting the R_ILIM resistor to set active sharing threshold during steady-state

R_ILIM is used in setting up the active current sharing threshold during steady-state. Each device continuously monitors the current flowing through it (I_DEVICE) and outputs a proportional analog output current on its own ILIM pin. This in turn produces a proportional voltage (V_ILIM) across the respective ILIM pin resistor (R_ILIM), which is expressed as Equation 24.

G_ILIM is the current monitor gain (I_ILIM : I_DEVICE), whose typical value is 20μA/A.

• Active current sharing during steady-state: This mechanism operates only after the device reaches steady-state and acts independently by comparing its own load current information (V_ILIM) with the Active Current Sharing reference (CLREF_LIN) threshold, defined as Equation 25.

  Equation 25. ${CLREF}_{LIN}=\frac{1.1\times V_{IREF}}{3}$

  Therefore, R_ILIM must be calculated using Equation 26 to define the active current sharing threshold as I_OCP(TOTAL)/N, where N is the number of devices in parallel. Using N = 4, R_IMON = 642Ω, and Equation 26, R_ILIM can be calculated to be 235.4Ω. The closest standard value of 234Ω with 0.1% tolerance and power rating of 100mW resistances are selected as R_ILIM for each device.

  Equation 26. $R_{ILIM}=\frac{1.1\times N\times R_{IMON}}{3}$
  Note:

  To determine the value of R_ILIM, Equation 27 must be used if a different threshold for active current sharing (I_LIM(ACS)) than I_OCP/N is desired.

  Equation 27. $R_{ILIM}=\frac{1.1\times V_{IREF}}{3\times G_{ILIM}\times I_{LIM\left(ACS\right)}}$

Selecting the C_ITIMER capacitor to set the overcurrent blanking timer

An appropriate capacitor must be connected at the ITIMER pin to ground of the primary or standalone device to adjust the duration for which the load transients above the circuit-breaker threshold are allowed. The transient overcurrent blanking interval can be calculated using Equation 28.

Where t_ITIMER is the transient overcurrent blanking timer and C_ITIMER is the capacitor connected between ITIMER pin of the primary device and GND. I_ITIMER = 2µA (typical) and ΔV_ITIMER = 1.3V (typical). A 4.7nF capacitor with 10% tolerance and DC voltage rating of 25V is used as the C_ITIMER for the primary device in this design, which results in 3ms of t_ITIMER. The ITIMER pin for all the secondary devices should be left open.

Selecting the resistors to set the undervoltage lockout threshold

The undervoltage lockout (UVLO) threshold is adjusted by employing the external voltage divider network of R₁ and R₂ connected between IN, EN/UVLO, and GND pins of the device as described in Undervoltage protection section. The resistor values required for setting up the UVLO threshold are calculated using Equation 29.

To minimize the input current drawn from the power supply, TI recommends using higher resistance values for R₁ and R₂. From the device electrical specifications, UVLO rising threshold V_UVLO(R) = 1.2V. From the design requirements, V_INUVLO = 46V. First choose the value of R₁ = 3.74MΩ and use Equation 29 to calculate R₂ = 100kΩ. Use the closest standard 1 % resistor values: R₁ = 3.74MΩ and R₂ = 100kΩ. For noise reduction, place a 100pF ceramic capacitor across the EN/UVLO pin and GND.

Selecting the resistors to set the overvoltage lockout threshold

The overvoltage lockout (OVLO) threshold is adjusted by employing the external voltage divider network of R₃ and R₄ connected between IN, OVLO, and GND pins of the device as described in overvoltage protection section. The resistor values required for setting up the OVLO threshold are calculated using below equation.

To minimize the input current drawn from the power supply, TI recommends using higher resistance values for R₃ and R₄. From the device electrical specifications, OVLO rising threshold V_OVLO(R) = 1.164V. From the design requirements, V_INOVLO = 60V. First choose the value of R₁ = 5.11MΩ and use Equation 29 to calculate R₃ = 101kΩ. Use the closest standard 1% resistor values: R₃ = 5.11MΩ and R₄ = 102kΩ. For noise reduction, place a 10pF ceramic capacitor across the OVLO pin and GND.

Selecting the R-C filter between VIN and VDD

VDD pin is intended to power the internal control circuitry of the eFuse with a filtered and stable supply, not affected by system transients. Therefore, use an R (150Ω) – C (0.22µF) filter from the input supply (IN pin) to the VDD pin. This helps to filter out the supply noises and to hold up the controller supply during severe faults such as short-circuit at the output. In a parallel chain, this R-C filter must be employed for each device.

Selecting the pullup resistors and power supplies for PG, FLT,

FLT, PG, are the open drain outputs. If these logic signals are used, the corresponding pins must be pulled up to an appropriate supply rail voltage through 33kΩ pullup resistances.

Selection of TVS diode at input and Schottky diode at output

In the case of a short circuit and overload current limit when the device interrupts a large amount of current instantaneously, the input inductance generates a positive voltage spike on the input, whereas the output inductance creates a negative voltage spike on the output. The peak amplitudes of these voltage spikes (transients) are dependent on the value of inductance in series with the input or output of the device. Such transients can exceed the absolute maximum ratings of the device and eventually lead to failures due to electrical overstress (EOS) if appropriate steps are not taken to address this issue. Typical methods for addressing this issue include:

1. Minimize lead length and inductance into and out of the device.
2. Use a large PCB GND plane.
3. Addition of the Transient Voltage Suppressor (TVS) diodes to clamp the positive transient spike at the input.
4. Using Schottky diodes across the output to absorb negative spikes.

Refer to TVS Clamping in Hot-Swap Circuits , Selecting TVS Diodes in Hot-Swap and ORing Applications, TVS Diode recommendation tool for details on selecting an appropriate TVS diode and the number of TVS diodes to be in parallel to effectively clamp the positive transients at the input below the absolute maximum ratings of the IN pin (20V). These TVS diodes also help to limit the transient voltage at the IN pin during the Hot Plug event. Four (4) SMDJ54A are used in parallel in this design example.

Selection of the Schottky diodes must be based on the following criteria:

• The non-repetitive peak forward surge current (I_FSM) of the selected diode must be more than the fast-trip threshold (2 × I_OCP(TOTAL)). Two or more Schottky diodes in parallel must be used if a single Schottky diode is unable to meet the required I_FSM rating. Equation 31 calculates the number of Schottky diodes (N_Schottky) that must be in parallel.
  Equation 31. $N_{Schottky}>\frac{2\times I_{OCP\left(TOTAL\right)}}{I_{FSM}}$
• Forward Voltage Drop (V_F) at near to I_FSM must be as small as possible. Ideally, the negative transient voltage at the OUT pin must be clamped within the absolute maximum rating of the OUT pin (–5V).
• DC Blocking Voltage (V_RM) must be more than the maximum input operating voltage.
• Leakage current (I_R) must be as small as possible.

4 B360-13-F are used in parallel in this design example.

TI recommends to add ceramic bypass capacitors to help stabilize the voltages on the input and output. The value of C_IN must be kept small to minimize the current spike during hot-plug events. For each device, 0.01µF of C_IN is a reasonable target. Because C_OUT does not get charged during hot-plug, a larger value such as 10µF can be used at the OUT pin of each device.