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Swiss Army Knife IC makes a buffered reference

10 Sep 2013  | Bruce Moore

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Swiss Army Knives are great. I bring a small one everywhere because it can do a million little tasks. In the analogue arena, there are some ICs that fall into the Swiss Knife category of devices, sporting a number of tools built into one package. The 555 timer comes to mind; it can be configured to do an amazing number of different tasks. As a result, it has gone into a huge variety of end products. As of 2004, one billion were being manufactured every year1, probably making it the biggest-selling IC ever. The LM10 is another good example, as the idea of gain-block plus reference in one package is definitely Swiss Army material.

For fooling around with analogue signals at super-low supply voltages, you need something other than a 555 or LM10, as they either die out below 4.5V, or draw too much battery current (270uA for the LM10). My new favourite knife, Touchstone Semiconductor's TS12011, is used in the following circuits to generate some oddball reference voltages for applications where battery current draw is critical. Like an LM10, it contains an op amp plus a reference, and additionally a comparator (unused in these circuits).

Figure 1: Buffered reference generators.

The gnat-power reference in the TS12011 is unbuffered, a design trade-off taken to absolutely minimise the supply current drain at all costs. You can only draw 100nA from it, according to the datasheet spec. That's fine, if the reference is being used to set a comparator threshold or op-amp virtual ground level, but if you need more oomph, buffering is required. When dividing the reference output with resistors, lower values are generally better from a noise and leakage standpoint, but take care not to overload it. In figure 1B, the total divider series resistance must be 5.8M or greater (0.58V/100nA). The TS12011 op amp has tighter offset current specs than input bias current specs, so it usually makes sense to add a series resistor at the other input to compensate for the high values used in the divider.

Figure 2: Circuit of Figure A, VDD = 1V; 0.666V VREF load regulation.

These basic reference circuits are a good starting point for all sorts of designs. Accuracy for figure 1A is ± 4.8% including temp drift, VOS, IOS, and 1% resistor tolerances, which is good enough for building a wake-up detector, timer, or power converter. The relatively high open-loop gain of the op amp keeps the load regulation characteristic flat up to 1mA. Supply current for either circuit is less than 2uA. The extra latching comparator is a bonus, shown employed as a voltage detector with 0.3V trip threshold in figure 1B. 

Ward, Jack (2004). The 555 Timer IC – An Interview with Hans Camenzind. The Semiconductor Museum.

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Bruce Moore contributed this article.

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