PPK2 - Voltage drop between input and output in ampere mode

[abernabeu]

Hi,

I tried to measure the current consumption of a system that consist of a supercapacitor supplying a MCU.

I placed the PPK2 between the supercapacitor and the MCU to measure the current consumption of the MCU and found out that the voltage is different between the input and the output of the PPK2. There is a drop of almost 0.6/0.8V.

We have the same behavior on 2 different PPK2 devices.

Is it a normal behavior ? I thought that in ampere mode, the PKK2 had no influence to the voltage.

The voltage used is within the operating range of the PPK2, ie. 3.3V / 1.8V. So to be specific, when the supercapacitor has a voltage of 3.3V, the output of the PPK2 is around 2.5V.

Is there a way to have the same voltage on the input and output of the PPK2 ?

Regards,

Antoine

[hmolesworth]

Not sure if this is what you are looking for, but depending on the current range there is a series resistor between the supercapacitor and the DUT which will drop V=IR volts. The series resistor R is low (51 mOhm) for high current, but large (110 Ohms) for lowest current measurements. I made these notes from the original PPK-2, it may have changed since then. Reduce the voltage drop by selecting a coarse measurement range on the PPK2.

// Resistor Rc for current sense is 1k0 in parallel with one or more of 110R, 11R0, 1R0 and 0R051
//
//              <- +ve Supply to DUT From VOUT                               External Battery +ve To VIN
//             +------------------------------+                             +-----------------------+
//             |                              |                             |                       |
//             |              +---------+     |                             |     +---------+       |
//             |              |     GND |     | VOUT                   VIN  |     | GND -ve |       |
//   +=========O=========+    |    +====O=====O=============================O=====O====+    |       | Battery
//   |        Vdd (+)    |    |    |    |     #----+                        |     |    |    |       | + 3 volts
//   |                   |    |    |    |     |    |Rc        AMPERE \      |     |    |    |     +-O-+
//   |                   |    |    |    |     |   +++                 \     |     |    |    |     |   |
//   |                   |    |    |    |     |   | |   +-----------O  O----#     |    |    |     |   |
//   | Device Under Test |    |    |    |     |   | |   |                   |     |    |    |     |   |
//   |       DUT         |    |    |    | 120k|   | |   |                   |120k |    |    |     |   |
//   |                   |    |    |    |    +++  +++   |   \  SOURCE      +++    |    |    |     |   |
//   |                   |    |    |    |    | |   |    |    \             | |    |    |    |     +-O-+
//   |                   |    |    |    |    | |   +----#--O  O-- PPK LDO  | |    |    |    |       | Gnd (-)
//   |                   |    |    |    |    | |                           | |    |    |    +-------+
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     |                             |     |    |
//   |                   |    |    |    |     #-- VDUT                VIN --#     |    |
//   |                   |    |    |    |     |  Sense                Sense |     |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     | 30k                     30k |     |    |
//   |                   |    |    |    +--#--+                             +--#--+    |
//   |                   |    |    |       |                                   |       |
//   |                   |    |    |     =====      <<== Common Gnd ==>>     =====     |
//   |                   |    |    |      ===           (Common -ve)          ===      |
//   |        Gnd (-)    |    |    |       =                                   =       |
//   +=========O=========+    |    +===================================================+
//             |              |     PPK-II
//             +--------------+

[abernabeu]

Hi, 

Thank you for the answer. Yes, the 110 Ohms must be the problem. Is there a way to force the resolution of the current measurement ? Within the power profiler from the nrfConnect app, I can't find any option that will modify the resolution.

[PaKa]

No, the ranging is automatic in HW. You should never see a drop that large as that would impact the application. The system should swap the resistance to keep the drop inside a predefined range. With 1A/51mOHMs I am tempted to say between 0V and 51mV. If you have damaged some of the circuitry inside then the switching may fail.

Not had any ESD issues, high inrush currents (above 1A) or done hot swapping of power sources?