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TPS62740: Using with CC430 and magnetometer

Dragos Ionescu
Intellectual 680 points
Part Number: TPS62740

Hi, I would like to use TPS62740 as a way to reduce the power consumption in a system comprising of a LiSocl2 battery one cc430 and one magnetometer. I'm not in electronics so much so I need some advice regarding the configuration of the system.

First, I need to decide if the magnetometer MMC5883MA (Datasheet) should be powered directly from the battery or from the TPS:

  • The magnetomer is very low power itself (7 measurements/second -> 40 uA)
  • As per application requirements, the magnetometer should detect approx 4 times a second in single detection mode

What's the best solution here? Is there any advantage if I power the magnetometer from the TPS? If I do, should I use the LOAD rail or the main rail? It seems to me that the LOAD rail activation would incur much more current consumption than the main rail and should be used only if I use a subsystem that can be disabled temporary so important power savings are obtain. On the other hand, if I power from the main rail wouldn't the normal activity of the radio and processor be affected by the magnetometer or the other way around?

Second, I can't see any usage in may case for the PG pin. If not used, what should be the value of the pull-up resistor?

Thanks,

Dragos

  • 0
    TI__Guru**** 150950 points
    Thanks for sharing your application. There are several reference designs, which may be of interest to you, here: www.ti.com/.../toolssoftware The Solar Dice uses a CC430 for a similar application (but with a different type of sensor).

    It is likely best to power the sensor from the output voltage of a DC/DC and to use the lowest output voltage acceptable by your system. Here is an app note which explains the power consumption benefit in detail: www.ti.com/.../slaa500.pdf It may also be required to power it from the lower DC/DC output voltage, if your battery voltage might go above the rating of the sensor (3.6V). I think most LiSocl2 can go above this level, over temperature, etc.

    It is not clear if you can power down the sensor between readings, but if you can, the load switch is useful for this purpose to save even more power.

    Per page 3 of the datasheet, PG may be left floating (without any resistor) if not used. You may want to route it to a test point with a spot for a resistor, in case you want to use it later.
  • 0 in reply to Chris Glaser
    Intellectual 680 points

    Thank you.

    I can power down the sensor between readings but I'm confused by this text from the datasheet:

    ". When CTRL pin is set to high, the
    Quiescent current of the DCS control block is increased to typ. 12.5μA. This ensures excellent transient response
    on both outputs VOUT and LOAD in case of a sudden load step at the LOAD output."

    12.5 uA is much more than 1uA consumed by the magnetometer in idle mode. Don't know for how much time the quiescent current is increased in order to to make a simple computation...

    CC430 works with 1.8V, this sensor works with minimum 2.16V so I believe 2.2V would be a good selection.

  • 0 in reply to Dragos Ionescu
    TI__Guru**** 150950 points
    Yes, you can connect the sensor to either Vout or load. If connected to Vout, it will draw 1uA for the time that it is off (let's assume 96%) and 40uA for the time when it is on (let's assume 4%). These percentages are entirely dependent on your application and sensor. They create an average power consumption.

    If you connect it to load, then it will consume 40 uA for 96% of the time plus the DC/DC will consume an extra ~12 uA from the battery during this time. However, for the 96% of the time, the 1 uA goes to 0 uA since the sensor is not powered. Based on this tradeoff, there is the possibility of further reducing the power consumption by using the load switch.

    You can also add other sub-systems to the load switch, if those can be switched on/off. Finally, you can always use a discrete load switch (instead of the integrated one) and not change the Iq of the DC/DC but still save some power.