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BQ24401 Datasheet(PDF) 4 Page - Texas Instruments |
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BQ24401 Datasheet(HTML) 4 Page - Texas Instruments |
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4 / 15 page 
Initiation and Charge Qualification The bq24401 initiates a charge cycle when it detects l Application of power to VCC l Battery replacement l Exit from sleep mode Immediately following initiation, the IC enters a charge-qualification mode. The bq24401 charge qualifica- tion is based on battery voltage and temperature. If voltage on pin BAT is less than the internal threshold, VLBAT, the bq24401 enters the charge-pending state. This condition indicates the possiblility of a defective or shorted battery pack. In an attempt to revive a fully depleted pack, the bq24401 enables the MOD pin to trickle-charge at a rate of once every 1.0s. As explained in the section “Top-Off and Pulse-Trickle Charge,” the trickle pulse-width is user-selectable and is set by the value of the resistance connected to pin RC. During this period, the LED pin blinks at a 1Hz rate, indicating the pending status of the charger. Similarly, the bq24401 suspends fast charge if the battery temperature is outside the VLTF to VHTF range. (See Ta- ble 4.) For safety reasons, however, it disables the pulse trickle, in the case of a battery over-temperature condi- tion (i.e., VTS <VHTF). Fast charge begins when the bat- tery temperature and voltage are valid. NiCd and NiMH Batteries Following qualification, the bq24401 fast-charges NiCd or NiMH batteries using a current-limited algorithm. Dur- ing the fast-charge period, it monitors charge time, tem- perature, and voltage for adherence to the termination criteria. This monitoring is further explained in later sec- tions. Following fast charge, the battery is topped off, if top-off is selected. The charging cycle ends with a trickle maintenance-charge that continues as long as the voltage on pin BAT remains below VMCV. Charge Termination Maximum Charge Time The bq24401 sets the maximum charge-time through pin RC. With the proper selection of external resistor and ca- pacitor, various time-out values may be achieved. Figure 3 shows a typical connection. The following equation shows the relationship between the RMTO and CMTO values and the maximum charge time (MTO) for the bq24401: MTO=RMTO ∗ CMTO ∗ 35,988 MTO is measured in minutes, RMTO in ohms, and CMTO in farads. (Note: RMTO and CMTO values also determine other fea- tures of the device. See Tables 2 and 3 for details.) Maximum Temperature A negative-coefficient thermistor, referenced to VSS and placed in thermal contact with the battery, may be used as a temperature-sensing device. Figure 4 shows a typical temperature-sensing circuit. During fast charge, the bq24401 compares the battery temperature to an internal high-temperature cutoff threshold, VTCO. As shown in Table 4, high-temperature termination occurs when voltage at pin TS is less than this threshold. DT/Dt When fast charging, the bq24401 monitors the voltage at pin TS for rate of temperature change detection, ∆T/∆t. The bq24401 samples the voltage at the TS pin every 16s and compares it to the value measured 2 samples earlier. This feature terminates fast charge if this voltage de- clines at a rate of V 161 V Min CC Figure 4 shows a typical connection diagram. 4 bq24401 Battery Chemistry Charge Algorithm NiCd or NiMH 1. Charge qualification 2. Trickle charge, if required 3. Fast charge (constant current) 4. Charge termination (∆T/∆t, time) 5. Top-off (optional) 6. Trickle charge Table 1. Charge Algorithm |
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