What you need to know about using general purpose op amps at low voltage

You need to design a basic op amp circuit at a low supply voltage and are tempted to use that high-voltage, low cost op-amp to save money. But will it work? I’ll show you how to tell.

l started with the LM324 device as my example because it is inexpensive (popular) and says it works down to 3V. The LM2902 supports -40C, and I’ll use this device instead as a personal challenge because cold temperature has the largest voltage diode drops. So, it is the temperature that will cause the most problems with input and output voltage range.

Step 1: Check the valid input and output voltage ranges for the VCC.

The LM2902 doesn’t have 3V parameters, so I’ll use the 5V parameters in figure 1.

Figure 1: 5 V parameters for TI’s LM2902

Input common mode is covered for 5V and higher, but for 3V I am own my own. The rules of electronics still apply at 3V, so I can use the “full range” VICR formula, 0 to Vcc-2V to see my input range is 0V to 1V.

VOH at 25C is pretty simple, VCC-1.5V is 1.5V, but the full temperature range specification is 23V which is VCC-3V for LM2902. That translates down to 0V at VCC=3V… I obviously can’t use that! Looking at the data sheet schematic, I see two VBE drops. Assuming a -2mV/C temperature coefficient, -40C VOH is reduced by 2*(-40C-25C)*-2mV/C=260mV. I’ll round up to 300mV for a little design margin. My new VOH is now VCC-1.5V-0.3V = 1.2V.

VOL is basically 0V for all temperatures, but there is a catch. This only applies for loads terminated to ground. For loads that require the output to sink current, it is a different story. For that, I’ll refer to the sinking current chart in the LM2904-N data sheet (figure 2). It is just as true for the LM2904.

Figure 2: Current sink for LM2904

VOL for moderate current is under 0.8V, but once again this is 25C data and a typical chart too. Looking back at the data sheet schematic, this performance makes sense. And this time, I see just 1VBE for higher current drive. Assuming a -2mV/C temperature coefficient, -40C VOL is increased by (-40C-25C)*-2mV/C = 130mV. I’ll round up to 200mV for design margin. My new VOL is now 0.8V+0.2V = 1.0V

Step 2: Look at the voltage range results and see if I can design anything with them.

Input range is 0V to 1V – this will be a challenge, but I’m OK with it.
Output range is 1.0V to 1.2V – this would be nightmare, I can’t do it.

Step 3: Get creative.

The good news is the output range can be 0V to 1.2V if the output never has to sink current more than a couple micro-amps. The output voltage range can also be increased by using a pull-up or pull-down resistor on the output pin.

A pull-down resistor’s effectiveness on VOL can be determined just by calculating the load driven (be sure to include the feedback networks as loads). The LM2904 will not oppose the resistor for VOL.

A pull-up resistor’s effectiveness on VOH can be determined by calculating the load driven. However the low-current sink (typically 30uA) in the LM2904 will add to the current that the pull-up resistor needs to provide.

So, the LM2902 and other high-voltage low cost op-amps can be used at their stated minimum supply voltage. However load current, load current polarity, input voltage and output voltage must be carefully calculated to avoid design errors.

Extended temperature options

TI’s portfolio of standard logic, power and amplifier functions are now more flexible for industrial designs, with more than 500 devices qualified for extended temperature ranges from –40C to 125C.

 Check out TI’s latest extended temperature logic functions.