Hello,

I'm designing a schematic of a buck converter using popular MC34063 IC.
It will power USB port (5V, 500mA) plus some additional devices.
Here is the schematic: http://www.nomad.ee/micros/mc34063a/

The input range will be quite large: 8 to 30V (it must work from 9V, 12V, 24V power supplies )
output voltage: 5V
output current: 0.75A
Fosc: 100kHz

In formulas you have to use the minimum Vin, so i used 8V.
I get about 296pF timing capacitor, and 9.4 uH inductor.

The main issue that worries me is what happens to the circuit operation if i end up using the 9.4uH inductor and 296pF and then feed bigger input voltage than i used in calculation? Will the circuit work ok?

Because if i use larger input voltage in calculations, let's say 24V
, the inductor then needs to be 28uH.. What inductor value to choose? 10uH or 28uH?

I wil be glad if someone could help me

 

First of all that schematics is not right for a step down converter. The inductor is not supposed to short the output.
Look here instead. (Or a million other places)
http://dics.voicecontrol.ro/tutorials/mc34063/

Secondly, a bigger inductor won't hurt anything but size and cost.

 

Typical input capacitor value is about 100uH, all electorlytics

-> forget about it.

10uH is a bit low for the 34063.

 

I think you will find calculators such as this one very helpful.
http://www.nomad.ee/micros/mc34063a/

 

Typical input capacitor value is about 100uH, all electorlytics

-> forget about it.

10uH is a bit low for the 34063.

What do you mean by saying forget about it?

 

First of all that schematics is not right for a step down converter. The inductor is not supposed to short the output.
Look here instead. (Or a million other places)
http://dics.voicecontrol.ro/tutorials/mc34063/

Secondly, a bigger inductor won't hurt anything but size and cost.

Sorry, i have drawn my schematics right from datasheet but haven't noticed that it is not step down when posting link.

I will use bigger value for the inductor, but what about timing capacitor? On what input voltage value should it be based? minimum (8V) or maximum (30V)?

 

Try a little experiment:

1. Get a cheap Chinese USB car phone charger for a few dollars. These are step down converters based on the MC34063A or an equivalent device and most are labeled to accept inputs from 12V to 24V.

2. Remove the inductor and replace with a 330μH torroidal inductor capable of 2A.

3. Load the output with a 5Ω 10W resistor.

4. Connect the input to a variable DC power supply. Monitor the output voltage and current while sweeping the input voltage from 24 down to 7.5.