Limit average input current (buck converter)

[JJalling]

Hello,

I'm trying to create a small circuit (3.3v) to interface my electricity meter wirelessly.
The meter has a serial port, and can also provide power for my circuit. The datasheet specifies 4V+- 250mV at 75mA.
My idea is to charge up a capacitor to handle the current spikes of the wireless module so I won't exceed the 75mA current limit.

What is the best approach to do this? I don't want to trigger any alarms in the meter by drawing too much current from the port.

How would you do it?

edit: buck, not boost converter

Thanks, Jonas

[ptricks]

If I understand you right you want to use something like and esp8266 as a wireless serial port to send the data and the supply is limited to 75ma ?
Things like the esp8266 use more current when transmitting so you can use a capacitor like you suggested but you need current limiting also in there somewhere.
If it were me I would figure out the max current that I need , 170ma + 10% overhead for the esp8266, so 177ma , or about 180ma.

180ma x 3.3v =.594 watts is what we need. 75ma x 4v = .300 watts is what we have supplied.
I would limit the current with a resistor then zener the voltage to 3.3V and charge a supercap.  Should enable you to remain in high power mode for a few seconds before needing to go to low power.
In the circuit below you may want to increase the resistor to 60 ohms to stay within the 75ma supply limit, the higher the value the less on time you will have to transmit as the capacitor time to charge will increase.  I chose 30 because it will allow the quickest charge time but will cause a very brief current spike when connected and most supplies can handle that fine.

[ogden]

Some switching regulator chips have input current limiter. Most popular example of such regulator is MC34063A.

[David Hess]

Also, some switching regulators intended for battery charging have a separate input current limit.  Or input current limiting can be added to an existing switching regulator controller.