Output voltage MC33879 is to low

dennis_restle · ‎03-27-2019

Hello,

We have the MC33879 in our Design. See Attachment. But we noticed that the Output Voltag is way below our Input Voltage.

A Small load of a single relais with ~20 mA drops the Voltag to 19V.

The Power supply is ok, the Voltage stays in a range of 23.90 to 24.20 V on the D-inputs of the MC33879. But the S-Outputs of the MC33879 is with 19.xx Volt quite low.

Where are those 4 to 5 Volts lost?

Best regards

Dennis

TomasVaverka · ‎03-28-2019

Hello Dennis,

Sound like the input supply voltage on the VPWR pin is not high enough. This voltage is used to generate a charge pumped voltage to drive the gates of the output MOSFETs. If this voltage is not sufficient, the MOSFETs are not correctly enhanced by the charge pump, so their Rdson and voltage drop is high.

So the solution is to increase the VPWR.

Best regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

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dennis_restle · ‎03-28-2019

Hello TomasVaverka‌

Thanks for your answer.

We put a Z-Diode infront of VPWR as you can see in the schematic.

When i bridged the Z-Diode the output had full Voltage.

On old designs we didn't had this problems with N-Mosfets becuase they have been in a lowside configuration.

Here we used them in a highside configuration.

The Nmosfet does not conduct as soon as the source voltage has reached the gate Voltage. Quite surprising, it seems that the Voltage from VPWR is directly connected to the NMOS Gate. I'm asking myself, what is the chargepump in the MC33879 for? We assumed that the chargepump would take care of that....