MC33978 switch to ground wetting current generator

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MC33978 switch to ground wetting current generator

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Tarkus72
Contributor I

Hi, looking to the internal block diagram, when inputs (both SG and SP) are switch to ground, current generator is between Vbat and the input pin; so I'm expecting that with high impedance the input pin voltage should rise up to (or very close to) Vbat in order to satisfy the required current setting.

But, unregarding Vbat value, voltage never rise more than ~8.0V.

When selecting switch to Vbat on SP instead, SP inputs operates very close to an ideal current generator, thus voltage swing from GND to Vbat in order to satisfy the current requested.

As I'm using this device to perform measurements (using it as current generator, measuring voltages with external op amp), I was expecting similar behavior happens also in switch to ground mode; but with high impedance, max voltage is as stated above, around 8V, thus not satisfying the wetting current request.

e.g., with a 2.2kOhm resistance to ground, Vbat to 15V and wetting current of 6mA, voltage across the resistor should be around 13.2V, but the device will not go more than 8V.

Am I wrong? Is it required some specific setting?

Thank you in advance.

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guoweisun
NXP TechSupport
NXP TechSupport

Hi

For this part if set the port as high-impedance mode, I don't think there is meaning for test its voltage.

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