# Choosing a current sensing resistor for boost converter IC AZ34063U

I'm looking to build a boost converter using the AZ34063U IC (Datasheet) and the example circuits in the datasheet use different values, ranging from 0.22 to 0.33, and I couldn't figure out how I should decide a value for my circuit. Here's a couple example circuits with the resistor in question boxed in red:

I've decided that values 10.5K and 1.2K 1% resistors would work for R1 and R2, aiming for 12vdc from 5vdc input (will be in the step up configuration of the first diagram). Any advice on choosing a reasonable current sense resistor in that case?

• See the "VIPK(sense)" parameter in the datasheet. You pick a resistor that gives you that voltage at the maximum current you want to allow. Sep 16, 2023 at 22:53

The 34063 has cycle by cycle current limiting. The current sense resistor for a 34063 sets the maximum current which will be permitted through the switch.

$$R_{sense} = \frac{0.3}{I_{max}}$$

If you use a controller chip other than the 34063, what is written above will, in general, not apply.

Also note that there are multiple suppliers of 34063s. While they all have the same general design, some parameters differ depending upon supplier. So, be sure to check the datasheet for the particular chip you are using when designing.

Edit. A question was raised in the comments as to whether the 34063 has cycle by cycle current limiting controlled by feedback from the current sense resistor.

Most of my understanding of the operation of the MC34063 comes from application notes, such as TI's slva252b.

Section 1.2 describes the oscillator, and states

The oscillator is composed of a current source and a current sink that charge and discharge the external timing capacitor $$\(C_T)\$$ between an upper and lower preset threshold.

and

The oscillator runs constantly, at a pace controlled by the value of $$\C_T\$$.

The phrase "runs constantly" is perhaps not as clear as one might like. However, my understanding, which has been verified by my own experience designing with this chip, is that the oscillator is either charging the timing capacitor, or discharging it. There is never a condition or time when it is not doing one or the other. So, when the timing capacitor reaches the upper threshold, the oscillator shifts to discharging the timing capacitor.

Section 1.3 describes the an overview of current limiting, and states

When the voltage drop across the sense resistor becomes greater than preset value of 330 mV, the current-limit circuitry provides an an additional current path to charge the timing capacitor $$\(C_T)\$$ rapidly, to reach the upper oscillator threshold and, thus, limiting the amount of energy stored in the inductor.

Note that the value of 330 mV conflicts with the formula given in the datasheet in numerous places in section 9

$$R_{SC} = \frac{0.3}{I_{pk(sense)}}$$

That is just one of the many examples of bad documentation surrounding the 34063, but I digress.

Section 1.4 describes the output switch. The description includes waveform diagrams which show that the switch is off whenever the timing capacitor is discharging.

Section 2 is titled "Functional Description". It makes explicit what was implicit in the previous section.

When the oscillator reaches its upper threshold, $$\C_T\$$ starts to discharge, and a logic 0 is present at the A input of the AND gate.

...the latch is always reset when $$\C_T\$$ begins ramping down.

• This is incorrect, I think: best guess is this AZ- variant is simply a 2nd-source of the original MC34063. Which does not have current limiting, but something that poorly approximates it: the oscillator frequency is skewed by the current sense voltage; it is not simply turned off as you would expect from a peak current limit control as such. I strongly discourage 34063s for this reason, among others. Sep 16, 2023 at 23:42
• @TimWilliams When the voltage across the sense resistor reaches approx 0.3V (0.33 in some datasheets, IIRC) two things happen. One, the oscillator capacitor is pulled up to its peak value, terminating the timing cap charging phase early and beginning the timing cap discharging phase. Two, the switch is turned off, as it always is during the discharging phase. Would be happy to cite you the relevant places in the datasheet where this is discussed, although the datasheet is written in an extremely user-unfriendly manner. Sep 16, 2023 at 23:51
• @TimWilliams Interesting, thanks. I skipped over the ADP3012 because it cost 20 cents more than the 34063, but I'm not that cost sensitive. I might just go with that chip instead and save on a few passives instead.
– nak
Sep 17, 2023 at 0:27
• @TimWilliams I have edited my answer with supporting references to my claim that when the voltage across the sense resistor reaches approx 0.3 or 0.33 V, the switch gets turned off. Incidentally,I have used the current limiting feature in creating a LED driver based on the 34063. Sep 17, 2023 at 0:41
• @TimWilliams, yes in fault conditions, current may rise higher than 0.3 V / R_sense because the chip takes some time to respond. But that's true of all chips. Your distinction between hastening the termination of a pulse, vs terminating per se, is merely one of speed. In all cases, there are some intervening components between the sense resistor and the switch. The only solid distinction is how fast those intervening components effect a shut-off of the switch. the 34063 is admittedly slow compared to more modern chips. But I'm not selling the 34063, just describing its operation. Sep 17, 2023 at 2:16