Primary Battery IoT Devices
Many small IoT devices require long-term operation with a primary battery.
Therefore, it becomes important to control and monitor the battery while supplying ultra-low-consumption operating and efficient power for each function of sensors, MCUs, and wireless communication. Here, we will demonstrate a solution that adds a power supply configuration that is general and suitable for long-term battery operation, as well as a function that cuts off power consumption during transportation and when not in use.
Note: About Lithium Primary Battery
3.0V is Manganese Dioxide type / 3.6V is Thionyl Chloride type
Solution Outline
About Boosting ICs
Block Diagram (a) shows the case where the MCU can be directly connected to the battery. Simple IoT/security/wearable/medical small devices are mostly of this structure.
In recent years, there has been an increase in the number of MCUs that operate over a wide range of 1.8V to 3.8V, in which case they can be directly connected to a battery for use without the need for a power supply IC. In this regard, RF and sensors need a fixed voltage of 3.3V, and even if the operating voltage is wide, in order to meet the specifications, most of them need more than a certain voltage, i.e., they need a booster IC. RF and sensors don't work all the time, and sometimes RF communicates once a day for a few seconds.
Additionally, even if it looks like it is working all the time, there are actually many cases where the current consumption is reduced by careful ON/OFF control to make the battery durable. In order to realize the above work, the MCU will ON/OFF control the work of RF and sensors when needed. In addition when stopping, not only the function of RF and sensor will be stopped, but also the boost IC and voltage regulator will be stopped, which can use the battery for a long time. To suppress the ripple during operation so that the noise frequency is constant, PWM fixed type is suitable.
If a lightly loaded operating condition exists, the PWM/PFM conversion (automatic switching of operating modes) type is used. In addition, to suppress EMI and make it compact, the coil-in-one type is suitable. Boost DC/DC
XCL102: PWM, Coil-in-One type XCL103: PWM/PFM, Coil-in-One type XC9141: PWM, External Coil XC9142: PWM/PFM, External CoilAbout LDOs
In order to make the power supply noise of RF and sensors lower, sometimes the back stage of the Boost IC is used with a Voltage regulator. High-speed LDOs with high ripple rejection ratio/low noise and good load transient response characteristics are best suited for RF parts with steeply varying consumption currents. In addition, there are cases where noise of 100kHz or more is important in sensor applications, and there are also cases where a low consumption type with low high-frequency noise is better suited than the high-speed type. Voltage regulatorXC6233: High-speed
XC6215: Low-consumption
About RESET IC
Monitors the battery voltage, and sends a signal to the MCU when the voltage drops. Using an ultra-low consumption type suppresses the burden on the battery.
The power supply voltage of the MCU is the same as the voltage being monitored, so a CMOS output type can be used; the CMOS output type does not require a pull-up resistor, so there is no consumption current flowing through the pull-up resistor. The N-channel open-drain product also reduces the parts, and when "L" is output when the battery voltage drops, the pull-up resistor used increases the current consumption, which affects the battery life, and there are also products that can monitor the voltage such as UVLO and A/D converter in the MCU. Voltage detectors are useful when monitoring functions are required. Voltage Detector XC6136 C Type: Iq ~ 100nA (C Type: CMOS Output)
Solution for Improving Battery Durability / Push Button Load SW
Block Diagram (b) shows a solution to add functionality and dramatically improve the battery's durability by adding a Push Button Load Switch. The solution is to add a Push Button load switch to the circuit. In order to **** enjoy MCU control and push button control a pull-up resistor is required for the SBD on the right side of the switch pin and the VDD of the MCU.
Push Button Load Switch XC6194: 1A SW Built-in XC6193: Supports External Pch Driving High CurrentThis solution has the following great advantages.
1. Prevents battery discharge from the time the product is shipped to the time it is first used
This is called "Storage Mode" or "Ship Mode". This is best suited for devices where the battery cannot be removed. In this case, the current consumption is almost zero, and you can start using the device by pressing the button. Of course, the button for MCU control can be used with this IC***.
2. Main power ON-OFF switch
Instead of a mechanical switch, a push button can be used for ON-OFF, which is best suited for waterproof devices, for example, where the MCU sends a signal to the SHDN pin and turns off the Push Button load switch. We have also prepared types of Push Button load switches that can be turned off by long-pressing the button.
3.Unlocking Dead Machines
The OFF function of the Long Push Button can be effectively utilized when the device is dead or other abnormalities. Selecting the type of up to 5 seconds or 10 seconds reduces the possibility of shutting down by mistake, and is suitable for countermeasures against crashes. After shutting down, press the button again to start it up normally. Furthermore, the Push Button load switch has the following features as a battery-friendly function.
The inrush current prevention function suppresses inrush current during startup
After startup, a PG pin output is provided to enable the next power supply IC and MCU to operate.
1.2V UVLO function allows the Push botton load switch to go into Shutdown state, which has the effect of preventing the battery from leaking, and when the VOUT drops dramatically, the output short-circuit protection function is used to provide Shutdown protection
As mentioned above, even for a simple IoT device centered on an MCU that operates with a direct battery connection, a little effort can further improve the battery performance. effort can further improve battery durability and easily meet small, high sensitivity requirements.
Li-ion Polymer Internet Devices
Although battery-operated, most of the IoT devices with high-frequency and complex functionality for sensors and communications use Li-ion/Polymer secondary batteries. Charge control of the primary battery and the addition of an ultra-low consumption buck DCDC that matches the supply voltage are representative power solutions.
Solution Summary
About CHARGER IC
IoT devices using Li-ion/Polymer require a battery charging IC for charging and a step-down DC/DC or regulator that reduces the voltage to within the supply voltage range of the MCU. First, I will explain the usage of battery charging IC. Charge Voltage (CV : Charge Voltage) and Charge Current (CC : Charge Current) are the basic choices. Select the charging IC and resistor RISET according to the desired charging current.
Battery Charging ICXC6808: 5mA ~ 40mAXC6803: 40mA ~ 280mAXC6804: 200 mA ~ 800 mALi-ion/Polymer batteries in this circuit box are built-in NTC with external PCM ( Battery Protection Circuit) case. The PCM is required for both internal and external mounting, and if the NTC is not built into the battery, please take care of where it is placed and mount it externally. If the NTC is not required, handle the NTC connection pin by the method specified by the battery charging IC. The CSO pin, which shows the charging status here, has been used to send the charging status to the MCU. the CSO pin is an N-Channel open-drain output that has been pulled up to the MCU's power supply through a resistor to match the signal's "H" level to the MCU's I/O voltage range.
If an LED is used to indicate the charging status, the LED is driven by a resistor through a current limit so that the power is drawn from VIN. In order to avoid driving the LED with the charging current supplied by the charging IC, a surge protection TVS is placed in VIN, and the TVS and Zener diode are used as countermeasures against surges such as ESD and high voltages that can be generated by poor-quality USB adapters even when there is no load because they are external pins.
In addition, if a load current is used while charging, or if a Li-ion/Polymer battery is supplied at 5V all the time and used as a backup, a high-function charging IC with a Current Path function that provides appropriate current from the output of both the VIN and the battery can be used. ICXC6806About Step-Down DC/DC and LDOs for MCUs
Li-ion/Polymer batteries up to CV = 4.2V or 4.35V generally require a step-down DC/DC or voltage regulator for MCUs with a maximum of about 3.8V. In IoT devices, the MCU operates in Sleep state for many periods, so the IOUT must be efficient from μA level (at Sleep) to over 100mA (at peak operation). Battery durability can be further improved by using a step-down DC/DC that is equipped with an output voltage switching (VSET) function along with ultra-low consumption for this purpose. By using the output voltage switching function, the operating voltage can be lowered even if the operating current is the same, and power consumption can be greatly reduced. Generally, MCUs require a high power supply voltage for operation due to built-in RF, analog-to-digital, and high-speed operations, etc., but they can be operated with a minimum voltage during Sleep. For example, by reducing VOUT from 3.0V to 1.8V during Sleep, the power consumption of the MCU can be reduced, dramatically improving battery durability. Step-Down DC/DCXC9276: Iq = 200nA, Output Voltage Switching FunctionXCL210: Coil-in-One Iq = 0.5μA (No Output Voltage Switching Function)For an inexpensive configuration of the solution, a voltage regulator is suitable.
Additionally in rechargeable applications, even inefficient regulators are sometimes judged to be okay to use. Regulator XC6504: Iq = 0.6μA, no output capacitance required
About Step-Down DC/DC and LDOs for RF/Sensor
RF and sensors also require step-down DC/DC and regulators due to high battery voltage.
In RF, it is important to have low ripple and low EMI. In addition, RF has steep current variations especially during transmit, so HiSAT-COT control with excellent transient response is suitable. Step-down DC/DCXC9281: PWM, world's smallest solution (3.52mm2)/Low EMIXC9282: PWM/PFM, world's smallest solution (3.52mm2)/Low EMIXCL221: Coil-in-One PWM, 1.2MHz/High-Efficiency/Low EMIXCL222: Coil-in-One PWM/PFM, 1.2 MHz/High Efficiency/Low EMIOnly when MCU is needed, set CE="H" to operate step-down DC/DC to supply voltage to RF and sensors to make them work. When stopping, not only the function of RF and sensor will be stopped, but also the operation of buck DC/DC will be stopped, which can make the battery use for a long time. To suppress the ripple during operation so that the noise frequency is constant, PWM fixed type is suitable. If there is a lightly loaded operating condition, use the PWM/PFM conversion (automatic switching of operating mode) type. If a voltage regulator is to be used, a high-speed LDO with high ripple suppression/low noise and excellent load transient response with steep changes in consumption current like RF is most suitable. In addition, in the case where noise above 100kHz is important in sensor use, there will be cases where a low consumption type with low high frequency noise is more suitable than a high speed type. Regulator XC6233: High-speed XC6215: Low-consumption
About RESET IC
Battery voltage can be monitored by using an ultra-low-consumption voltage detector.The power supply voltage of the MCU is different from that of the battery to be detected, so it is necessary to use the N-channel open-drain type to pull up the power supply voltage of the MCU through a resistor, and to pass the signal to the MCU.If you want to reduce the current consumed by the pull-up resistor after detection, it will be necessary to monitor the current consumption of the battery after detection, and to pass the signal to the MCU. pull-up resistor consumption current, separate the monitoring (VSEN) pin from the power supply (VIN) pin and use a CMOS output type. A CMOS output type can be used by obtaining power from the MCU's supply voltage. Voltage Detector XC6136 N type: Iq ~ 100nA (N type : N-Channel Open Drain Output)XC6135 C type: Iq ~ 100nA, Sensing Pin Separation Type (C type : CMOS Output)About the Push Button Restart Controller
About the Push Button Restart Controller, which is attached as a countermeasure against a crash. Button reboot controller.
Push Button Restart Controller XC6190Li-ion/Polymer IoT devices generally do not have removable batteries, so they require a function to reset and restart the device in case of a device abnormality such as a crash. In this example, there are two buttons for MCU control, and the Push Button reboot controller is used with it***. When the machine is dead, press the two switches continuously at the same time, and after the specified time has passed, the RSTB drops to "L", which resets the MCU. the RSTB is an N-channel open-drain output, so it will be pulled up to the power supply voltage of the MCU. In this case, the RESETB signal is sent to the MCU, and there are other methods such as controlling the CE of a step-down DC/DC that drives the MCU power supply, and forcing a restart by turning off the DC/DC with a long press of RESET. As mentioned above, high-performance IoT devices with low noise and long life required for simple and industrial devices can be realized by configuring ICs with the most suitable functions.
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