I need a circuit or preferable a ic that have a stable output of 5VDC. Input range may vary between 4 - 5.5V.
I want to "drive" a Vref on a ADC. The operating voltage for this ADC can be 2.7 - 5.5V, so no proplem here. But I would really like a stable external Vref, and I have seen a variation between 4.2 to 4.9V on the system.
Any ideas. I was looking to TPS61175 or TPS40304, but there have to be something simpler? ADCs to be used: MAX11614 or MCP3208 Kind regards
Answer
Since you say your supply voltage ranges from 4.9V - 4.2V, I'd think about choosing e.g. a 4.096V reference instead (or 2.048V reference as as Photon points out your supply swings a bit too close to 4.096V for many references), and divide the input voltage accordingly. Otherwise you will have to boost the input voltage and then use this to supply the reference and ADC.
Boosting the supply is certainly possible but adds complexity, since you don't really want to drive an ADC directly from a switching regulator, so having either some pretty decent filtering or an LDO before the analogue section would be necessary.
If you do want to look at this option there are plenty of simple little boost converters which would be worth looking at (e.g. something like an MCP1640 set to 5.5V followed by a 5V LDO such as a TPS76350 with typical 180mV dropout - you would have to do some reading to work out worst case scenarios and see if things work for your specs - maybe a 6V switcher would be better for headroom but I just picked the first one I found which was max 5.5V) although I think this option is less preferable to simply manipulating the input voltage instead)
If you want an accurate and stable reference, then generally you would use a precision voltage reference IC with an opamp buffer to drive the reference input with a low impedance. Some reference ICs can drive the input directly.
Usually the ADC datasheet will have an example or two of different options for driving the ref input (some ADCs may have an internal reference you can use as an alternative also)
Precision references commonly come with voltages that are a power of two mV-wise, like 1.024mV, 2.048mV, 4.096mV, etc. This makes it easy as e.g. for a 10 bit converter and a 1.024 reference, you have 1mV per LSB.
For your two parts, the Maxim has an internal reference option of 4.096V, but the Microchip part needs an external reference. In the datasheet an example circuit is given using a reference IC:
This uses one of their 4.096V ICs, but if you need 5V, you can simply swap it for a 5V reference - check on one of the major vendors and do a parametric search for "5V reference". Here is an example using Farnell (~150 results)
The other option is to use the reference with a buffer opamp for lower impedance, which may be necessary in some circumstances, here is an example:
The above came from this TI app note.
Whatever you choose, make sure you decouple the reference input (and the ADC supplies also of course) well and pay attention to PCB layout (e.g. analogue and digital sections)
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