The power delivery environment for wearables and other devices for the Internet of Things (IoT) is complex. Power consumption is highly variable because of the need to save energy as much as possible. Typically, IoT devices spend much of their time asleep as this offers the lowest power consumption – only waking up when they need to take measurements or transmit data. Even the difference between waking modes can be significant.
For example, only a few subsystems may need to be awake to read a sample from an A/D converter that is taking a sensor reading at regular intervals. However, when sending data over a radio link such as Bluetooth Smart, much of the device from the processor down to the wireless front-end needs to be active and in relatively high power-consumption states.
Although Bluetooth Smart has features to reduce energy consumption overall, it still requires 15 mA of current for the low-energy mode and 30 mW for the higher data rate mode. The need to support these different modes means a PMIC needs to be able to switch rapidly from an ultra-low energy quiescent state to one that delivers high currents for several milliseconds and then power the system down again equally quickly. This puts stringent demands on power delivery.
When a processor moves from one of its lowest-power sleep states to active mode, the difference in current calls for fast startup times and transient response from the power-delivery subsystem.
The processors used in IoT gateways and hubs can also exhibit large differences in power consumption. Although they spend less time in sleep mode, the processors designed for these applications often use dynamic clocking and voltage shifting strategies that tune power consumption to the workload at that point in time. As a result, there can be wide variations in current, with the shifts between modes occurring rapidly.
Figure 1: Block diagram of a typical communications-centric IoT or wearable device based on TI's power and MCU components.
The requirement for rapid transient response can stretch the capabilities of traditional closed-loop control strategies for DC/DC converters based on pulse-width modulation (PWM). Typically, the PWM circuit subtracts a scaled version of the converter’s output voltage from a reference voltage to establish a small error signal. This error signal is compared to a regular ramp signal driven by an oscillator, which generally runs at a fixed frequency. The comparator outputs a digital output that operates the power switch.
When the circuit output voltage changes, the error signal also changes and thus causes the comparator threshold to change. Consequently, the output pulse width also changes. This duty cycle change then moves the output voltage to reduce the error signal to zero, which completes the control loop.
The basic PWM strategy has many advantages, but to ensure stability, the closed-loop control bandwidth is typically held at a relatively low frequency – very often ten times lower than the PWM switching frequency. As a result, transient response can suffer.
Hysteretic control provides a faster-responding alternative. In its simplest form, which does not use PWM, the controller is a comparator with a small hysteresis between its two input terminals. One input is the converter’s output voltage and the other is a voltage reference. When the output voltage sags due to the rising demand from the load, the comparator will generate a high output signal that activates the gate driver of the main output transistor that feeds power to the load. This will drive up the output voltage until it exceeds the reference by the amount allowed for hysteresis. The falling comparator output then shuts off the gate driver.
With any loop comparison or error amplifier to slow down the signal, the delay between the output voltage change is only that of the speed of propagation through the comparator and gate driver. As a result, the hysteretic control strategy offers very fast transient response. Another advantage is that hysteretic control naturally supports wide variations in load current, from the sub-milliamp range through peaks of more than 1 A.
Figure 2: Circuit diagram for a hysteretic power controller
This flexibility comes at the cost of a large variation in switching frequency, which could lead to issues with electromagnetic compatibility (EMC) and induce resonances in the passive circuitry around the power-supply network. There is also the potential for less precision in the output voltage, which can be problematic for today’s voltage-sensitive microcontrollers and sensors.
Hybrid converters mix the two strategies, with the hysteretic comparator being used to control the output rails at low loads typical of deep and light sleep modes, moving into a conventional PWM mode once the load current passes a set threshold to provide high efficiency and predictable frequency response. An example of this strategy is the LM2650M synchronous step-down DC/DC converter manufactured by Texas Instruments, which operates over the 3 A to milliamperes load range. The device also features a logic-controlled shutdown mode in which it draws at most 25 μA from the input power supply.
The LM2650 employs a fixed-frequency PWM and synchronous rectification to achieve high efficiency at higher loads. In many applications, the efficiency exceeds 95 percent for loads around 1 A and exceeds 90 percent for moderate to heavy loads from 0.2 A to 2 A. At lower loads, the device enters a hysteretic ‘sleep’ mode. The LM2650 enters and exits sleep mode automatically as the load crosses current thresholds that are programmed individually, to allow a degree of hysteresis for the mode switches themselves, using external resistors.
In the hysteretic mode, the LM2650 uses a comparator with a built-in hysteresis of 30 mV centered at 1.25 V. Because the hysteretic mode only operates with low loads, the switching frequency is on the order of a few hertz to several thousand hertz. An optional soft-start feature limits current surges from the input power supply at start-up and provides a simple means of sequencing multiple power supplies.
There is a tradeoff between noise and the power savings of hysteretic mode operation. It is easier to filter for EMC purposes a fixed frequency. So, a logic input allows the user to override the automatic sleep feature and keep the LM2650 in PWM mode regardless of the load level.
Designed for low-noise IoT devices such as fitness wearables, the TI TPS65720 can also operate in a low-power hysteretic mode or be forced into a fixed-frequency PWM mode using commands delivered using the built-in I2C interface. However, low-noise performance may not be as much an issue in low-power modes if the system is not sampling noise-sensitive analog inputs or operating the radio interface – where energy spikes from a local high-frequency interferer may be important. By using the system software to force the device into the lower-noise PWM mode whenever a noise-sensitive operation is needed, the designer can optimize the overall energy efficiency of the IoT device.
The PWM mode runs at 2.25 MHz to allow for smaller inductors and capacitors than DC/DC converters that operate with a lower core switching frequency. The result is that the total footprint of external components can be reduced to an area of less than 11 mm2 if typical 0402 packages are used for resistors and capacitors. The high frequency also provides for better transient response. The switching frequency allows a high-Q bandpass filter to be used to suppress spurious frequencies.
The IC also provides a 200 mA low-drop regulator (LDO) output, which operates with an input voltage range between 1.8 and 5.6 V, allowing it to be supplied from the output of the step-down converter or directly from the system voltage. The IC supports a shutdown current of less than 1 μA for situations where the device is operating only the most basic power requirements, such as powering a real-time clock that counts down to the next sensor-input sampling period.
As the IoT develops, we can expect a greater focus on the transient performance and efficiency of low-power modes in DC/DC converters but devices such as the LM2650 and the TPS65720 from Texas Instruments provide effective support for this class of design.
