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Analog and digital power supply in ADS7263

Michal Tyc
Prodigy 180 points
Other Parts Discussed in Thread: ADS7263, TMS320F28335, TPS2103, TPS2101

Hello,

Is it possible to switch off analog power supply while maintaing digital power supply? In the ADS7263 manual, there is said that DVDD to AVDD must be >= 1.2 x AVDD, otherwise:

"Exceeding the specified limit causes an increase of the DVDD leakage current and leads to malfunction of the device."

This note does not say that:

"Stresses above these ratings may cause permanent damage to the device"

as with some other absolute maximum ratings values.

What DVDD leakage current increase can we expect? Will this increased leakage occur as well, if ADS7263 is put into Power Down mode before the analog power supply is switched off?

The reason for such a solution is to use a single analog supply for the entire analog front-end of our device and to achieve maximum energy saving.

I would be grateful for any information.

Michal

  • 0
    TI__Guru**** 182371 points

    Hi Michal,

    Thank you for your interest in the ADS7263!  You cannot have the digital rail powered up without the analog rail.  Note (1) in the absolute MAX ratings table actually applies to all items in the table.  The note (2) regarding the DVdd supply should not be read as greater than or equal to, but rather 'must not exceed' the AVdd by more than 1.2x.  If the analog supply is 0V, the digital supply limit is 0V *1.2.  

    Have you considered powering the digital side of the ADS7263 from the same source as the analog rail?

  • 0 in reply to Tom Hendrick
    Prodigy 180 points

    Hi Tom,

    Thank you very much for your quick answer.

    We would like to avoid powering both sides from the same source because of possible noise propagation. On the other hand, ADS7263 is connected to a microcontroller (TMS320F28335) via McBSP ports, and the microcontroller will not be powered down. Will it be safe to power down ADS7263 entirely while McBSP ports are connected? We have serial resistors in McBSP lines between ADS7263 and McBSP, what value of resistance will make powering down ADS7263 safe?

    Michal

  • 0 in reply to Michal Tyc
    TI__Guru**** 182371 points

    Hi Michal,

    The inputs to the TMS320F28335 are not 5V tolerant, so I'll assume you intend to run DVdd at 3.3V.  What you can do in this case perhaps is use something like the TPS2101 or TPS2103 and control it from one of the GPIOs of the TMS320F28335.  The digital side of the ADS7263 draws 2.5mA MAX, so there should be plenty of headroom using these small auxiliary power switches.  If you don't have an available GPIO pin, you might consider using a pull-up to the AVdd rail on the TPS210x EN (enable) pin.

    As for the McBSP control lines, is it safe to assume that the serial port will be reset when you are in your inactive state?  If you set the port pins to GPOin you won't have any issues.  The typical source current for the McBSP port pins of the TMS320F28335 is 4mA, so a series R of 750ohm should keep the pin voltage of the ADS7263 around the 300mV limit if you can't disable the McBSP for some reason.

  • 0 in reply to Tom Hendrick
    Prodigy 180 points

    Hi Tom,

    Yes, we are using 3.3 Volts supply. The power switches you have suggested are very interesting and may be useful for our future designs.

    In the meantime, we have decided to keep the both sides of the ADS7263 powered all the time. The power consumption in power-down mode is acceptable. The only problem was to find some room in our PCB layout for an additional voltage regulator for the analog side (the main analog power line will be still switched off), but after some fighting we succeded.

    We have also considered setting the corresponding GPIOs as inputs, but then we cannot prevent a failure caused by a possible software bug. Unfortunately, there seems to be no sufficient short-circuit protection of output GPIOs, and we are afraid that series resistances big enough might interfere with transmissions at maximum speeds.

    Thank you very much,

    Michal