IoT Devices (Battery operation, Low power consumption)

• Optical Transceiver
• IoT Devices
  (Battery operation, Low power consumption)
• Industrial Sensors
  (12V / 24V power supply)
• Small Li batteries rechargeable by LDO
  (nominal 2V to 3V)

IoT modules are increasingly used in industrial equipment, medical care and home automation. Normally in a small form factor, IoT modules monitor their surroundings and communicate the information over the internet. An IoT module will typically be battery powered and include various sensors, an MCU and a Radio for wireless communication. The use of small batteries is common and the efficiency of the power supply is often very important for the designer.

* See "Industrial Sensors" for solutions for IoT devices that operate on 12V or 24V power supplies.

IoT Device (Primary battery)

Many IoT devices use primary batteries which are non-rechargeable. The expected lifetime can still be long, so the power management solution chosen must have ultra-low power consumption to ensure efficient use of the battery to extend the operational lifetime.

Torex offers may ultra-low power IC and in the example solution circuit we demonstrate a function that cuts the power consumption by disconnecting the battery during transportation and when the IoT module is not in use.

Solution Summary

Step-up IC

The block diagram (a) shows a typical application where the microprocessor (MCU) is directly connected to the battery. This architecture is often used for simple IoT devices, wearables and medical products.
In recent years, the operating voltage range for MCU has also become wider and 3.3 V ~ 1.8 V or lower is now common place. As a result may MCU can now be connected directly to the battery without needing additional power supply IC.
However, Radio IC and Sensors often still require a fixed 3.3V supply and even if the operating voltage is wider, the supply Voltage needs to be carefully controlled and often the devices work better at higher Voltages, therefore a step-up DC/DC is sometimes necessary to boost the battery Voltage.

Normally, Radio IC and Sensors are not operated continuously and the application firmware will be used to turn them ON and OFF as needed. For example a Radio IC may only need to transmit and receive one time per day, possibly only for a few seconds, so it is more efficient to turn off the IC for the rest of the time to prolong battery life.

During standby or sleep mode, when the Radio IC and Sensors are OFF and disabled, the operation of the power mangement IC used to supply them will also be turned OFF to conserve energy. This can be controlled easily using the devices CE or EN pin.

Fixed frequency PWM mode ensures maximum efficiency at higher output loads and offers the lowest output ripple with easier noise management.
Alternatively PFM/PWM automatic switching can be used for improved efficiency during light loads. An inductor built-in type is also available, with better EMI suppression and smaller PCB area.

Step-up DC/DCs

XCL102: PWM, inductor built-in type

XCL103: PWM/PFM, inductor built-in type

XC9141: PWM, external coil

XC9142: PWM/PFM, external coil

LDO

Low Drop Out (LDO) Voltage Regulators are often used to clean up a supply rail immediately after a DC/DC. The aim is to reduce noise and interference in the power supplies to the Radio and Sensors.
For these Regulators, High Speed operation, with high levels of Power Supply Ripple Rejection (PSRR) is important together with Low Output Noise and fast transient response performance. With Torex 'Green Operation' (GO) the designer can combine these attributes with ultra-low quiescent current, because GO LDO will adapt their performance to the required output load.
In some cases, the noise performance at higher frequencies can be important and in these instances a simple low speed, low quiescent current LDO may be a better choice.

Voltage regulators

XC6233: High speed

XC6215: Low power

RESET IC

A RESET IC or Voltage Supervisor, is often used to monitor the battery Voltage, if the Voltage drops below a predetermined level the RESET IC generates a signal for the MCU. These devices are always operating, so ultra-low quiescent current is an important consideration.
If the MCU Voltage is the same as the monitored Voltage, a CMOS Output type can be used. The CMOS Output does not require an external pull-up Resistor which helps to reduce unnecessary losses.
When the MCU Voltage is different to the monitored Voltage an N-ch Output type should be used. An N-ch Output type will need an external pull-up Resistor and this will result in slightly higher current consumption across the resistor during reset.

Some MCU have a built-in Under Voltage Lock Out (UVLO) or Analog to Digital Converters (ADC) which can be used to monitor the supply Voltage. However, it is often advantageous to have a separate IC to monitor critical Voltages independently of the MCU.

Voltage detector

XC6136 Type C: Iq - 100nA (Type C: CMOS output)

Solution with improved battery life / push-button load SW

Block diagram (b) illustrates how a Push-Button Load Switch IC can be used to help maximise battery life, by disconnecting the battery from the rest of the circuit.
The Schottky Barrier Diode (SBD) to the right of the SW pin in the diagram and the pull-up resistor to the VDD of the MCU are implemented so the push button can be shared with the MCU.

Push-button load switches

XC6194: 1A built-in SW

XC6193: External Pch-driven, compatible with large currents

This solution has the following significant benefits.

1. Prevents battery discharge during shipment or when the unit is stored in the warehouse before sale.

Often referred to as "Storage Mode" or "Ship Mode" the Shutdown function is ideal for devices where the batteries cannot be removed for safety reasons. The quiescent current in "Storage Mode" is essentially "0".
A device in "Storage Mode" can be enabled by pressing the push button SW.

2. Push-Button Load Switch IC can be used to turn the main power supply ON and OFF.

The ON/OFF control can be performed by push-button rather than a mechanical switch and this is often preferred for applications which need to be waterproof or water-resistant.
The MCU sends a signal to the SHDN pin of the Push-Button Load Switch IC to turn off the main system power supply rail. The power can later be restored when the user presses the push-button switch.
It's also possible to enable the Shutdown function by pressing and holding the push-button for a predetermined period of time. Different options are available, so please consult the date sheet for more details.

3. Escape from system freeze or system malfunction

When a device freezes and the system firmware locks up, the only remedy is often a fully system reset. This can be achieved by turning the power OFF and ON by disconnecting the battery or main system power using the Push-Button Load Switch IC. Normally this involves the user pressing the push-button for a predetermined period of time which will enable the SHDN function. Is a longer push button hold time is selected (say 5 or 10 seconds) this will lower the possibility of accidental resets.
The device can easily be re-started by pressing the push-button again.

Push-Button Load Switch IC also offer the following additional benefits:

• Suppression of in-rush current during start-up with in-rush current prevention function
• The Power Good (PG) pin can be used to enable the next phase of the start-up sequence once the main power rail is established.
• When the battery Voltage goes below 1.2V the UVLO will force the Shutdown fuction to disconnect the battery for the system to prevent deterioration and battery fluid leakage.
• If the V_OUT drops significantly, the Shutdown function will be activated automatically to protect against short circuits.

The performance of simple IoT devices where the MCU is directly connected to the battery can be enhanced by utilising a Push-Button Load Switch IC to prolong battery life and provide additional protection functions.

IoT Device (Li-ion Polymer)

Some IoT devices are powered by rechargeable batteries and Li-ion and Li-Polymer are popular choices. These chemistries require dedicated charging IC and in our example solution circuit we illustrate the different options available from Torex.

Solution Summary

BATTERY CHARGER IC

IoT devices powered by Li-ion or Li-Polymer batteries need a Charger IC and normally use step-down DC/DC or LDO Voltage Regulators to reduce the battery Voltage to a lower level required by the microprocessor (MCU) and peripheral IC.

Firstly, we will discuss the different types of Charger IC
When selecting a Charger IC we should consider the required Charge Voltage (CV) and Charge Current (CC). Often the CC is fixed externally using a Resistor (RISET) as shown in the block diagram. The CV is normally factory set by laser trimming.

Battery charging ICs

XC6808: CC 5mA - 40mA, CV 4.2V, 4.35V or 4.4V

XC6803: CC 40mA - 280mA, CV 4.2V

XC6804: CC 200mA - 800mA, CV 4.2V

With Li-ion or Li-Polymer batteries a PCM (battery Protection Circuit Module) is almost always required. These PCM can be intergated into the battery pack or placed on the PCB. The battery temperature can be monitored using a Thermistor (NTC) placed close to the battery on the PCB or inside the battery pack.
For smaller batteries, temperature monitoring is not always required and The Charge Status Output (CSO) function can be used to monitor the charge condition.
The CSO pin has an N-ch Open Drain Output with external pull-up resistor. The output signal will be pulled 'HIGH' to march the level of MCU I/O Voltage.
An LED can also be used to display the charge status and it is advisable to drive the LED through a current limiting resistor that is connected to V_IN, to ensure the LED is not driven by the charge current supplied by the Charger IC.

Although Charger IC do include ESD diodes internally, care should be taken to protect the V_IN pin because there are many brands of USB adaptors on the market and some are better than others. A poor quality adaptor can generate high voltage spikes under no-load conditions and therefore an external TVS Diode or Zenner should be implemented at the input as a countermeasure.

When the battery is used as part of a back-up circuit, a Charger IC with Current Path function may be desirable so that power can be supplied to the system whilst he battery is being charged.

Battery charging ICs with Current Path and Shutdown functions

XC6806: CC 10mA - 385mA, CV 3.5V - 4.45V

Step-down DC/DC and LDO Voltage Regulators for MCU

The CV for Li-ion or Li-Polymer batteries is normally around 4.2V and the maximum input Voltage for the MCU is typically 3.8V or less, so a step-down DC/DC or Voltage Regulator is required to lower the battery Voltage for use within the system.
Today's MCU can operate in sleep or standby modes for long periods of time so high efficiency is required over a wide range of I_OUT conditions from a few uA during sleep mode to 100mA or more when the MCU is operating fully.

Using DC/DC with ultra-low quiescent current helps ensure high levels of efficiency when the MCU is in sleep mode and further savings can be realised by lowering the DC/DC output Voltage using a VSET function when the MCU enters sleep mode. Reducing the operating Voltage, even with the same quiescent current will significantly improve the overall system efficiency.
When normal MCU operation is required, the DC/DC Voltage will need to be increased again, because many of the standard MCU functions require higher input Voltages to operate correctly. The MCU can switch between the different Voltage levels using the VSET pin.

By varying the DC/DC Voltage in this way, we maximse the efficiency and help to prolong battery life.

Step-down DC/DCs

XC9276: Iq = 200nA, output voltage selectable function

XCL210: Inductor built-in type, Iq = 0.5μA (no output voltage selectable function)

Voltage Regulators can also be used to step-down the battery Voltage in place of a DC/DC. They are simple to implement and are generally more cost effective when compared to a DC/DC solution, however the lower efficiency and power dissipation can be a concern.

Voltage regulator

XC6504: Iq = 0.6μA, output capacitor unnecessary

Step-down DC/DC and LDO Voltage Regulators for RF & Sensors

Radio IC and Sensors also typically operate a lower Voltages, so DC/DC and Voltage Regulators are often used for their power supply.
For Radio IC the power solution must normally deliver a clean Voltage with low noise and low EMI. Stability of the Voltage is also important, especially when the output current can change quickly during transmit and receive sequences. Therefore a power supply with fast transient respone is also desirable.

Step-down DC/DCs

XC9281: PWM, world's smallest solution (3.52 mm²) / low EMI

XC9282: PWM/PFM, world's smallest solution (3.52 mm²) / low EMI

XCL221: Inductor built-in type PWM, 1.2MHz / high efficiency / low EMI

XCL222: Inductor built-in type PWM/PFM, 1.2MHz / high efficiency / low EMI

The MCU will normally control the operation of the Radio IC and Sensors, turning them ON and OFF as required. This is normally achieved by controlling the CE pin of the DC/DC or Voltage Regulator powering the Radio IC or Sensor.
By controlling the operation via the DC/DC or Voltage Regulator, the user not only saves the power used for the Radio and Sensors, but we also turn off the power IC to concerve energy and prolong battery life.

A DC/DC with fixed frequency PWM mode ensures maximum efficiency at higher output loads and offers the lowest output ripple with easier noise management.
Alternatively PFM/PWM automatic switching can be used for improved efficiency during light loads. DC/DC with built-in inductor are also available, offering better EMI suppression and smaller PCB area.

As a general rule, if Voltage Regulator is used, a high speed LDO Regulator, with high Power Supply Ripple Rejection (PSRR) and fast load transient response should be chosen to ensure a clean supply.
However, if the output noise at higher frequencies (>100kHz) is an important consideration, then a low power LDO with lower IQ may be more suitable because their output noise can be surprisingly low at high frequencies.

Voltage regulators

XC6233: High speed

XC6215: Low power

RESET IC

Also know as Voltage Supervisors or RESET IC, these IC are used to monitor a voltage rail or battery level.
If the Voltage being monitored is at a different level to the MCU's supply Voltage a Voltage Detector with N-ch open drain output and an external pull-up resistor can be used. If the Voltage levels are the same then Voltage Detector with CMOS output can be used without any resistor.
To avoid the quiescent current associated with the N-ch output's pull-up resistor, an alternative method is to use a Voltage Detector with a VSEN pin separated from V_IN. If the V_IN level is the same as the MCU a Voltage Detector with CMOS Output can be used and the Voltage being monitored is connected to VSEN.

Voltage detectors

XC6136 Type N: Iq ~ 100nA (Type N: Nch open drain output)

XC6135 Type C: Iq ~ 100nA, sense pin isolation type (Type C: CMOS output)

Push-button reboot controller

These IC are added to a circuit as a countermeasure against system freezes (when the system ceases to respond to inputs, due to an unforeseen software or hardware error)

Push-button reboot controller

XC6190

Modern Li-ion and Li-Polymer Batteries are not normally accessible to the user, so they cannot easily be disconnected from the system to force a reset. As a result, reboot controller IC are now commonly used to perform this reset function.
In the example circuit we show two external push-button switches which are used to control the MCU in conjunction with the Push-Button Reboot Controller IC. 
After a system freeze, the user can initiate a full reset by pushing and holding down both push button switches for a predetermined period of time. This action makes a RSTB "L" signal which is used to instruct the MCU to initiate a full system reset.
In our circuit, RSTB is connected directly to the MCU, but the signal could also be used to drive the CE pin of the main system power supply IC, which is another method of resetting the system (turning the power off and on).

As described above, placement of ICs with the most appropriate functions will make it possible to construct high-performance but simple IoT devices with low noise and long life required for industrial equipment.

• Optical Transceiver AOC（Active Optical Cable）
• IoT Devices (Battery operation, Low power consumption)
• Industrial Sensors (12V / 24V power supply)
• Small Li batteries rechargeable by LDO (nominal 2V to 3V)