Low-Power Hardware Design Application Note

Table of Contents

• Introduction
• General Design Guidelines
  • Notecard Modes of Operation
• Host MCU Recommendations
  • STM32
  • ESP32
  • Reference Design Example
• Power Supply Selection
  • Minimizing Phantom Current
• Component Recommendations
  • Level Shifting
    • Level Shifting 1V8
      • MAX1722x
      • ADP1606a
  • Boost Converter
  • Buck Boost Converter
• Measuring Power Consumption
• Notecard Electrical Characteristics
• Design Review Program
• Ordering Information
• Terms & Conditions
• Contact Information
• Revision History

Introduction

This application note covers how to design a low-power system that uses the Notecard for wireless communication. The Notecard is a family of system-on-module devices that can be used to add drop-in wireless connectivity for products.

Whether you are designing a product from scratch or adding the Notecard to an existing product to supply wireless connectivity, this document will help you navigate the design process and help you overcome some of the common challenges of designing a low power integration.

The architecture of a typical Notecard integration with custom hardware is shown above. To learn more, see the Notecard datasheets.

Notecard is designed to always be powered. Moreover, Notecard power architecture is designed so that it will gracefully enter and leave its low-power idle state on its own, with modem activity controlled in software (see Modem Power Management). This has the advantage of reducing wear on the components from power cycling as well as keeping the modem ready at all times.

General Design Guidelines

This application note focuses on low power; however, if you are new to Notecard, we strongly recommend you start with the Carrier Design Guide for an overview of our recommendations.

As detailed in that guide, Notecard-based products should be designed with an M.2 Key E connector such as Amphenol MDT420E01001 and secured to a grounded standoff with mounting screw. Blues reference designs use a Würth Elektronik 9774025151R paired with an M2.5x4 metric machine screw.

The M.2 mounting screw and standoff are in the same location for all Notecards, but Notecard lengths vary between 35mm and 42mm depending on the model. Design your carrier to accept a Notecard that is at least 42mm long.

Notecard Modes of Operation

The Notecard can be configured to operate in five different modes: periodic, continuous, minimum, off, and DFU. Most of these modes control how often the modem connects to the network. Periodic mode can be configured in software to communicate with Notehub at set intervals. This is ideal for low-power operation because every modem transmission expends joules.

Continuous mode keeps an always-on network connection to Notehub. It draws far more power than periodic mode, and is appropriate for certain customer applications that need low latency.

To minimize device power consumption, it’s important to understand the different modes of operation and how to set them with the Notecard API hub.set request. For a deeper discussion of the low-power software features available to Notecard, see the Low-Power Firmware Design Guide.

Host MCU Recommendations

The host microcontroller you select will affect your overall power consumption dramatically. While you can use almost any host microcontroller with Notecard, we have had good experience with the following MCUs for low power designs.

Regardless of which MCU you select, the overall integration with the Notecard will be relatively similar.

ST STM32

The STM32 from ST is a family of 32-bit microcontrollers based on the Arm Cortex-M with low power consumption and solid performance. The STM32L4 Series of MCUs pair nicely with low-power design due to the following key features:

• Ultra-low-power mode (ULPM): 8nA with backup registers without real-time clock (5 wakeup pins)
• ULPM + RTC: 200nA with backup registers (5 wakeup pins)
• ULPM + 8 Kbytes of RAM: 195nA
• ULPM + 8 Kbytes of RAM + RTC: 340nA

For more technical information, see the STM32 microcontroller system memory boot mode application note.

Espressif ESP32

The ESP32 from Espressif is another 32-bit MCU with low-power sleep states: about 10μA in deep sleep and 5μA in hibernation. It also features onboard WiFi and Bluetooth/Bluetooth LE, which may be appealing for certain product use-cases.

See the Espressif ESP32 Design Guide for their recommendations on working with the MCU.

Reference Design Example

This schematic shows a barebones design for integrating Notecard with your MCU of choice. It is important to note how Notecard interfaces with the host MCU and the power tree that must be provided for proper functionality.

A barebones low-power reference design for integrating Notecard with an external MCU.

Notecard needs these pins (AUX3, AUX4, AUX_RX and AUX_TX, or the equivalent ALT_DFU pins on Notecards that have them) to update your MCU's firmware over the air from Notehub. Without them, updating the MCU's firmware requires physical access to the device. The same reset connection also lets Notecard restart the host remotely.

In the schematic above, the power source is shown as a Scoop, which is an ideal way to provide power back-up to your product, but could just as easily be a LiPo battery or line power that meets Notecard power supply requirements — the decision of which power source to use is very much use-case specific.

Power Supply Selection

We recommend selecting and designing your power supply conservatively to accommodate all possible Notecard SKUs. For some designs, consider our Scoop as a better alternative to LiPo battery for your design.

Notecard's main supply voltage (VMODEM_P) is used for the cellular modem and associated circuitry. The Notecard has on-board regulators designed for direct connection to a battery so any voltage in the range of 2.5V to 5.5V may be provided.

Notecard's logic voltage (VIO_P) is provided by the Notecarrier or host system for digital communication; it will be either 1.8V or 3.3V Although the Notecard typically draws very little current, this supply should be designed with a 150mA budget allocated to the Notecard. Blues reference designs provide for a 300mA draw from this supply to leave room for peripherals. Utilizing a 1V8 MCU (and potential peripherals) will give you the best efficiency and no level shifting is required. If you hope to interface a 3.3V or 5V MCU (for example) with Notecard, or if you're hoping to interface peripherals at various logic voltages (for example) with Notecard, level shifting is required.

Cellular Notecards have specific power requirements to satisfy the power demands of the cellular radio. Your design must supply: 2V5 to 5V5 and be capable of a sustained 750mA draw and brief 2A bursts. This voltage must be applied to the VMODEM_P pins of the M.2 connector (pins 70, 72, and 74).

Keep in mind that a powered Notecard at idle consumes roughly 8–18µA@5V, depending on the model.

Minimizing Phantom Current

The "phantom current" of your device is whatever the board draws when the host MCU is sleeping and no work is in progress. It is the single biggest determinant of battery life for a mostly-idle product. Here are three suggestions to keep it low:

1. One always-on 3V3 rail, chosen for ultra-low quiescent current. This rail powers Notecard VIO and any MCU circuitry that must survive host sleep. Prefer regulators with an Iq of a few µA or less — for example the Richtek RT6160A buck-boost draws about 2µA when not switching, yet can supply up to 3A. Every other rail in the design should be switched on and off via an EN input driven by a host GPIO, so only the one always-on rail contributes to idle draw.

2. Leave VIO powered at all times and do not reset the Notecard after boot. Notecard is designed to be continuously powered, and power-cycling it is counterproductive as its idle draw is already measured in microamps.

3. Be deliberate with pull-up, pull-down, and voltage-divider resistor values. A 10kΩ pull-up on a 3V3 rail burns 330µA whenever the pin is low, which is far more than Notecard itself. For pins that spend most of their time in one state, size resistors in the 100kΩ–1MΩ range, or use a MOSFET to gate the divider only when you need to sample it.

Component Recommendations

Level Shifting

Depending on your connecting MCU and various peripherals, you may require level shifting the logic between devices; Notecard logic requires 1V8 OR 3V3. e.g. If your design has peripherals, they may not be supported by the Notecard without level shifting to 1.8V or 3.3V.

VMODEM (pins 70, 72, 74) operates at 2V5 ~ 5V5, whereas VIO (pins 2 and 4) typically operates at either 1V8 or 3V3.

Because the operating voltages may differ, a logic shifter must be implemented; this doubles as protection between your Notecard and external peripherals — which is strongly suggested.

• TXS0102DCUR
• NVT2002TLH

PMGD175XNE is a dual N-channel MOSFET used on the Notecard in conjunction with on-Notecard i²c devices for level shifting and isolation purposes.

The i²c Serial interface uses the SCL_P (pin 40) and SDA_P (pin 42) signals.

Notecard provides 10KΩ pull-ups on SCL_P and SDA_P, so no additional pull-ups are necessary.

You should provide appropriate termination resistance on SCL_P and SDA_P. Some Notecarriers, such as Notecarrier F, use 100Ω series resistors for this purpose. For more information, please consult the Notecard Carrier Board Design Guide.

Level Shifting 1V8

For 1V8 we recommend the MAX17225 boost converter paired with a logic shifter such as the TXS0102DCUR or the NVT2002TLH. These parts have a low shut down current, high efficiency, and small quiescent current giving a broad feature set. The MAX17225 has a shutdown current of only 0.5nA, far lower than most parts, which are in the µA range.

MAX1722X

We selected the MAX1722X because its job is to power the Notecard MCU, accelerometer and temp sensor, and the crypto engine. It has a very small quiescent current and has 95% peak efficiency. It also meets the 150mA spec in the block diagram above, plus a bit of overhead.

MAX17225 block diagram from the MAX17225 datasheet

MAX17225 Features:

• 300nA Quiescent Supply Current Into OUT
• True Shutdown Mode
• 0.5nA Shutdown Current
• Output Disconnects from Input
• No Reverse Current with VOUT 0V to 5V
• 95% Peak Efficiency
• 400mV to 5.5V Input Range
• 0V88 Minimum Startup Voltage
• 1V8 to 5V Output Voltage Range
• 100mV/Step
• Single 1% Resistor Selectable Output
• 1A Peak Inductor Current Limit

Three specs to keep in mind while selecting a regulator for a 1V8 design are efficiency, shutdown current, and quiescent current. It is also important to keep protection and any kind of leakage current in mind when selecting your regulator.

ADP1606A

When selecting a power supply for your low-power design, it is critical to select a DC/DC with low quiescent current and high peak efficiency. The MAX1722X and ADP1606ACPZN1.8 appear like similar parts, and one may be cheaper, available, and easier to source than the other, but for the use-case of low-power design, the MAX1722X would be a better option.

ADP1606 schematic diagram from the component datasheet.

ADP1606 Features:

• 25µA quiescent current (1V8 output)
• True Shutdown Mode & Power Isolation
• 96% Peak Efficiency
• Fixed 1V8 output
• 1A switch current limit

The reason is because the MAX1722X mates perfectly with the power profile within Notecard that was specifically designed to cater to the health of cellular modem. In addition to that, our aim is for sub 1µA quiescent draw on this always-on rail, so the MAX1722X uses 0.3µA of that 1µA budget whereas the seemingly similar part (ADP1606A) would draw 25µA, far exceeding it.

Level Shifting 3V3

Now that you are familiar with some of the low-power considerations in selecting components for level-shifting, the schematic from Notecarrier F's 3V3 level-shifting is provided for guidance.

Notecarrier F 3V3 logic level shifter schematic.

Boost Converter

A boost converter raises a lower supply voltage to the level a Notecard rail needs. For example, it can keep VMODEM_P above its 2.5V minimum as a battery discharges, or generate a 3V3 VIO_P from a 1V8 rail (Notecarrier F does this with the MAX17225).

The modem's power requirements change with its activity, from microamps at idle to 2A bursts while transmitting, so choose a converter with low quiescent current that can still deliver the current your rail needs. The table below compares some low-Iq boost converters.

Datasheet current ratings for these parts are switch or inductor current limits, not output current. Check each part's achievable output current at your input and output voltages, especially for VMODEM_P, which must support 2A bursts. Power supply manufacturers often have a product comparison matrix that lists each part's Iq, maximum output current, output voltage, and so on.

TPS61021A

TPS61021A schematic from the Texas Instruments datasheet.

The TPS61021A typical application circuit, from the Texas Instruments datasheet, is shown above as a starting point.

Buck boost

A buck-boost converter holds its output steady whether the input is above or below it, which is useful when the supply can be either USB or a discharging battery. Notecarrier F uses a TPS63020 buck-boost to produce a steady 3.6V supply for the host Feather from its main input (the Notecard's VMODEM_P is fed from the main input directly). The schematic from Notecarrier F shows how to implement the TPS6302x in your design.

Notecarrier F schematic where the TPS6302x buck boost converter is used.

Measuring Power Consumption

Idle draw is the number you optimize against. Component datasheets give you the lower bound, but the only way to know what your assembled board actually pulls is to measure it.

Bench measurement with a Joulescope

For the most accurate idle-energy measurement during bring-up, use a Joulescope in place of the battery. Connect a 3V8 bench supply (to approximate a healthy Li-Po) to the Joulescope's input, and wire its output to where the battery would connect. The Joulescope can log current over wide dynamic ranges (nA to A) without range-switching artifacts, so it will catch both the µA-level idle floor and the 2A modem bursts.

In-field measurement with Blues Mojo

To measure power draw in the field under real workloads, add a Blues Mojo in line with the battery. Mojo is a small coulomb-counter board that Notecard detects automatically over I2C (Qwiic), and Notecard reports its power consumption readings to Notehub (see card.power). This is especially useful for validating voltage-variable behaviors under realistic battery curves.

The Power Usage Estimates section of the Low-Power Firmware Design Guide contains measured reference values (taken with a Mojo under standardized conditions) that you can use as sanity checks against your own measurements.

Notecard Electrical Characteristics

Both "absolute maximum ratings" and "DC characteristics" may be found in each Notecard's datasheet.

Design Review Program

The Blues Design Review Program is a complimentary service offered to customers building Notecard and Notehub-based solutions. Customers can take advantage of this service to reduce implementation costs, product spins, and overall time-to-market.

As part of the service, customers will collaborate with Blues technical support and applications engineers to define their product architecture, select key components, and ensure that their application is designed to utilize the best of what the Notecard and Notehub have to offer.

The service includes:

• Product architecture consulting
• A Product ideation session with Blues product experts
• Schematic review
• PCB layout review
• Component placement consulting
• Component selection guidance for BOM cost optimization

For additional information about this service and to schedule an introductory conversation, please contact the Blues Team.

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Contact Information

Blues Inc. https://blues.com
50 Dunham Ridge Suite 1650
Beverly, MA 01915
support@blues.com

Revision History