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Posted on 01 August 2019

IC Tailored for Battery Charging

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 Single Li-Ion/Polymer cells are in focus

Single cell Li-Ion/Polymer batteries are the preferred voltage source for various portable equipments such as PDAs, cellular phones and digital still cameras (DSCs) because they offer the highest energy density, the lowest self discharge rate and the thinnest profile.

By Merisio Massiniliano, STMicroelectronics

 

The biggest challenges in defining an IC for Li-Ion battery charging are the choice of the best charging approach and the ability to provide monitoring and control function to the host system in order to optimize the charging process and protect the battery from harmful conditions.

Considering a single Li-Ion/Polymer cell,the main recharge requirements are:

1 high accuracy voltage loop, (±1% or better) to regulate the fully charged battery
2 voltage (the voltage termination depends on the anode material)
3 medium accuracy current loop (±5%)
4 medium charge rate (approximately 1C, where C is the capacity of the battery expressed in A)

Additional useful features for the battery charger IC are:

1 the ability to pre-charge deeply discharged batteries
2 the ability to terminate the charge process based on a minimum current level or a safety timer
3 the management of a battery temperature sensor in order to stop the charge process if these parameters are out of a specified safety window

Until the battery voltage is lower than a specified threshold (and so it can be considered deeply discharged) it is charged with a light current. Then, the charge current abruptly increases to the fast-charge level (usually close to 1C, as already mentioned). Once the battery voltage is close to the regulated output voltage, the voltage regulation takes place and the charging current decreases. Finally, when the charging current goes lower than a set threshold Iend (usually approximately 10% of the fast charge current), or when a set timer expires, the battery charging process is terminated.

Recharge profile for single cell Li-Ion-Polymer batteries

Recharge Techniques for single cell Li-Ion/Polymer batteries

There are two main recharge control techniques for single Li-Ion/Polymer cells:

1 linear approach
2 pulse approach

Figures 2 and 3 describe the two different approaches. When operating in Linear mode, the device works in a similar way to a linear regulator with a constant current limit protection. During the Pre- Charge and Fast- Charge phases, the battery charger regulates the charge current to the set value. So, in these phases it works like a linear regulator in constant current limit protection. Its output voltage drops to the battery voltage.Once the battery voltage is close to the regulated output voltage VO, the voltage regulation phase takes place and the charging current is reduced. Finally, when the charging current goes below a set threshold or when a set timer expires, the charge process is terminated. The graph shows the current and voltage profiles during the different phases, as well as the power dissipated inside the charger.

Linear recharge approach for single cell Li-Ion-Polymer batteries

As can be seen, the worst case in power dissipation occurs when the device starts the Fast-Charge phase. The battery voltage is at its minimum (which means maximum voltage drop across the charger) and the charge current is at its maximum. When operating in Pulse mode, the device works in a similar way to a controlled switch. When the battery voltage is low, the switch is completely closed and directly connects the power source to the battery. Once the battery voltage is close to the regulated output voltage VO, the charger starts turning the switch ON and OFF, thereby charging the battery by current pulses. The voltage control loop gradually reduces the duty cycle of the pulses so that the average charge current is reduced as well. The charge process terminates when the duty cycle of the pulses goes below a set value or a set timer expires. The period of the pulses is much shorter than the chemical time constants of the battery, and so the low pass filter is provided by the battery itself. The graph shows the current and voltage profiles during the different phases, as well as the power dissipated inside the charger.

Pulse recharge approach for Li-Ion-Polymer batteries

The great advantages of the Pulse approach are the very low power dissipation inside the charger and its simplicity. In fact, the power dissipation actually occurs only when the internal switch is closed, and it is only due to its Rdson conduction losses. The drawback is that the charger does not control the instantaneous current, and so the upstream adapter must behave like a constant current source when the switch is closed. Thereby, the upstream adapter must be matched to the battery characteristics and cannot be a cheap general purpose part. This is sometimes an unacceptable limit. For example, this kind of approach cannot be used to recharge batteries from a USB bus since it is not possible to rely on its current limit.

A new device for single cell Li- Ion/Polymer battery chargers

The L6924D is a new device dedicated to battery chargers for single Li-Ion/Polymer cells and it is designed with the advanced BCD6 (BiCMOS-DMOS version 6) fabrication. Figure 4 shows the simplified block diagram of the part. The device consists of a fully integrated solution, including the power pass transistor with reverse blocking structure and sense element. It is dedicated to linear battery chargers but can also be used in a different recharge approach (see the following paragraph). The device also includes a closed loop thermal control to avoid overheating.

L6924D block diagram

Finally, it offers the possibility to adjust many parameters such as:

1 pre-charge current threshold
2 fast-charge current threshold
3 end-of-charge current threshold
4 pre-charge voltage threshold, as well as a flexible way to terminate the charge process, to implement a thermal battery protection, and to set a charge timer. The package is a small VQFN (3x3mm).

The next paragraphs will describe in more detail:

1 a new recharge approach
2 the closed loop thermal control
3 the charge process termination

 A New Charge Approach

Even if the device is thought to be used in Linear mode, it can also manage a different approach. This is a combination of the Linear and Pulse ones (for this reason here it will be called Quasi-Pulse). When the battery voltage is low, the internal switch is completely closed and directly connects the power source to the battery (like the Pulse approach). Once the battery voltage is close to the regulated output voltage VO, the voltage regulation phase takes place. Figure 5 shows the current and voltage profiles during the different phases, as well as the power dissipated inside the charger. place. The charger takes the control of the current, and the charging current is reduced (as in a Linear approach).

Quasi-Pulse recharge approach for Li-Ion-Polymer batteries

As it can be seen, the worst case in power dissipation occurs when the device starts the Constant Voltage phase that is, when the charger starts behaving like a linear regulator. However, when this happens, the battery voltage is close to its maximum, and so the voltage drop is much lower than at the beginning of the charge process.

To make the device operate in this mode, it is sufficient to set the charging current higher than the current limit of the upstream adapter. However, neglecting the voltage drop across the charger, its input voltage is equal to the battery one and so a very low operating input voltage (down to 2.5V) is required. The main advantage of this approach is that it has the same simplicity of the Linear approach with lower power dissipation. The drawback is that like the pulse charger, during the Fast-Charge phase it does not control the current, and so it must rely on the current control of the upstream adapter.

Performance Results

One of the most significant performance results in a linear battery charger is the thermal behaviour. As already mentioned, the device can manage a new recharge approach thanks to its very low minimum input voltage. Figure 6 shows the thermal benefit of this new approach (Quasi-Pulse). The picture shows the maximum device temperature when a battery is charged with a fast charge current of 500mA and an input voltage (output voltage of the upstream adapter) of 5V. As explained, in Quasi-Pulse mode, the worst case from a thermal stand point is at the beginning of the constant voltage phase but even in this case, the junction temperature is much lower than in Linear charge approach.

Thermal performance. VIN=5V, ICHG=500mA, TAMB=25C°

 

 

 

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