The importance of lithium battery protection IC

In recent years, PDA, DSC, Cellular Phone, Camcorder, Portable Audio, Advanced Game, Assist Bicycle, Electric Scooter, Bluetooth Device... More and more products are rapidly using lithium batteries as their main power source, which are nothing more than: small size, high energy density, no memory effect, long cycle life, high voltage battery, low self-discharge rate... and other advantages. Because it is different from nickel-cadmium and nickel-metal hydride batteries, it is necessary to consider the safety of charging and discharging to ensure the prevention of characteristic degradation. However, because of this, the protection of lithium batteries against overcharge, over-discharge, over -current and short-circuit current is more important, so usually a protection circuit is designed in the battery pack to protect the lithium battery. This shows the importance of lithium battery protection IC.

Lithium-ion batteries have high energy density, making it difficult to ensure battery safety. Specifically, when overcharged, the battery temperature rises and the energy becomes excessive, causing the electrolyte to decompose and generate gas, which can cause a risk of ignition or rupture due to increased internal pressure. Conversely, when over-discharged, the electrolyte decomposes, causing degradation of battery characteristics and durability (i.e., a reduction in the number of charge cycles).

Lithium-ion battery protection circuits are designed to ensure safety during overcharge and discharge and to prevent degradation of characteristics. Lithium-ion battery protection circuits are composed of a protection IC and two Power - MOSFETs . The protection IC monitors the battery voltage; when overcharge and discharge occur, the external Power-MOSFET is used to protect the battery. The functions of the protection IC are: (1) overcharge protection, (2) overdischarge protection, and (3) overcurrent/short circuit protection. The following describes the protection actions of these three functions.

(1) Overcharging:

When a lithium battery is overcharged, the electrolyte inside the battery will be decomposed, causing the temperature to rise and gas to be generated, which will increase the pressure and may cause a risk of spontaneous combustion or explosion. The purpose of the lithium battery protection IC is to prevent overcharging from happening.

Overcharge protection IC principle:

When an external charger charges a lithium battery, charging must be terminated to prevent the internal pressure from rising due to temperature rise. At this time, the protection IC needs to detect the battery voltage. When it reaches 4.25V (assuming the battery overcharge point is 4.25V) and activates overcharge protection, the Power MOS is turned from ON to OFF, thereby stopping charging.

In addition, for overcharge detection, false operation caused by noise must also be paid attention to, so as not to judge it as overcharge protection. Therefore, a delay time setting is required, and the delay time cannot be shorter than the noise time.

In the case of over-discharge, the electrolyte decomposes, causing the battery characteristics to deteriorate and resulting in a reduction in the number of charging times. The lithium battery protection IC is used to protect the battery from over-discharge and achieve protection action.

Over discharge protection IC principle:

In order to prevent the lithium battery from over-discharging, if the lithium battery is connected to a load, when the lithium battery voltage is lower than its over-discharge voltage detection point (assuming it is set to 2.3V), the over-discharge protection will be activated, and the Power MOS will be turned from ON'OFF to cut off the discharge, achieving protection to avoid battery over-discharge and keep the battery in a low static current state (standby mode). At this time, the power consumption is 0.1uA

When the lithium battery is connected to the charger and the lithium battery voltage is higher than the over-discharge voltage, the over-discharge protection function will be released.

In addition, in order to deal with pulse discharge situations, over-discharge detection is provided with a delay time to prevent such malfunctions from occurring.

(3) Overcurrent and short-circuit current:

Because of unknown reasons (discharging or the positive and negative poles being accidentally touched by metal objects), overcurrent or short circuit current occurs. To ensure safety, stop discharging.

Current protection IC principle:

When the discharge current is too large or a short circuit occurs, the protection IC will activate the over (short circuit) current protection. At this time, the over-current detection uses the Rds(on) of the Power MOS as an inductive impedance to monitor the voltage drop. If it is higher than the set over-current detection voltage, the discharge is stopped.
The formula is:

V-(overcurrent detection voltage)=I(discharge current)*Rds(on)*2

Assuming V-=0.2V, Rds(on)=25mΩ, the protection current is I=4A

Similarly, overcurrent detection must also be equipped with a delay time to prevent false operation when there is a sudden current inflow, causing false operation of protection.

Usually after an overcurrent occurs, if the cause of the overcurrent can be removed (for example, immediately disconnecting the load), it will return to its normal state and normal charging and discharging operations can be performed again.

New functions of lithium battery protection IC:

In addition to the above-mentioned lithium battery protection IC functions, there are some new functions that deserve our attention. Take the "Ricoh" lithium battery protection IC exclusively represented by Dongrui Electronics as an example --- R5426

(1) Overcurrent protection during charging :

When an overcurrent suddenly occurs during charging (charger damage), overcurrent detection occurs during charging. At this time, Cout will change from High to Low, and Power MOS will change from ON to OFF to achieve protection.

V-(Vdet4 overcurrent detection voltage ) = I(charging current)*Rds(on)*2

Note: Vdet4 is -0.1V

(2) Shorten the test time:

Assuming that it takes 1 second to test one PCB, then 1 million PCBs will take 1 million seconds, which is very time-consuming and inefficient. Therefore, we can use the following functions to shorten the test time.

(A) When we open the DS pin of R5426, the delay time is as shown in the specification.

(B) When we connect the DS pin of R5426 to VDD, the delay time will be only 1/90.

(C) When we connect the DS pin of R5426 to Vim (min=1.2V, max=VDD-1.1V), the delay time can be ignored.

(3) Overcharge lock mode (Latch):

Usually, the protection IC will turn off the Power MOS (Cout) after a delay time during overcharge protection to achieve the purpose of protection. When the lithium battery voltage drops to the release point (Overcharge Hysteresis Voltage), it will recover. At this time, it will continue to charge, protect, and discharge. This situation is not a good situation and the safety problem cannot be effectively solved.

Lithium batteries are constantly charging and discharging, and the gate of the Power MOS will repeatedly switch between High and Low. This may cause the MOSFET to heat up and also affect the battery life. This shows the importance of Latch Mode.

If the lithium battery protection circuit has a latch mode when overcharge protection is detected, the MOSFET will not heat up and the safety will be much improved. After overcharge protection is detected, as long as the charger is connected to the battery pack, the current state and the overcharge latch mode are reached. Therefore, although the voltage of the lithium battery drops, recharging will not occur. To resolve this situation, just remove the charger and connect the load to restore the charge and discharge state.

(4) Reduce the size of protection circuit components:

The delay capacitor for overcharge and short circuit protection is built into the protection IC.

Requirements for protecting IC:

(A) High precision of overcharge protection:

When a lithium-ion battery is overcharged, the charging state must be turned off to prevent the internal pressure from rising due to temperature rise. This protection IC checks the battery voltage. When overcharging is detected, the overcharge detection Power-MOSFET is turned OFF to cut off the charging. At this time, it should be noted that the overcharge detection voltage should be highly accurate. When charging the battery, it is a problem that users are very concerned about charging the battery to a full state. At the same time, in order to take into account the safety issue, the charging state must be cut off when the allowable voltage is reached. To meet these two conditions at the same time, a very high-precision detector is required. The current accuracy is 25mV, but in the future, there will be a demand for higher accuracy.

(B) Reduce the power consumption of the protection IC to achieve over-discharge protection:

As the lithium-ion battery is charged, its voltage will gradually decrease over time, and finally fall below the standard value. At this time, it needs to be recharged. If it is not charged and continues to be used, it may not be able to be recharged (over-discharge state). In order to prevent over-discharge, the protection IC must detect the battery voltage state. Once it reaches below the over-discharge detection voltage, the Power-MOSFET on the discharge side must be turned OFF to stop the discharge. However, at this time, the battery itself still has natural discharge and the consumption current of the protection IC, so the current consumption of the protection IC needs to be reduced to the minimum.

(C) Overcurrent/short circuit protection requires low detection voltage and high accuracy:

When a short circuit is caused by unknown reasons and a large current is consumed, the discharge is stopped to ensure safety. In overcurrent detection, the Rds(on) of the Power MOS is used as the inductive impedance to monitor the voltage drop. If the voltage at this time is higher than the overcurrent detection voltage, the discharge is stopped. In order to effectively apply the Rds(on) of the Power MOS during charging and discharging currents, the impedance value must be as low as possible (currently about 20mΩ ~30mΩ). In this way, the overcurrent detection voltage can be lower.

(D) Achieved withstand voltage value:

When the battery pack is connected to the charger, high voltage will be generated instantly. Therefore, the protection IC must have the requirement of "high voltage resistance" (Ricoh's protection IC can withstand 28V)

(E) Low power consumption:

When protection is reached, the static current consumption must be small (0.1uA)

(F) Zero volt rechargeable:

Some batteries may be stored for too long or for abnormal reasons, causing the voltage to drop to 0V. Therefore, the protection IC needs to be able to charge at 0V.

Future Development of Protection IC Function

Future development will be as mentioned above, improving the accuracy of voltage detection, reducing the current consumption and packaging of protection ICs, integrating MOS, improving the malfunction prevention function, etc. At the same time, the high voltage resistance of the charger connection terminal is also a development focus.

In terms of packaging, it has gradually shifted from SOT23-6 to SON6. In the future, there will be CSP packages and even COB products to meet the current emphasis on lightness, thinness and compactness.

Not all functions of the protection IC must be used. A single protection function (such as only overcharge protection or over-discharge protection function) can be developed according to different lithium battery materials, which can greatly reduce costs and space, which is not a bad thing for us.

Of course, the goal of functional components is to make them single-crystal. For example, mobile phone manufacturers are currently moving towards integrating peripheral circuits such as protection ICs, charging circuits, and power management ICs into single chips, and forming a dual-chip chipset with logic ICs. However, it is currently difficult to reduce the open-circuit impedance of Power MOS and combine it with other ICs. Even if a single chip is made using special technology, the cost may be too high. Therefore, it will take some time to solve the problem of single-crystal protection ICs.