Why low quiescent current matters for op amps

BY ROBERT NICOLETTI
Principal Member of Technical Staff, Core Products Group
Maxim Integrated
www.maximintegrated.com

Operational amplifiers — or op amps, as you probably call them — have long been a key building block in analog circuits. It’s true even in our digital age as op amps are tapped to handle useful functions like feedback control, differentiation, addition, multiplication, and integration.

In digital systems, they are handy components in applications such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), buffers, and regulated power supplies. Think of them as analog insurance — they play an important role in ensuring that the voltage levels in your design are where they ought to be.

They also perform an important signal conditioning role, ensuring that analog signals are clean before they are converted into digital signals.

Now that our design world has many more battery-powered products — many of these in small-form-factors — the power consumption of op amps is under more scrutiny. For portable applications, op amps must operate from a lower, and typically single, positive supply voltage. They must also consume less current. But, despite these specifications, some op amps are required to operate at higher frequencies or with lower noise, even while drawing less current.

So what’s a designer got to do? Fortunately, op amps are continuing to evolve and advance. They are getting more precise and providing better thermal drift. Supply current is reducing, and the parts are getting smaller.

Some chipmakers offer op amps geared toward specific purposes. For example, you might find one that is designed for precision and low noise or another part that offers high voltage. There might be flavors that are low-power and available in small packages, and yet others with CMOS inputs with low input bias current. All good news for an array of applications, including small, battery-powered designs.

Lower I_Q for longer battery life
Another specification that bears close consideration is quiescent current, a circuit’s quiet state when it’s not driving any load, and its inputs are not cycling. While typically nominal, quiescent current does have a big impact on battery life, especially in devices like wearables, hearables, and IoT sensor nodes.

Such products are typically designed to wake periodically to perform some action and then slip back into standby mode. Other product types, such as medical patches, could remain on stockroom shelves for a long period before being called to use. For all of these products, a good user experience relies on long battery life.

Battery life is calculated based on active, sleep, and hibernate currents of the central controlling unit like a microcontroller. The power supply provides energy to all of the system’s functional blocks. Active current consumption has an important impact in extending battery life, but run-time is ultimately influenced by how much time is spent in each power mode.

So as sleep and hibernate modes occupy lengthier periods of time in a device, the standby current of each component becomes more critical. For these cases, the power supply’s quiescent current is the biggest contributor to standby power consumption in the system. That’s why it’s a good idea to build your power supplies with components that have low quiescent current.

Consider, as an example, a small device powered by a lithium coin-cell battery with these specifications:

• 34 mAh from 3-V to 2-V terminal voltage

• 1%-per-year self-discharge, which translates into a 39-nA self-discharge current

• 10-year operating life, 390-nA average load

Now, for a device that’s in idle mode for extended periods of time, using an op amp with low quiescent current (at nano-ampere levels, for example) can yield substantial energy savings. For instance, an IoT sensing system that powers up for 15 ms every minute to take a measurement would use an average of 2.5 uA an hour.

Using the lithium coin-cell battery from our IoT sensing system example — 34-mAh rating — should supply the circuit for 18.6 months. Adding in the losses from an op amp even at a low 1.5 uA, and you can see that there’s a very substantial loss of 60%. By comparison, if we were to use an op amp with nano-ampere current levels, losses contributed by this part would only be around 30%.

Meeting time-to-market requirements
In addition to being used as a discrete component, op-amp functionality can also be integrated onto a system-on-chip (SoC) device. While this provides another option, designing with an SoC minimizes the flexibility that discrete components provide. It can also extend the design cycle, as the application developer has to work with — and wait for — an SoC vendor to create a chip that fits the design’s particular specifications.

Yet developers of products like wearables, hearables, and IoT nodes face considerable pressure to get their solutions into customers’ hands ahead of the competition. When faster time-to-market is essential, it’s prudent to use a small, discrete op amp with low quiescent current.

Take, for instance, Maxim’s MAX40007 nanoPower op amp, which features specifications suited for small, battery-powered portable products such as wearables, smartphones, tablets, and medical devices. Shown in Fig. 1 , the MAX40007 op amp consumes just 750 nA and is available in a 1.1 x 0.76-mm wafer-level package (WLP). So it can be designed in quickly.

Fig. 1: The nano-power op amp that provides a maximized ratio of gain bandwidth to supply current.

With its small size and low quiescent current, the discrete op amp can serve early-generation designs. And using discrete parts that won’t take up too much board space or add any significant current drain can be a significant time-to-market advantage. Operating from a single 1.7-V to 5.5-V supply, the op amp can be powered by the same 1.8-V, 2.5-V, or 3.3-V nominal supply that powers the system’s microcontroller.

As an example of an end application, consider the portable patient monitoring design shown in Fig. 2 . In the signal chain block to the left of the processor, you can see that two op amps can be used to filter the signals from the pulse-oximetry, secure authentication, and blood pressure sensors before they are processed by the ADC.
Fig. 2: The block diagram of a patient monitoring design that employs op amps with low quiescent current.

As a portable device, this patient monitoring design can benefit from low quiescent current as well as small-form-factor.

Summary
The humble op amp has come a long way since its emergence in its early form during the 1940s. Some even believed that this part would become irrelevant as the electronics industry progressed toward digital designs.
But op amps continue to be a critical analog component, and their designs continue to evolve to meet the needs of new and emerging applications. Very small op amps with low quiescent current, in particular, are well-suited for supporting the design challenges of battery-powered devices such as wearables, hearables, and IoT nodes.