Robust System Design for Linear Chargers

As portable devices are placed in increasingly harsh environments, portable end users will continue to face robustness and stability challenges. The main difference between some products and others is the stability, reliability and robustness of the device in various harsh environments. The bq2406X product family has unique input overvoltage protection, thermal regulation and DTC functions to achieve a robust system design.

Modern portable devices are quite popular, and more and more A and portable media players have begun to use lithium-ion (Li-Ion) batteries. The high demand for these rechargeable batteries has prompted many third-party manufacturers to continue to develop accessories such as charging cradles and charging adapters. Although these charging adapters are inexpensive, their charging characteristics and protection functions are often inferior to those of the original OEM adapters and reliability. If the following characteristics are kept in mind during the design process at the charger level, a robust system that works for all external AC adapters can be implemented:

1. Protection against transient voltage peaks;

2. Unregulated adapter;

3. Example of a thermal management and fault protection product family showing a range of linear chargers designed to meet the above requirements.

A wide variety of AC adapters

One of the main issues in today's portable system design is to accommodate the need for a wide range of input power sources, such as AC adapters, USB cables, or DC outputs from vehicles/no-load. The introduction of adapters to the market at low prices, replacing expensive original equipment manufacturer (OEM) adapters, has promoted the development of many manufacturers and made rechargeable portable devices a great success. The most popular power sources, such as AC adapters, can generally be divided into two types based on their characteristics: regulated adapters (original accessories) and unregulated adapters (ordinary accessory adapters).

The output voltage of a regulated adapter provides very good line and load regulation through internal circuitry. An unregulated adapter provides an output voltage that depends on the load. Line and load regulation is not as strong, and the adapter behaves differently in overcurrent conditions. Regulated adapters typically have a steeper transition region when entering the overcurrent region.

Input transients and overvoltage conditions

Today, portable devices powered by these adapters must be designed with protection features to minimize the risk of overvoltage damaging the end device. Overvoltage can be of two types: DC overvoltage and transient overvoltage.

Typically, DC overvoltages are caused by incorrect output voltages on plugged-in accessories or nonstandard adapters. On the other hand, transient overvoltages occur when an adapter is hot-plugged into an end device. Transient overvoltages can easily reach 2 times the normal adapter output voltage, as shown in Figure 1.

Figure 1: Multiple features combine to create a robust system design for linear chargers.

If designed properly, the charger stage can be used to isolate the external power source from the battery and system, as shown in Figure 2. In this topology, the positive terminal of the battery pack and the system are connected to the output of the charger. The charger power stage effectively isolates the external adapter from the system power bus.

Implementing the topology shown in Figure 2 results in a robust design where the charger stage incorporates input overvoltage protection (OVP) to monitor the input voltage and shut down the charger stage when a DC overvoltage is detected. When the charger stage is shut down, the system's power bus is completely isolated from the adapter's output. For a 5V regulated adapter, the input overvoltage protection threshold is typically set at 6.5V. The bq2406X family offers 6.5V and 10.5V options for both regulated and unregulated adapters.

Figure 2: The charger stage isolates the external power source from the battery and system.

To isolate and protect the system and battery from external power supply transient overvoltage damage, a charger stage with a wide input voltage range of about 2 times the rated voltage of a typical adapter can be used.

It is important to note that in the topologies discussed above, a latch-up condition may occur if the minimum system current (e.g., in standby mode) is above the termination current threshold. If the system current is above the termination current threshold, the termination current cannot be detected. The safety timer will be activated and the charger stage will power down before the battery is fully charged. To address this possible issue, the bq2406X family provides an option to turn off the safety timer and charge termination when the device is in high power mode and the battery charger is in on mode.

Thermal Management and Fault Protection

The high input voltage differential to the system power bus in a linear charger can cause the die temperature to rise, even exceeding the maximum junction temperature and causing thermal damage. To avoid this problem, such designs must consider using a robust thermal management solution that includes thermal shutdown and thermal regulation.

Typically, all integrated charger ICs must have an internal thermal shutdown function. Once the internal junction temperature of the IC exceeds the maximum junction temperature value, the thermal shutdown function will be triggered to ensure that thermal damage does not occur during operation. In a typical application, the thermal shutdown function will be activated when the charging current is about 1A and the charger input voltage is 2~3V higher than the battery voltage. In the activated state, the thermal shutdown circuit will turn off the charger power stage to avoid thermal damage. The usual thermal shutdown circuit has a hysteresis effect in the design. When the IC die temperature drops, the power stage is turned back on. The die temperature will continue to rise until the thermal shutdown circuit is activated again. Its heat dissipation can be maintained for up to several seconds, depending on the PCB layout. This operating mode is generally called "flashing" mode when indicated by the charging status LED.

To solve this type of heat dissipation problem, a heat dissipation loop can be added to reduce the charge current and ensure that the IC junction temperature is below the thermal shutdown threshold. The operation of adding a heat dissipation loop to the bq2406X series linear charger is shown in Figure 2. The heat dissipation loop can effectively reduce the charge current and reduce the power dissipation of the charger stage power MOSFET when activated.

It is important to note that a linear charger with a thermal loop can drop the charge current to very low values ​​when the input voltage is too high. In such cases, false termination can detect if the charge current drops below the termination threshold. To avoid this problem, the bq2406X turns off the termination function when the thermal loop is activated.

Dynamic timer control

The charging safety counter is used to detect fault conditions. If the charging cycle duration exceeds the total time expected under normal conditions and the charging current is equal to the rated fast charging current, a fault is determined. When the thermal loop is activated, the charging current is reduced. If the thermal loop is activated for a longer period of time, then the fault state of the fault safety counter can be observed. To avoid the occurrence of error conditions, the bq2406X charger IC activates the dynamic timer control (DTC), which is a built-in circuit that reduces the clock efficiency of the safety counter by programming the time value of the timing termination output. The DTC circuit will be turned on at the same time when the thermal loop is activated.

Figure 3: Operation of the thermal loop during input voltage transients.

Thermal regulation along with DTC circuitry provides a robust thermal management and fault protection method to protect the charger stage and system from thermal failures caused by transients or other overvoltage conditions.

Conclusion

The portable devices discussed here are in a market where there is intense competition, innovation, and differentiation among manufacturers. As portable devices are placed in increasingly harsh environments (e.g., leaving the phone in a car to charge on a hot summer day, or plugging in cheap accessories or the wrong adapter), end users will continue to face challenges with the robustness and stability of portable devices. The main difference between one type of product and the others is the stability, reliability, and robustness of the device when these undesirable conditions occur. The unique input overvoltage protection, thermal regulation, and DTC features of the bq2406X product family create a robust system design to address these issues.

It is the job of system design engineers to take such factors into account to ensure that their products are smarter and more robust, so that their products stand out from the crowd rather than being "indistinguishable from the crowd."