Solutions for Lithium-Titanate Oxide Batteries, rechargeable with an LDO

For IoT, Wearable and as Backup for Industrial Applications

There are many different types of batteries and they often need specialised charging IC and dedicated protection devices. Below we discuss the new generation of re-chargeable batteries which can be charged simply, with inexpensive Linear LDO Voltage Regulators. Before moving to the main topic, let us first review and compare some of the different types of batteries.

In the tables below (Fig.1), we show some of the widely used battery chemistries that are readily available in the market. For each we show the operating voltage and provide a summary of the strengths and weaknesses associated with each type of cell. The table is intended as a quick overview and we acknowledge that there are other chemistries available and that the specifications of each battery will vary from manufacturer to manufacturer.

Primary Batteries

Battery Name	Type (Positive electrode)	Nominal Voltage	Voltage Range	Features
Lithium battery	Manganese dioxide (Li-MnO2)	3.0 V	1.8 V ~ 3.2 V	CR battery. Lower impedance than Li-SOCl2.
Lithium battery	Thionyl chloride (Li-SOCl2)	3.6 V	2.4 V ~ 3.6 V	ER battery. Large capacity for incorporation into smart meters, etc. Beware of high impedance.
Alkaline battery		1.5 V	0.9 V ~ 1.6 V	High capacity, high current output dry cell.
Zinc-carbon battery		1.5 V	0.9 V ~ 1.6 V	Dry cell suitable for long-term use with low output current

Rechargeable Batteries

Battery Name	Type (Positive electrode)	Nominal Voltage	Voltage Range	Features
Li-ion battery (incl. Li-polymer)	Lithium Cobalt Oxide (LiCoO2) Lithium Manganese Oxide (LiMn2O4)	3.7 V 3.8 V	2.8 V ~ 4.2 V, 4.35 V, 4.4 V	General lithium rechargeable battery. Solid-state type is ready. Large capacity, low impedance for various applications. Protection circuit (PCM) is required for safety. Manganese type is used for automobiles.
	Lithium iron phosphate (LiFePO4)	3.2 V	2.8 V ~ 3.6 V	Lower capacity than above, but safe and long life.
	[Negative node] Lithium-titanate (Li4Ti5O12)	2.3 V	1.6 V ~ 2.6 V	Rechargeable by LDO. Semi-solid state types are ready. Safe, long life, compatible for constant voltage charging and fast charging, good for IoT etc.
Ni-MH battery	Nickel oxide hydroxide (NiOOH)	1.2 V	1.0 V ~ 1.3 V	Higher current, larger capacity and smaller memory effect than Ni-Cd
Ni-Cd battery	Nickel oxide hydroxide (NiOOH)	1.2 V	1.0 V ~ 1.3 V	Be careful to the memory effect.

(Fig.1: Typical primary and rechargeable batteries)

Typically, conventional Li-ion batteries have a nominal voltage of 3.7 V and they require a dedicated CC / CV charging IC plus a dedicated external protection circuit. They often have a narrow operating and charging temperature range, making them difficult to use in some industrial equipment.

New Low Voltage Li-ion Batteries

A new generation of Li-ion rechargeable batteries with a nominal voltage between 2 V and 3 V are now emerging and they offer many benefits over the traditional Li-ion products. Some of these new batteries can be charged with a constant voltage between 2.5 V to 3.0 V and these offer exciting opportunities for the designer as outlined below:

Benefits include:

	Ready for constant voltage charging by LDO. No need for a dedicated expensive CV / CC charging IC.
	Resistant to over-discharge and can be used with simple low voltage detection
	Because it is a battery, it can maintain a constant voltage around 2.2 V to 2.3 V for a long time. Energy can be used more easily and efficiently than a Supercap, which drops the voltage linearly.
	There are also products that can handle high temperatures such as 70 °C and 105 °C.
	Reflow / Hot laminating compatible products are also available.
	Semi-solid-state battery types and Coin battery-shaped types that support reflow soldering are readily available.

Simple Charging Solution

In Fig.2 below we illustrate a typical circuit for charging a small Li battery and offer an explanation of the operation during different states.

(Fig.2: Typical circuit for charging rechargeable batteries)

OPERATION EXPLANATION

When charging the battery:

The battery is charged by a constant voltage provided by the XC6240/42/15 LDO Voltage Regulator.
The battery voltage rises to the LDO voltage regulator output voltage in a short period of time after the start of charging, and then the battery is gradually charged.
It is not necessary to detect full charge, and it is generally not necessary to turn off the voltage regulator after full charge.

While using the battery:

When the VIN is cut, the CE of the XC6240/42/15 becomes ‘Low’, the LDO turns off, and the leakage current from the Li battery to the LDO becomes very small, 0.24 µA.
Reverse current from the battery to VIN is prevented by the SBD.

Battery voltage detection during discharge:

This battery is generally resistant to discharge, and even in applications where over-discharge is repeated, it is enough to stop the stage after the battery with simple discharge detection.
The voltage is detected by the XC6140/36 RESET IC which has an extremely low quiescent current of only 110 nA. The circuit in the following stage is stopped by the RESETB output.

Selecting the correct Regulator and Detector

The operation of the Voltage Regulator circuit is shown in detail below:

(Fig.3: Typical Voltage Regulator Circuit)

The XC6240/42 and XC6215 typically consume 0.8 µA when operating and they sink only 0.24 µA in standby (battery sink current). The XC6240/42 has a fixed V_OUT of 2.63 V ± 1.5 % specifically for NGK’s EnerCera products. Whereas the XC6215 is available with V_OUT selectable from 0.9 V to 5.0 V ±2 % in 0.1 V steps.

If size is important, both series are available in ultra-small USPN-4 (1.2 x 0.9 x Max 0.40 mm) or low profile USP-6B06 (1.5 x 1.8 x Max 0.33 mm) as well as the larger SSOT-24 (2.1 x 2.0 x Max 1.1 mm).

The XC6136 and XC6140 typically consume 110 nA when operating and the detect voltage threshold (V_DF) is user selectable, normally V_DF is between 1.6 V and 2.2 V for these batteries’ types.

With the XC6140 the Detect Release Voltage (V_DR) is fixed at 2.475 V and this is the optimum value for this type of cell (see notes in Fig.2). The V_DR for the XC6136 is calculated with a fixed hysteresis of 5 % (V_DR=V_DF+5%) so it is dependent on V_DF. Both series are available in the ultra-small USPQ-4B05 package (1.0 x 1.0 x Max 0.33 mm).

The Operating Temperature Range for the XC6240 is -40 °C ~ 85 °C whereas the XC6240 is -40 °C ~ 105 °C.

Application example: Back-up Circuit

In the circuit example below, we illustrate a typical use case for these new batteries.

Many industrial applications need a back-up circuit to provide an emergency supply in the event of a main power failure. Typically, a non-rechargeable coin cell or Supercap is be used today, but now these can easily be replaced by a small rechargeable Li cell as shown in Fig.4.

(Fig.4 Back up circuit for 5V to 3.3V or unstable 5V to 5V cases)

OPERATION EXPLANATION

For VIN = 5 V to VOUT = 5 V

XC9142 C or F type keeps VOUT 5 V.
Backup capacitor or battery is always charged by the XC6240/42/15 LDO.
When VIN is cut, the lower side Step-up DC/DC provides voltage.
XC9142 C or F type is ready for “OR” connection of VOUT.
XC9140/XCL101 can be used for the lower side step-up as well.

For VIN = 5 V to VOUT = 3.3 V

XC6227 is suitable for the upper side LDO to make 3.3V because it includes reverse current prevention, so an external blocking diode is not required.

Notes:

Step-up DC/DC operates every time and VOUT2 should be lower than VOUT1 by 0.1 V ~ 0.2 V
A Supercap is usually 2.5 V max, so the engineer needs to be wary of over voltage.
If Li rechargeable batteries by LDO are used, they can maintain 2.2 V ~ 2.3 V for a longer period and all the stored energy can be used (unlike with a Supercap).

The ease of charging combined with superior strength against over discharge and the extended operating temperature range all mean these batteries are a good choice for use as a back-up in industrial equipment.

Application example: IoT module, Battery module and Smartcard

Other common applications include small IoT modules, battery power systems and next generation smartcards. Often space is constrained in these applications and our charging solution fits perfectly due to the small size and low profile of our USP packaging. A solution can easily be implemented with a height ≦ 0.35 mm, making it ideal for smartcard applications.

In the circuit example below, we illustrate a typical use case:

(Fig.5 Circuit for IoT modules, battery power systems and smartcards)

Notes:

While the voltage of Supercap drops linearly when discharged, this Li battery keeps the output voltage constant at about 2.3 V, making effective use of energy easy.
Li battery rechargeable by LDO and Li capacitor can be charged with a constant voltage of 2.5 to 3.0V by LDO and can be used easily, making them ideal for backup applications.
Higher operating temperatures type than Li primary batteries such as CR2032, which are ~ 70 ^OC and ~ 105 ^OC, have also been developed and are suitable for industrial equipment applications. Also supports reflow and hot lamination.
XC6140 is ready as a dedicated IC that matches the discharge detection circuit of Li battery rechargeable by LDO and Li capacitor.

Manufacturers of batteries rechargable by LDO

There are a growing number of manufacturers promoting these new batteries and you can find out more by exploring the links below:

NGK insulators Chip-type Ceramic Rechargeable Battery "EnerCera" Series

https://www.ngk-insulators.com/en/product/electron/enercera/

XC6240/42 and XC6140 are approved by NGK for use with the EnerCera series.
EnerCera Coin is suitable for reflow soldering and higher temperatures.
EnerCera Pouch is suitable for hot lamination manufacturing of smartcards.

Murata Small Li-ion secondary batteries

https://www.murata.com/en-eu/products/batteries/small/

XC6215 VOUT = 2.60 V is compatible with CT04120.
Quick charge. High-rate charging (10C) is possible.

Nichicon Small Li-ion Rechargeable Batteries

https://www.nichicon.co.jp/english/products/slb/about/

XC6215 VOUT = 2.70 V is compatible with SLB03070.

Samples and Evaluation Boards

Samples for all the Torex products mentioned in this feature are readily available, so please ask your local Torex representative for details or contact us directly via our website.

We also offer dedicated evaluation boards for testing the circuits with NGK EnerCera batteries. These evaluation boards include two portions, one for the charging function and one for the power supply. Customers can use the Power Supply Unit to test the rest of their circuit.

The Power Supply Unit has a Boost Micro DC/DC (with integrated coil) and an LDO fitted as standard. Layout for an an optional Buck Micro DC/DC (with integrated coil) is also included.

Our standard Power Supply Unit features a 3.3 V Boost Micro DC/DC and a 1.8V Low Power LDO, but the Buck Micro DC/DC is not mounted.

Semi-custom versions of the Power Supply unit are also available for customers with special Voltage requirements or if a Buck Micro DC/DC is required.

Please note Torex does not supply batteries, but these can be sourced from the battery manufacturers directly.

The Torex IC included are shown in the following table

Charging unit		Power supply unit
LDO for charge	XC6240A2638R-G	Low lq LDO	XC6215B182NR-G
RESET IC for battery voltage monitor	XC6140C20A9R-G	Step-up Micro DC/DC	XCL103D333CR-G
Wireless power receiver	XCM414B050D2-G	Step-down Micro DC/DC (Optional)	XCL206xxx3CR-G

The circuit for these evaluation boards are shown below:

Charging unit

The evaluation board is ready for both DC and ANT inputs, however if only the DC input is used the Schottky Barrier Diode on DC input side is not necessary.