TI Power Switch Design Tips 4 - Single-chip bq2403x Power Path Manager provides battery power while powering the system

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Lithium-ion batteries are widely used in portable devices because of their high energy density in both weight and capacity. Users who use devices such as smartphones, PDAs, and MP3 players want to power the device with input power without using a battery. This requires a power architecture called "power path management" to power the device system and charge the battery in separate paths.

Dynamic Power Path Management (DPPM) Battery Charger

In the most common battery charging and system power configurations, the system load can be directly connected to the output of the battery charger. Although this architecture is not only simple and easy to use but also low in cost, invalid control of the battery charging current may cause abnormal termination of charging and false alarm of safety timer.

The bq2403x Series DPPM Battery Charger has a power sharing feature that charges the battery while powering the system. This avoids problems such as charge termination and safety timers, thereby minimizing the AC adapter's power rating and improving system stability. This feature also allows the system to work properly while charging an over-discharged battery.

The schematic diagram of the power path management battery charger is shown in Figure 1. When the AC adapter is powered up, MOSFET Q1 pre-regulates the system bus voltage, VOUT, which is higher than the maximum battery regulation value, VBAT. This establishes a direct path between the adapter input and the system. MOSFET Q2 is designed for battery charging, so the battery does not interfere with the system. When USB is turned on and selected, MOSFET Q3 is fully enabled, the Q3 output provides almost the same amount of output voltage as the USB output, and MOSFET Q2 controls battery charging.

Figure 1 Schematic diagram of the power path management battery charger

The DPPM dynamically monitors the system bus voltage. If the system bus voltage drops to a preset value due to a small input current from the adapter or USB, the battery charging current is reduced until the output voltage stops dropping. Only when the DPPM control is as steady-state as possible, the system can get the required current and charge the battery with the residual current. Because of this, the adapter is designed based on the average power of the system, not the maximum peak power of the system. This allows designers to use adapters that are less powerful and less expensive.

A typical DPPM application circuit is shown in Figure 2. When the total current of the system and battery charger exceeds the current limit of the AC adapter or USB, the capacitor connected to the system bus begins to discharge and the system bus voltage begins to decrease. When the system bus voltage drops to a predetermined threshold set by the DPPM pin, the charge current is reduced to prevent system crashes due to overload of the AC adapter. If the system bus voltage is still not maintained when the charge current drops to 0 A, the battery will temporarily discharge and power the system to prevent the system from collapsing. This is the “battery replenishment mode”, and Figure 3 shows how this mode works with the DPPM experiment waveform.

Figure 2 DPPM Battery Charger

Figure 3 DPPM experimental waveform

The device's three power MOSFETs and one power controller are integrated into a 3.5 x 4.5 mm thermally enhanced QFN package. The thermal regulation loop reduces the charge current to prevent the silicon chip temperature from exceeding 125 °C. Whether the active thermal regulation circuit or the active DPPM causes a reduction in the charging current, the safety timer time is automatically extended to prevent accidental occurrence of a safety timer false alarm. The charge termination feature can be disabled when the DPPM or thermal regulation loop is active. This method prevents the abnormal termination of charging.

in conclusion

When the system bus voltage drops to a preset threshold due to insufficient input current, the DPPM will reduce the battery charging current while continuing to power the system load. DPPM also completely eliminates the problem of battery and system interference such as abnormal charging termination and false alarm of safety timer. The DPPM battery charger is ideal for applications that need to charge both the battery and power the system.