Is your optical-sensing wearable saving as much power as it can as it provides continuous, real-time monitoring under varying measurement conditions? You’ve probably designed your device to accommodate different use cases. Chances are, you’ve also built into your wearable the ability to tune the current up and down to minimize current consumption as the device does its work. But imagine how much more power you could be saving if you had a method to also dynamically adjust the voltage level at the same time.
Some simple examples—with simple math—demonstrate the potential for additional power savings. Optical-sensing systems are designed to run under more challenging, unfavorable measurement conditions. For example, capturing vital-sign measurements from a runner working up a sweat on a trail dotted with sunshine and shade will be more challenging than doing so from someone who is working at a desk inside a temperature-controlled office. Under the more challenging conditions, an optical sensor algorithm aiming to capture a more accurate, continuous heart-rate measurement will need to turn the LED current up to its highest rating to achieve a better signal-to-noise ratio (SNR). A typical optical-sensing circuit will have an LED in series with an optical analog front end (AFE) driven from a voltage VLED. For our simplified example, let’s take a look at a system based on green LEDs where the VLED powering the LED and AFE chain is fixed to 5V. Let’s say that, at the highest current rating (100mA), the forward voltage drop across the LED is 4V, which leaves 1V drop across the optical AFE.
- VLED = 5V
- VF = 4V at 100mA
- VDRV = 1V
Now, let’s imagine this system under favorable measurement conditions—like when the person is working at a desk or sleeping. In such conditions, the algorithm will drop the optical sensor current down significantly. At the lower current (5mA, for instance) the forward voltage drop across the LED will drop to 3V, which leaves 2V across the optical AFE.
- VLED = 5V
- VF = 3V at 5mA
- VDRV = 2V
At the lower current, the voltage required across that AFE to operate correctly (its compliance voltage) drops. Given the fixed 5V on VLED, however, VDRV is actually higher than when under the unfavorable conditions. Say the compliance voltage of the AFE is VDRV_COM = 0.16V; this scenario leaves an excess of 1.84V across the AFE. As such, the system is dissipating 1.84V x 5mA more power than actually needed.
Wearables that provide continuous, real-time monitoring of vitals such as heart rate are designed to operate reliably under varying conditions and use cases. Dynamic voltage scaling can complement other techniques to minimize power and extend battery life.
Basically, any system that uses a fixed VLED architecture that needs to be scaled for unfavorable conditions ends up burning too much power under favorable conditions. But what if VLED were set by a regulator with dynamic voltage scaling (DVS)? Then, VLED could be adjusted up or down in tandem with the current settings, minimizing the power consumption. The system could adjust VLED to minimize, with appropriate headroom, the following expression for every current setting.
VLED – (VF + VDRV_COM)
Even with a simple look-up table method, setting the voltage appropriate to certain current ranges could result in a noticeable savings in power and, as a result, longer battery life for the wearable.
Calculating the Power Savings
The power savings possible due to DVS should be significant, especially when you consider the fact that most wearables will be worn in unfavorable measurement conditions for only a relatively short time. Of course, varying blood perfusion levels, skin types, and use cases will influence how much is actually saved. To illustrate this point, take a look at this typical scenario:
- LED pulse current = 38.9mA
- Pulse duty cycle is 117µs at 100sps, which is 1.17%
- Buck-boost efficiency = 95%
Say these parameters apply for 4 hours in a 24-hour cycle. If we can achieve a DVS savings at 1V, we can calculate the battery savings as follows:
1V × 38.9mA × 1.17% × 95% × 4 hours = 1.73mWHr per 24-hour day
For a 100mAHr battery (370mWHr), that’s a power saving of 4.67% over 10 days of use. And this example doesn’t factor in the other 20 hours in a full day.
Maxim offers a power management IC with DVS for 24/7 monitoring wearables and IoT applications. The MAX20345 includes voltage regulators with ultra-low quiescent current, a linear battery charger, three buck regulators that support DVS, three low-dropout (LDO) linear regulators, and a buck-boost regulator. So, if you’re eager to generate more power savings from your wearables, the technology is available to help you get there.
A similar version of this blog appeared on Embedded Computing Design in May.
