MC34063 buck with large variable input? help

Hi,

I want to use a MC34063 switching chip (or similar) for it's low cost in bulk, of about 0.20 USD.
Edit: I found a new chip, see next post.

I can find plenty of documentation on how to design the circuit with a constant input voltage, but I can't find anything on what happens if the input needs to be variable from 7v-24v. (it won't change during operation, but I want the board to have the flexibility)

I need 5v 500ma output (1000ma would be even better). Should I design the circuit based on the highest input voltage or the lowest input voltage? or is this not the right chip?

There is drop in SMPS I like that does exactly what I need, but it's $2.20 USD, which is too much for what i'm doing.

Thanks
Mike

Would ACT4088 chip be a good option?
http://www.mouser.com/ds/2/631/ACT4088_Datasheet-345917.pdf
I really like the price and minimal components on this chip.

I know this chip has a wide input range based on the datasheet, but I still can't determine if this means you choose the external components based on a fixed input within the allowed range, or can the input be variable to the completed circuit for flexibility to the end user?

I need an input range of about 7v-24v DC, to provide 5v output, ranging from 100ma to 500ma, but 1000ma would be nice to have if possible.

any suggestions would be great.
thanks
Mike

miketedeschi:
Would ACT4088 chip be a good option?
http://www.mouser.com/ds/2/631/ACT4088_Datasheet-345917.pdf
I really like the price and minimal components on this chip.

I know this chip has a wide input range based on the datasheet, but I still can't determine if this means you choose the external components based on a fixed input within the allowed range, or can the input be variable to the completed circuit for flexibility to the end user?

I need an input range of about 7v-24v DC, to provide 5v output, ranging from 100ma to 500ma, but 1000ma would be nice to have if possible.

any suggestions would be great.
thanks
Mike

as far as I can see, the output voltage only depends on Rfb1 and Rfb2 -

but, the efficiency decreases as the input voltage increases, and is not good at low load current values , you should have a look to the typical performances curves, to see if it suits your needs .

http://dics.voicecontrol.ro/tutorials/mc34063/

Enter it as the middle of the voltage range, and a percentage. So:
24 - 7 = 17
17/2 = 8.5
7 + 8.5 = 15.5V middle of the range
8.5/15.5 = 0.548 or 54.8% range

Calculation Results For Step-Down Converter

Vin = 15.50 ±54.8% V
Vout = 5.00 V
Iout = 0.50 A
Vripple = 0.05 V

Vin min = 7.01 V
Ton / Toff = 3.59
Ton + Toff = 10.00 µs (micro seconds)
Toff = 2.18 µs (micro seconds)
Ton = 7.82 µs (micro seconds)
Ct = 312.77 pF
Ipk = 1.00 A
Rsc = 0.30 ?
Lmin = 11.78 µH
Co = 25.00 µF
R1 = 10.00 k?
R2 = ((Vout - 1.25) / 1.25) * R1 = 30.00 k?

If you design for the highest input voltage, then the design should be OK at lower voltages too. If you design for a lower input voltage, then you may choose an inductor with a value that is a little too low at higher voltages.

The limiting factor apart from the chips voltage and current limits, is the inductor saturation current.
If you design the circuit based on the highest input voltage at maximum output current , then it will be Ok at lower input voltages.

OK, perfect. I figured there was a way to make sure it worked across the range, but it's tough to google because the word "variable" usually finds info on variable outputs, not variable inputs!

thanks for the tips, this just saved me $2 per board!

mauried:
The limiting factor apart from the chips voltage and current limits, is the inductor saturation current.
If you design the circuit based on the highest input voltage at maximum output current , then it will be Ok at lower input voltages.

Assuming the same load, wouldn't the inductor current be the same for all cases of input voltage for a buck converter? The only difference would be the duty cycle, which may have implications for the actual inductance value, but not the maximum current.

To be safe, I'd perform the design calculations for both extremes, and pick the most conservative component values.

Hello, chopped liver here. Did anyone read my posts?

I want to design a dc to dc converter. I want :

Vin = 5-25 V
Iout= 1.5 A

four Vout : 5V , 12V , 15V and another one is adjustable .

Dose it possible to gt these outputs (5V , 12V , 15V , adjustable ) by changing R1 and R2 only ?