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Home > EDA/IC Design > How to choose a single-cell battery charging IC

How to choose a single-cell battery charging IC

tempo di aggiornamento: 2022-09-19 17:47:09

In the power management ICs, battery charging ICs are a class of devices that regulate battery charging current and voltage and are widely used in various portable devices. Today, the application of battery charging ICs is becoming more and more in-depth as lithium batteries are making a big splash.


Diverse battery charging IC topologies


The voltage and current requirements of charging ICs differ for different end devices. Laptops require higher charging power levels than wireless headphones in our common portable devices. The different power levels require a variety of battery charging topologies, each of which provides trade-offs and optimizations.


Linear charging ICs adjust the charging current and charging voltage by adjusting external resistors, and there is also Flash direct charging, which directly modulates the input voltage source. These two modes share the same architecture, where the charger consists of a device that acts as a short circuit resistor, and the battery charging system must be connected to the input source to achieve a complete charge cycle. The ICs in these topologies require an input voltage higher than the battery voltage to operate properly.


Switch-mode charging ICs can adjust the duty cycle and use a low-pass inductive-capacitive (LC) filter to regulate the charging current or voltage. With this topology, variations in the input supply have little effect. By rearranging the switching elements and LC filters, this topology type of charger can complement batteries with higher or lower voltages relative to the input voltage.


As with all power supply design-related issues, linearity is certainly the most straightforward choice if charging is to be accomplished at relatively low currents, and the simplicity and low cost of the charging scheme need to be balanced. Where higher currents are needed to charge larger capacity batteries, switch mode charging ICs maximize efficiency and minimize system heat.


Single-cell switch-mode charging ICs


In single-cell battery charging ICs, buck switch-mode charging ICs are often used. Typical buck switch-mode charging ICs have an architecture where the system is powered by either the buck converter output (when the input is present) or the battery. Buck-switching mode charging ICs solve the efficiency limitation by expanding the external component area. The larger circuit area has higher efficiency (>90%) at higher charge currents and is mostly used in various handheld devices and portable power packs.


Three-stage buck switch-mode charging ICs have higher efficiency (>95%) and power density through higher switching frequency. Of course, many switch-mode charging ICs can now be set to buck, boost, or buck-boost configurations depending on the input source and battery condition. A wide enough operating voltage range increases the flexibility of switch-mode charging ICs for single-cell battery charging in various application scenarios.

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Single-cell charging, linear to direct charging


A typical linear charging IC consists of two bi-directional blocking switches that isolate the input and output terminals, with a pin between the two switches called the PMID. During normal operation in the presence of input, the first switch opens and shorts the input to the PMID. In contrast, the second switch adjusts its resistance to regulate the current and voltage at the battery output.

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Linear charging ICs are simple designs that can be very small and have very low quiescent currents. This charger can achieve high regulation accuracy (e.g., 0.5% regulation voltage accuracy) at low charging currents. Because there is no high-frequency switching circuit, EMI problems are greatly reduced compared to switching mode. However, since its efficiency depends only on the ratio of input voltage to battery voltage η=VBAT/VIN, the efficiency will be relatively low.


In charging applications below 1A, linear charging ICs are very much used, and now the linear charging IC package is getting smaller and smaller, the quiescent current is getting lower and lower, which can significantly extend the battery life and is a very popular type of charging design for small wearable devices. The whole system operates extremely reliably with the introduction of temperature monitoring and thermal regulation.


Flash direct charging greatly improves charging efficiency by keeping VIN close to VBAT in the same architecture as linear charging ICs. The direct charging IC offloads the regulator to an external adapter and connects the input directly to the charger to achieve over 95% efficiency. In this scheme, all regulation is performed by the main processor, minimizing losses inside the mobile device with a very high charging current (>4A). The direct charging IC needs an adapter that can achieve high precision regulation with a dedicated host.


Although the Flash direct charging architecture is already very efficient, another direct charging architecture, SC direct charging, which is more efficient, can reach 8A charging current.


Direct charging in this architecture does not require cable replacement as the charging current rises compared to Flash direct charging. From the market demand for single core high power charging, there is still much room for developing SC direct charging architecture charging ICs.


Summary


In various single-cell charging scenarios, linear charging ICs are popular for their tiny size and low quiescent current in small wearable devices. Switch-mode charging ICs are unique for their good thermal performance and versatile design flexibility at various power levels, not to mention the ultra-high charging efficiency of direct charging. Different charging architectures have been developed to suit various charging applications with different needs.


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