A Compact Rapid Charger for Li-Ion Cells

Introduction

There are many off-the-shelf ICs available for charging Li-Ion batteries. In this Note we present a Li-Ion charger design based on a GreenPAK mixed-signal IC and a GreenFET load switch from Dialog Semiconductor. There are some advantages in using the GreenPAK approach. For one, we may integrate the solution with other circuit functions — related or unrelated — in the same IC. This helps in reducing the overall footprint in an application. Secondly, we can customize the solution by incorporating context-specific control mechanisms in the same GreenPAK chip; for example, in some contexts the battery need not always be charged to full capacity, and the design can be tailored to terminate suitably early so as to prolong battery life. Thirdly, the inherent design flexibility can be exploited — the power supply used for supplying the bulk charge current can be from a different domain than the supply used to power the GreenPAK chip. All of these features are especially useful in systems powered by renewable energy or harvested energy where the charging source may have varying characteristics, and there may sometimes be a tradeoff between using the source for powering a load or charging a battery.

1. Broad Design Strategy

One way to implement the design is to choose a GreenPAK chip such as the SLG46533V that has at least 3 ACMPs in it so that we can configure things to detect the voltage thresholds as well as the current threshold (by using a small resistor). This would require an external GreenFET.

Figure 1: Overall hardware schematic

2.Realization in the GreenPAK Designer

Figure 2 shows the design realized using SLG46533V. While this design uses an external PFET, all the blocks used are available in the SLG46116V which allows for easy porting to the latter.

Figure 2: SLG46533V Designer diagram for the Li-Ion charger
Figure 3: Timing diagram illustrating charge termination logic when ACMP0 = DOWN

3. Test Results and Notes

Lab tests were performed on the SLG46533V version of the design and a 3.7V 18650 battery. Just as the present design does not have any check for testing for underflow of the duty cycle, it does not have an explicit check for duty cycle overflow — i.e., a situation where ACMP0 outputs an nUP when the duty cycle is already near 100%, causing the next SET pulse to appear “just before” the RESET pulse which would result in the duty cycle attempting to increase beyond 100% and therefore dropping to nearly zero. Since the duty cycle updates start only when the Absorb state is triggered, and since this state is triggered only when ACMP0 has output a DOWN, the above situation is normally not expected to happen. However, it is possible for such a situation to be triggered by the ripple level at the Cell Monitor pin, which is why we have a lowpass filter consisting of R3 and C2 at the Cell Monitor input. Values shown were found to work satisfactorily during tests. If the problem nevertheless manifests itself, higher values of R3 and C2 can be experimented with. When increasing the value of R3 remember that the ACMPs have an input impedance of about 1MΩ and gain error of about 3% at a gain of 0.25x. Therefore, the IN- voltage must be suitably lowered for higher values of R3 and the circuit recalibrated.

4. Conclusion and Extensions

This Note presented a compact yet capable Li-Ion battery charger. There is considerable scope for experimentation with this design — for example, we may try different PWM frequencies and see what frequency leads to the best results for different Li-Ion chemistries. While this is a subject of ongoing research it is difficult to find definitive answers to this question in the trade literature. More complex designs would include temperature monitoring and the ability to vary charge current and termination threshold with battery temperature to ensure optimum charging and battery life. An even more sophisticated “parametric” charger could be designed that would include an I2C interface to allow some degree of user control over charger parameters such as the termination voltage with battery chemistry so that the charger acquires more universality and can be adapted to newer Li-Ion chemistries as they are released into the market.

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Renesas’ Family of Programmable Mixed-Signal ASICs

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