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I have a current-shunt monitor using INA202 between a 5V power supply and loads. The current monitor should turn off a MOSFET switch and disconnects the loads if the load current goes over 150mA.

I have problem when I connect one of the loads. It is an accelerometer (which should consume only 350µA), powered via a 2.5V LDO regulator. Whenever I connect it, the current monitor shows overcurrent and disconnects the loads. But it doesn't happen if I first connect the accelerometer and then power on the power supply. Also it doesn't have the problem with other resistive loads.

I solved the problem by putting a resitor and zener diode, making a 4.3V regulator, in front of LDO. Although it solves the problem, I didn't understand what was causing the problem. I would like to know what could be the cause.

Edit: Here is the rough circuit:

enter image description here

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  • \$\begingroup\$ rough sketch of schematics \$\endgroup\$ – User323693 Jul 10 '15 at 8:28
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    \$\begingroup\$ Inrush current, maybe? Your series resistor is limiting that. \$\endgroup\$ – Nick Johnson Jul 10 '15 at 9:45
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The accelerometer datasheet states that the turn-on time is proportional to the values of Cx, Cy and Cz (Note 7 on page 3 pasted below)

(7 Turn-on time is dependent on CX, CY, CZ and is approximately 160 × CX or CY or CZ + 1 ms, where CX, CY, CZ are in microfarads (μF)).

This implies that these capacitors have to charge up at power-up, giving your circuit a short term inrush current. This is not at all unusual in a Hot Swap environment.

When the device is connected at normal power-up, these capacitors are charging at the same time as the voltage is rising, masking the effect of the output capacitors.

If you want to do hot swap, I would suggest using a device specifically designed for the task. A number of vendors have these, and this is a typical device.

HTH

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