mc34063 current sense resistor question

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
Hello! I'm working on a buck converter that uses the MC34063 IC. It's regulating the voltage perfectly, but the current is nowhere near what I want. I want to pull at least 700 ma, but I've only been able to get about 70 ma. I understand that this is because I'm using a current limiting resistor that's too large, but the current limiting resistor is the battery connector. It's about 1 ohm. Any ideas on how I can lower the resistance even more? Thanks. :)
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
Here's a schematic of the circuit. The component values are, for the most part, identical. R1 is about 1145 ohms. R2 is 3300 ohms. Ct should be 560 pf (has the number 561 on it, which should mean 56*10^1 pf). Co is 100 microfarads. Input capacitor is 660 microfarads (2 330 in parallel). Rsc is a battery connector with about 1 ohm of resistance.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
I calculated that to get 700 ma out, Rsc, the current sense resistor, needs to be just under 0.22 ohms. I discovered that the connector that I was using to power the circuit was about 1 ohm. I decided that that was the closest I could get to the 0.22 ohms of resistance that I needed.
 

R!f@@

Joined Apr 2, 2009
9,734
And you figured 1Ω is same as 0.22Ω?
And how long do you think that a contact will stay at that resistance over usage and time.

Find a low value and put it there.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
It's the closest I could get. My multimeter says it's 1.2 ohms. I have resistors that should be 0.27 ohms, but my multimeter says that they are 1.5 ohms. I'll double check with another multimeter.
 

R!f@@

Joined Apr 2, 2009
9,734
1.2 + 0.27 = 1.47
Do you see where I am getting at ?

If you do not, try adjusting the 0Ω adjust of the VOM (If analog)
if DMM, try subtracting the probe resistance from the measured value
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
The current output isn't what it would be with such low resistance. My calculations say that there is just over 2 ohms on Rsc. I found the equations on the MC34063 datasheet. Do you think the breadboard could be increasing the resistance?
 
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