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I'm building a device that contains microcontroller (Arduino) and several other modules. What makes me worry is the fluctuation of voltage at the Vcc input of one of the modules. High level scheme of power distribution is shown below: enter image description here

When I measure voltage with my portable voltmeter then I get stable 5.0V at OUT1, however when I measure voltage at the Vcc input of 'Arduino MEGA Sensor Shield 2.0' (which is just providing a convenient way to connect all modules to Arduino microcontroller) then I see a continuous fluctuation of voltage in the range 4.9-5.0V. So I started to power off all modules one by one and I found out that it is CO2 gas sensor that is causing this fluctuation. When CO2 sensor is powered off then Vcc is stable. Then I powered CO2 sensor directly from OUT2 of X705 like shown below:

enter image description here

In such setup voltage is stable and no fluctuation is observed.

While it seems that I solved the problem I still would like to understand why the problem got solved this way and here come the questions:

Question 1: From specification of X705 and all ICs it consists of I understood that despite there are 4 outputs there is still only one voltage converter (boost). So in other words this is not 4-channel power converter, but rather only 1-channel, and these 4 outputs are all connected to this single channel. If so, then why reconnecting CO2 sensor from Sensor Shield directly to X705 solves the issue with voltage fluctuation? Or I understood something incorrectly from specifications?

Question 2: X705 is rated to provide up to 8A, but all modules connected to it consume max 500mA. CO2 sensor itself consume max 85 mA. Why CO2 sensor is causing such voltage dropouts? I understand CO2 sensor has internal heater which is being switched on/off frequently, but why it has such effect on Vcc line?

X705 consists of the following ICs:

  • Injoinic IP5310 Power Bank SoC with 3A charger and 3.1A discharger
  • TI TPS61088 10-A Fully-Integrated Synchronous Boost Converter
  • AO 4447A 30V P-Channel MOSFET
  • TI NE555 Precision Timers
  • TI CD4011B CMOS NAND Gates

Some links:

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2 Answers 2

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The sensor 85mA rating is the average consumption. It draws current in pulses, and it needs a power supply of at least 150mA. The 100mV drop during a 150mA pulse is not unreasonable for boost converter load regulation, especially if the boost converter switches from light load mode to normal load mode.

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  • \$\begingroup\$ Why this drop is happening only when sensor is connected as shown in the first schematics and is not happening when sensor is connected directly to X705 ? \$\endgroup\$
    – Dmitri
    Commented May 6, 2020 at 14:15
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    \$\begingroup\$ I don't know with what you are measuring, but the INA has 0.1 ohm shunt, so that can add up to 15mV fluctuation from the 150mA current. And since the current goes through your wiring before it connects to Arduino input, the resistance from wires and connectors add up too. \$\endgroup\$
    – Justme
    Commented May 6, 2020 at 20:55
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The link you provide for the power management board does not provide a complete specification for the device, but since 4.9V is within 2% of 5V I would guess that the XC705 is working correctly. What makes you think that there is any kind of malfunction or error?

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  • \$\begingroup\$ Unfortunately there is no better specification for X705, - that's why I also indicated ICs that are used in X705. I want to understand why when I connect sensor directly to X705 then there is no voltage fluctuation, but when I connect sensor in parallel with other modules then voltage starts to fluctuate? \$\endgroup\$
    – Dmitri
    Commented May 6, 2020 at 14:22

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