"Artificial" load on boost switching regulator to keep inductor small


Are you set on using that regulator ?

What about a little ap3012 for loads like that, along with a 10uH surface mount inductor.

Or better yet what about a voltage reference chip?


How about using a 555 timer to generate 12V from 5V? It can be done, see attachments.

The 555 is quite versatile. It can be used as a pulse width modulator (PWM) which is just what you need to generate 12V fro 5V.

In the circuit attached, the 555 is set to run at about 75KHz. The output of the 555 then drives a logic level N Mosfet which in turn waggles one side of inductor L1. R1 on the schematic is the DC resistance of L1 as is there for the benefit of the simulator and you do not need to add it to the circuit.
When Q1 is on L1 is pulled towards 0V. L1 charges up during this period. When Q1 is turned off, the output of L1 goes positive. D1 becomes forward biased and L1 then dumps its charge into C2.
PWM is achieved by varying the control voltage of the 555. Q2 and its associated component provide voltage feedback for the 555. As shown the circuit will control to about 12V. Adjustment can be made to this value by varying the value of R3. Make it larger for more volts or smaller for less volts.
Resistor R2 is required to make the circuit regulate but may not to be so small in value.
The circuit will be a little temperature sensitive due to Q2.
The value of L1 is not very critical and anything in the range of 15 to 33 micro henries works. Ensure that the DC resistance of L1 is not excessively high and that the saturation current of the choke is adequate.
If you decide to have a go then experiment a little and see what you may learn.

The graph attachment shows what you may expect with the circuit provided.

Hope this helps

5 to 12V converter.pdf (13.3 KB)

5 to 12 graph.pdf (88 KB)


Hi again. Thanks for another comprehensive reply, that is helpful for me in my learning quest :slight_smile: the circuit you have drawn is a little complicated for me at this stage (I don't quite understand it all) and it is perhaps a little component-heavy for my application, but it is interesting. How much current would you expect to be able to draw from such a circuit?

I would also like some clarification on my original question, in that what is the relationship between the inductance of the inductor and the available current I can draw from my MC34063?


Yes.. this is the relationship it is inversely proportional .. has to do with the duty cycle of the regulator and the voltage across the inductor a whole lot of things that are over my head, I believe you can add a resistor in order to use a smaller inductor if that's the goal. Maybe someone with more knowledge will add more to this.

Switchmode regulators all need a minimum load to be able to regulate their output, as there is a minimum on time for the switcher chip based on the switching frequency.
The greater the minimum load, the smaller the inductor can be to still enable output regulation.
The maximum current that can be drawn will be set by the maximum current the switcher
chip can handle, and the maximum current the inductor can handle before it saturates.


Modern chips work with much higher frequencies.



Inductor value for the lowest current, and wire size (physical size) for the highest current.
10x higher switching frequencies means 10x lower inductor value.