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F28335 power sequencing for sensors (possible hardware issue on reference designs)

BOGUS CRISTIAN
Prodigy 210 points

Other Parts Discussed in Thread: CONTROLSUITE, DRV8412, OPA2350

Hello,

I have a question concerning the power sequencing of a "real life" inverter.

 In such a case you often have a +/-15V or 5V power supply who feeds the current/voltage sensors (for instance LEM). The same power supply is also used to power indirectly the DSC (F28335 on a Control Card). That means the 5V, 3.3V supplies are made from the +15V supply.

I wonder if there is a risk that some sensors will be powered before de DSC core and will apply a voltage greater than 0.7V on some input pin of the F28335. This situation is contrary to the hardware recommendations:

According to the section 3.5.4 Power sequencing of the document SPRAAS1A (Hardware Design Guidelines for TMS320F28xxx DSCs):

"Additionally, it is recommended that no voltage larger than a diode drop (0.7 V) should be applied to any

input pin prior to powering up the device. Voltages applied to pins of an unpowered device can bias

internal p-n junctions in unintended ways and produce unpredictable results."

I looked at the schematics for the HV PFC (from the ControlSuite) inverter and also for the DRV8412 drive... In my understanding there is no special measure taken there to insure the operational amplifiers driving the AD inputs are powered after the DSC (on the control card).

This puzzles me especialy because I see in the HV MotorControl +PFC schematics that the DC bus voltage and the PFC input AC voltage are measured through a resistive divider (Vfb-Bus and VACfb) and applied directly to the AD inputs on the ControlCard. This means that if the HV DC capacitor is already charged at some voltage prior to 15V power up then the F28335 wakes up with more than 0.7V on an input pin. This leads to "unpredictable results"... according to the previous mentionned section.

So the recommendation in section 3.5.4 is still valid for F28335 (and other DSCs) or we can forget it?

Equally if a motor (connected to the kit) is turned (even by hand) when the aux power supply 15/5V are not present then the motor EMF can polarise the OPA2350 inputs although the op amps are not powered... Do you see an issue here?

Regards,

Cristian

  • 0
    Prodigy 210 points

    Hello TI support,

    I would like to know that the issue concerning the reference design kit is for you a real one or not. Does the hardware recommendations apply in this case ?

    Is any standard/recommended  method of protecting the microcontroller input pins in the case the sensors get powered up some ms BEFORE the microcontroller and polarise some A/D inputs at voltages superior to 0.7V?

     

    Regards,

    Cristian