Electronic – Why are there resistors on the input of this regulator


I am looking at an older circuit, and I was wondering why the designer added these two resistors in series with the input voltage of this 5 V regulator. The regulator would be powering a system that pulls 30 mA maximum.

I think I had seen that some people add resistors to try to dissipate heat, but the specified resistors are 1/4 W 0805 resistors. Is that really what they are for?

The regulator is MCP1703-5002E/DB (datasheet).

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Best Answer

I was wondering why the designer added these two resistors in series with the input voltage of this 5v regulator.

There are obvious possible (and interconnected) reasons: -

  1. The two resistors will limit the output current that the regulator can provide to the load. This may be regarded as important by the designer. The current limit will be about 120 mA and this will also...
  2. Reduce the power dissipation that the regulator has to deal with. This may be an important consideration regards potential fault conditions in the load. The normal running current may be 30 mA but, under fault, current will rise.

Drop-out voltage from data sheet: -

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So, at a load current of 120 mA the drop-out voltage is about 0.16 volts and the minimum voltage needed to run the regulator is 5.16 volts hence, the voltage across the resistors will be around 6.9 volts at a current of 115 mA.

There could be another reason too: -

If the load is fairly stable at around 100 mA there would be a guaranteed volt drop across the resistors of 6 volts and this means that if the input rail (called 12v_ISO) rose too high, the regulator would be somewhat better protected. The regulator has a maximum input voltage of 16 volts but the designer may be aware that this voltage might spike up to over 16 volts in certain situations.

A trick that the designer may have missed is not applying more input capacitance. With 100 nF in series with 60 ohms, this acts as a 27 kHz low pass filter but the main improvement to the device's PSRR is to be made at frequencies starting at a few hundred Hz: -

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So, based on that I'd be making the input capacitor more like 10 uF resulting in a cut-off of 265 Hz and this will improve PSRR quite a bit. The data sheet tends to favour an input capacitor of 1 uF for most of its performance graphs so this might be another little thing that the designer missed.

Data sheet quote: -

For most applications (up to 100 mA), a 1 µF ceramic capacitor will be sufficient to ensure circuit stability. Larger values can be used to improve circuit AC performance.