How to size the GND resistor for a PROFET+2 or SPOC+2 device

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All SMART7 devices are designed to have a resistor (RGND) connected between GND pin and module GND, as shown in Chapter "Application Information" in each product datasheet.

Although the datasheet recommends 47Ω for PROFET+2 and SPOC+2 products, such values depend on the system and on the test conditions set by Tier1 or OEM during voltage transient testing. Once the value in Ω of the resistor is fixed, it is necessary to choose the appropriate physical size so that the resulting power dissipation during voltage transient events can be withstood.

Purpose of RGND

The main purpose of RGND is to limit the current flowing through GND pin during voltage transient events.

Boundaries of SMART7 devices

The internal circuitry connected between VS and GND pins can withstand a defined amount of energy. In standard setups (like e.g. ISO7637 pulses 1 and 2a) such energy limitations can be translated into maximum values of current:

1. positive voltage transient (similar to ISO7637 pulse 2a): 1A
2. negative voltage transient (similar to ISO7637 pulse 1): -0.5A (the current is negative because it flows into GND pin)

The maximum allowed current value is different depending on the direction because the internal circuitry doesn´t have a symmetric behavior, due to design constraints. Any current outside of these values or outside of the timings of the mentioned pulses, can result in a damage for the device.

Where does the current flow during voltage transients

Voltage transients are applied to the module (ECU) connectors, therefore their energy is distributed across all devices and all loads connected to the connector where the transient is applied. The amount of current for each branch (either VS to GND or VS to OUT) depends mainly on the resistance connected to the pin (the higher the resistance, the less current flows through it) and if the load has some "direction". If a diode is connected to an OUT, this allows current flow in case of positive voltage transient, while there is no current flowing in case of a negative voltage transient.

How to size RGND according to the different application setups

In principle, the current flowing through RGND is

• directly proportional to
  • voltage amplitude of the transient
  • equivalent resistance of loads connected to outputs
• inversely proportional to
  • number of devices supplied by the ECU connector where the transient is applied
  • number of channels for each device
  • numbers of loads connected to outputs
  • internal resistance considered for the voltage transient generator
  • value of RGND of each device

The formulas attached show how to calculate the current flowing through GND pin according to different parameters. Please note that the formulas are different, depending if the pulse is positive or negative. The formulas use the following parameters:

• positive voltage transients:

n	total number of loads connected to outputs (without channels in Open Load)
m	total number of devices
R_Li	load resistance (averange per output)
R_GNDj	ground resistance (average per device)
Vx(CLAMP)	clamping voltage during positive pulses (from VS to GND: VS(CLAMP) - P_8.6.0.7, from VS to OUT pins: VDS(CLAMP))
VISOpos	positive voltage applied by the generator
RI	internal resistor of the generator
I_GNDj	positive current during transient voltage

 Condition to ensure device reliability: I_GNDj < 1 A

• negative voltage transient

n	total number of loads connected to outputs (without channels in Open Load)
m	total number of devices
R_Li	load resistance (averange per output)
R_GNDj	ground resistance (average per device)
VS(REV)	clamping voltage (from GND to VS pins) during negative pulses - P_6.4.0.9
VISOneg	negative voltage applied by the generator
RI	internal resistor of the generator
I_GNDj	negative current during transient voltage

Condition to ensure device reliability: I_GNDj < 0.5 A

(the sign of the current is inverted for simplicity and clarity).

How to estimate the size of RGND in terms of power dissipation

This is not a straightforward process, as it depends on the design rules and discrete components availability at each Tier1. With the formulas presented before it is possible to calculate the peak power dissipation during the voltage transient and, from this, derive the necessary size and power capability of RGND.

Is there a "correct" value of RGND?

As mentioned before, it depends on the system characteristics.

In a typical, realistic case (several devices supplied by the same connector, all loads connected), the amount of current for each GND circuitry is rather limited (tens of mA) therefore the stated values in the datasheet are sufficient to protect the device. In such conditions, it is very likely that the power dissipation on each RGND is limited therefore a size of 0603 or 0402 (imperial standard) is appropriate.

In case the test requirements are "only 1 device connected, no load connected to OUT", then the value for RGND can be increased up to e.g. 220Ω.

In principle there is no upper limit for RGND, at least for the device. It is important to consider the ground shift between GND pin and module GND. If RGND = 220Ω is used, the corresponding ground shift can reach IGND(ACTIVE) * RGND = 4mA * 220Ω = 880mV for a PROFET+2 device, and much more for a SPOC+2. This value has to be taken into consideration for the value of RGND.