# how voltage references work - LM336

In the datasheet for the LM336-5.0 5V reference it uses a 5k resistor fed by 10V. Am I correct in saying that I can give it 12V and it wouldn't matter as long as the current is limited to less than 10mA? It's absolute maximum forward current is 10mA. It's easier for me to feed it 12V because of how I laid out my board.

In normal operation, you would be feeding reverse current through the device, so the Absolute Maximum Rating would be 15mA; the line on the reverse-voltage graph stops at 10mA, but it probably extends pretty straight up to 15mA. Not that one should operate the part there, but one probably shouldn't worry if current gets up to 11mA.

On the other hand, in most cases there's probably not much point in driving the thing with more than 1mA. The graph suggests that the voltage drop will be pretty constant provided there's at least ~0.4mA flowing through it, but the voltage drop falls off very rapidly below that threshold, so one needs to ensure one is operating safely above it. To allow for part variations, 1mA seems like a good safe operating level. The only time I can see usefulness to going above that would be if one has a load whose demand might vary e.g. from 0-8mA. In that case, one might drive (part plus load) with 10mA and figure that the device would have 2-10mA passing through it. Voltage should remain constant provided the load current stays below about 9.5mA.

• you highlight an interesting point. If this was a "real" zener the reverse current would be relevant. This is an IC that "looks like" a zener. At a glance I'd expect an IC to be operated in foward current mode even when modelling a zener. BUT it is clear from the datasheet that your interpretation is correct - their terminology shows all currents when in regulation as a reverse current. Interesting. Apr 2, 2012 at 23:56
• @RussellMcMahon: I thought the forward/reverse distinction significantly important to merit a response. Interesting that the Absolute Maximum Rating for reverse is higher than for forward, though both graphs stop at 10mA. As for what I'd "expect" with regard to forward or reverse, I would expect that if the schematic symbol looks like a zener, the forward/reverse conventions would match one, though it might have been helpful to use terms like "Forward (diode conduction) current" and "Reverse (voltage regulating) current". Apr 3, 2012 at 14:31

Yes you are correct. A series 6k8 resistor would result in the same forward current (about 1mA) with a 12V supply. Note that although 10mA is the absolute maximum current, the device is characterized at 1mA and this would be the preferred operating current.

Yes, but ...

As Supercat has noted, the absolute maximum rating is 15 mA as the device is considered to be a zener diode, and zeners diodes wok in reverse breakdown mode when regulating. As it is REALLY an IC this terminology is misleading, but a look at the data sheet shows that the "reverse" current is the normal operating current in this instance.

You should aim at a maximum current of somewhat less than 15 mA so that the 15 mA absolute maximum is not exceeded ever.

Absolute maximum specs are meant to be just that.
If your
power supply can be 5% high worst case and your
resistor is 1% tolerance worst case, then if you design for exactly 10 mA you could end up with
10mA x 1.05 x 100/99 =~ 10.6 mA.
That does not seem like much over the limit, and it's not (especially as the REAL limit is 15 mA), and you would probably have no problems at say 11 mA (if rating was in fact 10 mA and not 15 mA) but it is never wise to exceed specification values.