9.1 Application Information

The bq2412x battery charger supports precision Li-ion or Li-polymer charging system for single cell or two cell application. The design example below shows the design consideration for a 1-cell application.

9.2 Typical Application

TVS Zener diode is optional for devices with date codes effective March 2008

Figure 17. Standalone 1-Cell Application

9.2.2 Detailed Design Procedure

This section provides a detailed system design example for the bq24120.

• V_IN = 16V
• V_BAT = 4.2V (1-Cell)
• I_CHARGE = 1.33 A
• I_PRECHARGE = I_TERM = 133 mA
• Safety Timer = 5.0 hours
• Inductor Ripple Current = 30% of Fast Charge Current
• Initiate Charge Temperature = 0°C to 45°C

1. Determine the inductor value (L_OUT) for the specified charge current ripple:
Equation 10.

Set the output inductor to standard 10 μH. Calculate the total ripple current with using the 10 μH inductor:

Equation 11.

Calculate the maximum output current (peak current):

Equation 12.

Use standard 10 μH inductor with a saturation current higher than 1.471A. (that is, Sumida CDRH74-100)

2. Determine the output capacitor value (C_OUT) using 16 kHz as the resonant frequency:
Equation 13.

Use standard value 10 μF, 25V, X5R, ±20% ceramic capacitor (that is, Panasonic 1206 ECJ-3YB1E106M)

3. Determine the sense resistor using the following equation:
Equation 14.

In order to get better current regulation accuracy (±10%), let V_RSNS be between 100 mV and 200 mV. Use V_RSNS = 100 mV and calculate the value for the sense resistor.

Equation 15.

This value is not standard in resistors. If this happens, then choose the next larger value which in this case is 0.1Ω. Using the same equation (15) the actual V_RSNS will be 133mV. Calculate the power dissipation on the sense resistor:

Equation 16.

Select standard value 100 mΩ, 0.25W 0805, 1206 or 2010 size, high precision sensing resistor. (that is, Vishay CRCW1210-0R10F)

4. Determine ISET 1 resistor using the following equation:
Equation 17.

Select standard value 7.5 kΩ, 1/16W ±1% resistor (that is, Vishay CRCWD0603-7501-F)

5. Determine ISET 2 resistor using the following equation:
Equation 18.

Select standard value 7.5 kΩ, 1/16W ±1% resistor (that is, Vishay CRCWD0603-7501-F)

6. Determine TTC capacitor (C_TTC) for the 5.0 hours safety timer using the following equation:
Equation 19.

Select standard value 100 nF, 16V, X7R, ±10% ceramic capacitor (that is, Panasonic ECJ-1VB1C104K). Using this capacitor the actual safety timer will be 4.3 hours.

7. Determine TS resistor network for an operating temperature range from 0°C to 45°C.
Figure 18. TS Resistor Network

Assuming a 103AT NTC Thermistor on the battery pack, determine the values for RT1 and RT2 using the following equations:

Equation 20.
Equation 21.

9.2.2.1 Inductor, Capacitor, and Sense Resistor Selection Guidelines

The bqSWITCHER provides internal loop compensation. With this scheme, best stability occurs when LC resonant frequency, f_o is approximately 16 kHz (8 kHz to 32 kHz). Equation 22 can be used to calculate the value of the output inductor and capacitor. Table 3 provides a summary of typical component values for various charge rates.

Equation 22.

Table 3. Output Components Summary

CHARGE CURRENT	0.5 A	1 A	2 A
Output inductor, LOUT	22 μH	10 μH	4.7 μH
Output capacitor, COUT	4.7 μF	10 μF	22 μF (or 2 × 10 μH) ceramic
Sense resistor, R(SNS)	0.2 Ω	0.1 Ω	0.05 Ω

9.2.3 Application Curves

Figure 19. Efficiency 1-Cell

Figure 20. Switching Waveforms in Fast Charge Mode

9.3 System Examples

TVS Zener diode is optional for devices with date codes effective March 2008

Figure 21. Standalone 2-Cells Application

Zener diode not needed for bq24125.

Figure 22. Externally Programmable Application

Using bq24125 to charge a LiFePO4 battery

Zener diode not needed for bq24125.

Figure 23. 1-Cell LiFePO4 Application

The LiFePO4 battery has many unique features such as a very high thermal runaway temperature, high discharge current capability, and high charge current. These special features make it attractive in many applications such as power tools. The recommended charge voltage is 3.6 V and termination current is 50 mA. Figure 23 shows an application circuit for charging one cell LiFePO4 using bq24105. The charge voltage is 3.6 V and recharge voltage is 3.516 V. The fast charging current is set to 1.33 A while the termination current is 50 mA. This circuit can be easily changed to support two or three cell applications. However, only 84 mV difference between regulation set point and rechargeable threshold makes it frequently enter into recharge mode when small load current is applied. This can be solved by lower down the recharge voltage threshold to 200 mV to discharge more energy from the battery before it enters recharge mode again. See the application report, Using the bq24105/25 to Charge LiFePO4 Battery, SLUA443, for additional details. The recharge threshold should be selected according to real application conditions.

Figure 24 shows charging a battery and powering system without affecting battery charge and termination.

TVS Zener diode is optional for devices with date codes effective March 2008.

Figure 24. Application Circuit for Charging a Battery and Powering a System Without Affecting Termination

The bqSWITCHER was designed as a stand-alone battery charger but can be easily adapted to power a system load, while considering a few minor issues.

Advantages:

1. The charger controller is based only on what current goes through the current-sense resistor (so precharge, constant current, and termination all work well), and is not affected by the system load.
2. The input voltage has been converted to a usable system voltage with good efficiency from the input.
3. Extra external FETs are not needed to switch power source to the battery.
4. The TTC pin can be grounded to disable termination and keep the converter running and the battery fully charged, or let the switcher terminate when the battery is full and then run off of the battery via the sense resistor.

Other Issues:

1. If the system load current is large (≥ 1 A), the IR drop across the battery impedance causes the battery voltage to drop below the refresh threshold and start a new charge. The charger would then terminate due to low charge current. Therefore, the charger would cycle between charging and termination. If the load is smaller, the battery would have to discharge down to the refresh threshold resulting in a much slower cycling. Note that grounding the TTC pin keeps the converter on continuously.
2. If TTC is grounded, the battery is kept at 4.2 V (not much different than leaving a fully charged battery set unloaded).
3. Efficiency declines 2-3% hit when discharging through the sense resistor to the system.