VN820 datasheet(19/43 Pages) STMICROELECTRONICS

19 / 43 page

VN820

Application information

Doc ID 7370 Rev. 5

19/43

3.1.2

Solution 2: diode (DGND) in the ground line

A resistor (RGND = 1 kΩ) should be inserted in parallel to DGND if the device drives an

inductive load.

This small signal diode can be safely shared amongst several different HSDs. Also in this

case, the presence of the ground network produces a shift (

≈ 600 mV) in the input threshold

and in the status output values if the microprocessor ground is not common to the device

ground. This shift does not vary if more than one HSD shares the same diode/resistor

network.

Series resistor in INPUT and STATUS lines are also required to prevent that, during battery

voltage transient, the current exceeds the absolute maximum rating.

Safest configuration for unused INPUT and STATUS pin is to leave them unconnected.

3.2

Load dump protection

Dld is necessary (Voltage Transient Suppressor) if the load dump peak voltage exceeds the

VCC max DC rating. The same applies if the device is subject to transients on the VCC line

that are greater than the ones shown in the ISO 7637-2: 2004(E) table.

3.3

MCU I/Os protection

If a ground protection network is used and negative transient are present on the VCC line,

the control pins are pulled negative. ST suggests to insert a resistor (Rprot) in line to prevent

the microcontroller I/O pins from latching-up.

The value of these resistors is a compromise between the leakage current of microcontroller

and the current required by the HSD I/Os (Input levels compatibility) with the latch-up limit of

microcontroller I/Os.

-VCCpeak/Ilatchup ≤ Rprot ≤ (VOHµC-VIH-VGND) / IIHmax

Calculation example:

For VCCpeak= - 100V and Ilatchup ≥ 20mA; VOHµC ≥ 4.5V

Ω ≤ Rprot ≤ 65kΩ.

Recommended values: Rprot =10kΩ .

3.4

Open load detection in off-state

Off-state open load detection requires an external pull-up resistor (RPU) connected between

OUTPUT pin and a positive supply voltage (VPU) like the +5V line used to supply the

microprocessor.

The external resistor has to be selected according to the following requirements:

no false open load indication when load is connected: in this case we have to avoid

VOUT to be higher than VOlmin; this results in the following condition

VOUT= (VPU / (RL+RPU)) RL < VOlmin.

no misdetection when load is disconnected: in this case the VOUT has to be higher than

VOLmax; this results in the following condition RPU < (VPU – VOLmax) / IL(off2).

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