Device Schematic
The schematics shown in Figure 4 on page 7 and Figure 5 on page 8 constitute a complete two-cell battery charger.
A signal from the PWM goes to the RC-filter, which consists of resistor R4 and capacitor C4. A constant voltage signal proportional to the PWM duty cycle value forms at the Q2 gate. Therefore, the PWM and RC-filter is a simple implementation of a PWM-DAC. The bipolar transistor Q2 is driven by an analog signal from the PWM-DAC. This bipolar transistor and resistors R1 and R5 form a resistive divider. Therefore, the voltage drop on the resistor R1 is directly dependent on the Q2 base voltage; that is, on the PWM- DAC level. The MOSFET transistor Q1 is driven by the voltage drop on resistor R1 and regulates the battery charge current. The PWM period was set to 2048 for an accurate current level setting, and can easily be adjusted in the firmware.
Note that the charger proposed in this application note is based on a linear current regulator. The advantages of this regulator are low cost and small size. However, to charge a battery with a capacity of over 1000 mAh with a charge current of 1 CA (where CA is the nominal battery capacity) the linear regulator can be nonoptimal due to the large voltage drop on the MOSFET and the consequent high MOSFET temperature. In this case, a step down regulator is preferable to a linear current regulator. The step-down regulator is explained in detail in Application Notes AN2107 and AN2258.
Diode D1 is used to prevent a reverse current that can discharge the battery when the charger is disconnected from the supply voltage. The cell-balancing circuit is represented by MOSFETs Q4 and Q5, and by balancing resistors R11 and R14. The MOSFETs are directly controlled from the PSoC device port (high level - close, low level - open). The resistors R8-R10 and the bipolar transistor Q3 act as a level translator and allow opening the MOSFET Q4 by a logic signal from the PSoC.
AN2309
The resistive network (R6, R7, R12, R13, R15, R16, and
R18-R22) and the reference voltage Vbias from the divider on R29 and D8, allow transformation of the battery current,
voltage, and temperature into signals suitable for the PSoC device. The 100 mΩ resistor R23 is a current-sense resistor that is in the battery pack current path.
The two-cell charger user interface uses two LEDs to display internal status. In this application configuration, the green LED indicates the charge phase, and the yellow LED indicates the discharge phase. The Error state is indicated when both LEDs are on and the idle status is indicated when both LEDs are off.
To provide a processor power supply from a high voltage level, the linear current regulator U2 is used. Alternatively, a switching regulator can be used, as explained in AN2258. Or, the regulated step-down converter from an internal SMP can be used, as explained in AN2180, “Using the PSoC Switch Mode Pump in a Step-Down Converter.” An external voltage supply is applied to the connector J4. The SW1 switch allows the device to be disconnected from the external power supply. Two diodes in the D6 package allow the processor to operate during the charge phase from the external power supply and during the discharge phase from the battery pack power supply. The external load is connected to the connector J3 LOAD. The diodes D4 and D5 provide an uninterrupted power supply (UPS) to the LOAD connector, much as D6 provides power to the processor. The switch-on transistors Q6 and Q7 allow the power supply to be disconnected from the LOAD connector and protect the battery from overdischarge. This switch is optional and can be removed to reduce total device cost further. The ground level is connected to the external ground level POWER (during the charge phase or discharge phase) and to the battery pack ground that follows the current- sense resistor. Only in this way can the charge battery pack current and the total battery pack discharge current pass through the current-sense resistor. This ground-level position is used to supplement the battery fuel gauging functionality in the PSoC software, as shown in AN2294.