Principle and Application of Dynamic Power Path Management (DPPM) Battery Charger

Dynamic Power Path Management (DPPM) Battery Charger

In the most common battery charging and system powering configuration, the system load is connected directly to the output of the battery charger. While this architecture is easy to use and low cost, it can cause abnormal charging termination and safety timer false alarms due to ineffective control of the battery charging current.

The bq2403x series DPPM battery chargers feature a power sharing function that can charge the battery while powering the system. This avoids the problems of charge termination and safety timer, thereby minimizing the AC adapter power rating and improving system stability. This feature also allows the system to operate normally while charging an over-discharged battery.

The simplified block diagram of the power path management battery charger is shown in Figure 1. When the AC adapter is connected to the power supply, MOSFET Q1 pre-regulates the system bus voltage VOUT, which is higher than the maximum battery regulation voltage VBAT. This establishes a direct path between the adapter input and the system. MOSFET Q2 is dedicated to battery charging, so the battery and system do not interfere with each other. When the USB is turned on and selected, MOSFET Q3 is fully turned on, and the Q3 output provides an output voltage that is almost equal to the USB output, and MOSFET Q2 controls the battery charging.

Figure 1 Simplified diagram of the power path management battery charger

DPPM can dynamically monitor 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 will be reduced until the output voltage stops dropping. Only when the DPPM control is in a steady-state condition as much as possible can the system obtain the required current and use the remaining current to charge the battery. For this reason, the adapter is designed based on the system average power rather than the system maximum peak power. This allows designers to use adapters with smaller power ratings and lower costs.

The typical DPPM application circuit is shown in Figure 2. When the total current of the system and the 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 also begins to decrease. When the system bus voltage drops to the predetermined threshold set by the DPPM pin, the charging current is reduced to prevent the system from crashing due to overload of the AC adapter. If the system bus voltage cannot be maintained when the charging current drops to 0A, the battery will be temporarily discharged and power will be supplied to the system to prevent the system from crashing. This is the "battery supplement mode", and Figure 3 shows the operation of this mode along with the DPPM experimental waveform.

Figure 2 DPPM battery charger

The DPPM voltage threshold VDPPM is set by resistor R3 and is usually lower than the regulated voltage of the OUT pin to ensure safe operation of the system. R3 can be calculated as follows:

The function of R1 is to set the fast charge current, which can be calculated as follows:

R2 is used to set the safety timer value. The charging temperature range of lithium-ion batteries is usually required to be between 0℃ and 45℃. RT1 and RT2 are programmed and can be used in other temperature ranges.

The battery charger can select AC or USB power as the main power supply through the PSEL pin. If the USB port is selected, the maximum current can be selected through ISET2.

Figure 3 DPPM experimental waveform

The device's three power MOSFETs and a power controller are integrated into a 3.5x4.5mm thermally enhanced QFN package. The thermal regulation loop reduces the charge current to prevent the silicon die temperature from exceeding 125°C. Whether the active thermal regulation circuit or active DPPM causes the charge current to be reduced, the safety timer time will automatically be extended to prevent unexpected safety timer false alarms. The charge termination function can be disabled when the DPPM or thermal regulation loop is active. This approach prevents abnormal charge termination from occurring.

Conclusion

When the system bus voltage drops to a preset threshold due to insufficient input current, the DPPM reduces the battery charge current while continuing to power the system load. DPPM also completely eliminates battery-system interference issues such as abnormal charge termination and safety timer false alarms. DPPM battery chargers are ideal for applications that need to charge the battery and power the system at the same time.