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[Torex] How to use LDO regulator to charge small lithium-ion batteries (1/3)

Author:Torex
Release on:2021-01-12

LDO rechargeable small lithium-ion battery solution

For the Internet of Things, wearable devices and backup of industrial applications

There are many types of batteries, and they usually require dedicated charging ICs and dedicated protection equipment.Below we discuss a new generation of rechargeable batteries that can be easily charged using cheap linear LDO regulators.Before turning to the main topic, let us first review and compare some different types of batteries.

In the table below (Figure 1), we show some of the widely used battery chemistries that are readily available on the market.For each battery, we will display its operating voltage and provide a summary of the advantages and disadvantages associated with each battery type.This table is intended as a quick overview, and we acknowledge that there are other chemicals available, and the specifications of each battery vary from manufacturer to manufacturer.

Primary battery

Battery name	Type (positive electrode)	Q	voltage range	feature
lithium battery	manganese dioxide (Li-MnO2)	3.0V	1.8V~3.2V	CR battery.The impedance is lower than Li-SOCl2.
lithium battery	Thionyl Chloride (Li-SOCl2)	3.6V	2.4 V to 3.6 V	ER battery.Large capacity incorporated into smart meters, etc. Beware of high impedance.
Alkaline battery		1.5V	0.9 V to 1.6 V	Dry battery with high capacity and high current output.
Zinc Carbon Battery		1.5V	0.9 V to 1.6 V	Dry battery is suitable for long-term use under low output current

rechargeable battery

Battery name	Type (positive electrode)	Q	voltage range	feature
Lithium Ion Battery (Including lithium polymer)	Cobalt acid lithium(LiCoO2)manganeseacidLithium (LiMn2O4)	3.7V 3.8V	2.8 V~4.2 V, 4.35 V, 4.4 V	Ordinary lithium rechargeable battery.The solid type is ready. Large capacity, low impedance, suitable for various applications. For safety, a protective circuit (PCM) is required. Manganese type is used in automobiles.
	Lithium Iron Phosphate (LiFePO4)	3.2V	2.8V~3.6V	The capacity is lower than the above-mentioned capacity, but it is safe and has a long life.
	[negative electrode] Lithium titanate (Li4Ti5O12)	2.3 volts	1.6V~2.6V	It can be charged via LDO.The semi-solid type is ready. Safe, long life, compatible with constant voltage charging and fast charging, suitable for IoT, etc.
NiMH batteries	Nickel hydroxide (NiOOH)	1.2V	1.0 V to 1.3 V	Compared with nickel-cadmium batteries, the current is higher, the capacity is larger and the storage effect is smaller
Nickel-cadmium batteries	Nickel hydroxide (NiOOH)	1.2V	1.0 V to 1.3 V	Pay attention to the memory effect.

(figure 1: Typical primary battery and rechargeable battery)

Generally, the nominal voltage of traditional lithium-ion batteries is 3.7V, and they require a dedicated CC/CV charging IC and a dedicated external protection circuit.They usually have a narrow operating and charging temperature range, making it difficult to use in certain industrial equipment.

New low voltage lithium ion battery

A new generation of lithium-ion rechargeable batteries with a nominal voltage between 2V and 3V have emerged. Compared with traditional lithium-ion products, they have many advantages.Some of these new batteries can be charged with a constant voltage of 2.5V to 3.0V, and these provide exciting opportunities for designers as described below:

Benefits include:

	readyVia LDOConstant pressureRecharge.No need for dedicated expensive CV/CC charging IC.
	Over discharge resistance, Can be used for simple low pressure detection
	Since it is a battery, it canMaintain a constant voltage of 2.2V to 2.3V for a long time. Compared with supercapacitors (linear reduction in voltage), energy can be used more easily and efficiently.
	There are some products that canWithstand high temperatures such as 70°C and 105°C.
	Also offersReflow/thermal lamination compatibleproduct.
	Support reflow solderingSemi-solid battery typewithType of button cell shapeIt's easy to get.

Simple charging solution

In Figure 2 below, we illustrate a typical circuit for charging a small Li battery and provide instructions for operation in different states.

(figure 2: Typical circuit of rechargeable battery)

Instructions

When charging the battery:

The battery is charged by a constant voltage provided by the XC6240/15 LDO regulator.
Within a short time after charging starts, the battery voltage rises to the output voltage of the LDO regulator, and then gradually charges.
There is no need to detect a full charge, and there is usually no need to turn off the regulator after full charge.

When using batteries:

After the VIN is cut off, the CE of XC6240/15 becomes "low", the LDO is turned off, and the leakage current from the lithium battery to the LDO is very small, 0.24µA.
SBD prevents reverse current from the battery to VIN.

Battery voltage detection during discharge:

This kind of battery is usually resistant to discharge, even in repeated over-discharge applications, it is sufficient to stop the battery's work through simple discharge detection.
This voltage is detected by XC6140/36 RESET IC, and its quiescent current is extremely low, only 110nA.RESETB output will stop the next level circuit.