# How to accommodate a range of input voltages

How does a designer generally accommodate for a range of input voltages in their circuit? For example, my design needs to be able to accept 12-24V where it uses 12V and 9V at seperate areas as shown below. Current no more than 200mA.

The 12V supply does not need to be very accurate as it is being used for two things:

1. A power supply for a 4-20mA sensor transmitter (As far as I know, the sensor has its own regulation to maintain a constant current)
2. A supply for the 9V regulator. The 9V supply needs to be precise, say under ±1% but a regulator will be able to step down a slightly noisy/innacurate 12V supply if needed so i'm not concerned.

simulate this circuit – Schematic created using CircuitLab

Is there an IC that output a regulated 12V with a range of input voltages from 12V up to 24V. I've found a buck boost converter that seems to do the job (TPS55165-Q1). Will it do well in a 12V - 12V situation scenario?

• Just curious right now, but is your input voltage requirement precisely $12.0\:\text{V}-24.0\:\text{V}$? Or are those numbers somewhat flexible? Similarly, what room do you actually have on the two supply rails? With enough slop available, almost anything can be made easier. :) In short, can you specify some accuracy and precision figures? (Understood on the current compliance.)
– jonk
Commented Aug 28, 2019 at 0:39
• @jonk Thanks for the reply, i've edited the question to include accuracy requirements. Commented Aug 28, 2019 at 1:18
• Just off the cuff, that seems so much easier than before. If I get a moment and others don't jump in because of how easy it is, then I may write something. But this feels right now almost too easy to be true. Must be I missed something.
– jonk
Commented Aug 28, 2019 at 1:48
• If you know you need 12V and less, and for sure your supplies will be between 12V and 24V, a buck regulator should do (you dont need the boost). Commented Aug 28, 2019 at 1:51
• Are you trying to program these voltage regulators? You'll have to use the "Power Good" signals from this IC.
– user103380
Commented Aug 28, 2019 at 2:21

You lack much of the detail necessary to provide a viable answer.

1. What is the range of 12V output that can be tolerated?
2. Can the 12V supply go above 12V or is that an absolute upper limit?
3. What is the range of 9V output that is expected?
4. What is the current range for the 12V output?
5. What is the current range for the 9V output?
6. Are you expecting low loss (switching) supplies for both the 12V output and the 9V output? Is linear regulation a possibility for either/both?

You give only one current figure, 'no more than 200mA'. If all you are driving is a single 4-20mA loop (at something around 12V) plus 9V supply ….does this means the 9V supply is about 180mA maximum?
If you draw a maximum of 200mA from a 24V supply then linear regulators would dissipate about 3W combined, whereas using a switching regulator for both the 12 an 9V regulators would drop this to a little under 1W combined.
You need to make a choice about what you need to use.

In terms of solutions the biggest challenge is to provide 12V output when the input is 12V (it's lowest). To do this you would need to use a Boost/Buck switching regulator which would be in Boost mode when the input AND output are 12V. Typically you would expect this Boost mode as the input gets closer to 12V since there are voltage losses to cope with internally.

Since it appears that the majority of your current is consumed in the 9V supply line, the best solution may be to use a linear regulator for 12V supply and use a switching regulator for the 9V supply ...running both from the raw input (12-24V).

simulate this circuit – Schematic created using CircuitLab

The 12V supply could be a simple TLV431 and a TO92 transistor that would run to about 50c in cost. This would be ideal if you set the regulated output to about 11.3V (well within 10%).

The 9V regulator could be an LM2596 based regulator that costs about 80c.

• Thanks for the detailed reply. I've reconsidered my circuit and the 9V supply will only need 50mA. Is there a reason why you'd take the 12-24V input for the 9V supply and not the 12V? I was thinking of using the 12V as an input for the 9V regulator and using a LDO (ADP7102 looks good). The sensor does not have min/max ratings that I can find (It's an old gas sensor) So i'd rather keep a low tolerance on the 12V supply. The 9V supply tolerance as per OP. As for the 12V supply, I'll do more digging to find the supply tolerances for the Gas sensor so I can choose between a buck-boost or TLV431. Commented Sep 6, 2019 at 4:12
• @dos584 Cascading power supplies may increase complexity and is usually only necessary if you want to ensure power supplies come up in a certain order. Since very cheap switching regulators that can withstand your 24V exist ...why would you think there is a benefit from cascading the regulators? Commented Sep 6, 2019 at 13:54
• Good point. Thanks for your advice. Should I have any concerns regarding the TL431 and the possibility of it constantly sinking current in the event of a permanent 24V supply installation. Also, would you still recommend that LM2596 (9V) Supply for a low current (50mA) application? Commented Sep 9, 2019 at 1:39
• @dos584 The LM2596 works down to minimal currents. The board itself ensures that the minimum current required for regulation is flowing, so I see no issues. Commented Sep 9, 2019 at 4:29

I don't know about the TPS55165 specifically, but buck-boost controllers are specifically designed to smoothly transition from the input being lower (or about equal to) the output to the input being higher. One would hope they'd do the job with ease, but then, it's not the easiest job.

Scrutinize the data sheet, while reminding yourself that it was written to (A) sell lots of chips chips, while (B) keeping TI from getting sued. So if they say something definitive, you can trust it (see B), but if they only seem to say something definitive, be suspicious (see A). And note that I'm not picking on TI -- all semiconductor datasheets work pretty much the same way.

They do have a "step down to step up transition" graph in there that looks pretty good to me, but check for yourself. I am, after all, just some guy on the Internet.

The 12V supply does not need to be very accurate as it is being used for two things

The above 12 V, If it can be say 11 V (10% tolerance) I believe the 4 - 20 mA sensor can still work.

Consider, LMR36006 4.2-V to 60-V, 0.6-A Ultra-small synchronous step-down converter for example.

The drop out voltage is about a couple 100 mV. This is measured by setting output voltage as 5, but I believe the performance for 12 V will be similar. The circuit is simple and there are multiple options available from many vendors for the buck regulator. I would argue not to be really sticking to buck boost.

Isn't it is okay when you get 11.5 V supply instead of 12, when the input is at 12 V? Check once again.

For 9V, you can have a linear regulator to convert 12 V to 9V for the best noise performance (you spoke about noise again, so).

• Thanks for the reply, I will check on the supply tolerances for the sensor. Commented Sep 6, 2019 at 4:13