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MC33340 Datasheet(PDF) 7 Page - ON Semiconductor |
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MC33340 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 12 page MC33340 MC33342 7 MOTOROLA ANALOG IC DEVICE DATA Fast Charge Timer A programmable backup charge timer is available for fast charge termination. The timer is activated by the Time/Temp Select comparator, and is programmed from the t1/Tref High, t2/Tsen, and t3/Tref Low inputs. If one or more of these inputs is allowed to go above VCC – 0.7 V or is left open, the comparator output will switch high, indicating that the timer feature is desired. The three inputs allow one of seven possible fast charge time limits to be selected. The programmable time limits, rounded to the nearest whole minute, are shown in Figure 10. Over/Under Temperature Detection A backup over/under temperature detector is available and can be used in place of the timer for fast charge termination. The timer is disabled by the Time/Temp Select comparator when each of the three programming inputs are held below VCC – 0.7 V. Temperature sensing is accomplished by placing a negative temperature coefficient (NTC) thermistor in thermal contact with the battery pack. The thermistor connects to the t2/Tsen input which has a 30 µA current source pull–up for developing a temperature dependent voltage. The temperature limits are set by a resistor that connects from the t1/Tref High and the t3/Tref Low inputs to ground. Since all three inputs contain matched 30 µA current source pull–ups, the required programming resistor values are identical to that of the thermistor at the desired over and under trip temperature. The temperature window detector is composed of two comparators with a common input that connects to the t2/Tsen input. The lower comparator senses the presence of an under temperature condition. When the lower temperature limit is exceeded, the charger is switched to the trickle mode. The comparator has 44 mV of hysteresis to prevent erratic switching between the fast and trickle modes as the lower temperature limit is crossed. The amount of temperature rise to overcome the hysteresis is determined by the thermistor’s rate of resistance change or sensitivity at the under temperature trip point. The required resistance change is: DR(T Low ³ T High ) + VH(T) I in + 44 mV 30 mA + 1.46 k The resistance change approximates a thermal hysteresis of 2 °C with a 10 kΩ thermistor operating at 0°C. The under temperature fast charge inhibit feature can be disabled by biasing the t3/Tref Low input to a voltage that is greater than that present at t2/Tsen, and less than VCC – 0.7 V. Under extremely cold conditions, it is possible that the thermistor resistance can become too high, allowing the t2/Tsen input to go above VCC – 0.7 V, and activate the timer. This condition can be prevented by placing a resistor in parallel with the thermistor. Note that the time/temperature threshold of VCC – 0.7 V is a typical value at room temperature. Refer to the Electrical Characteristics table and to Figure 3 for additional information. The upper comparator senses the presence of an over temperature condition. When the upper temperature limit is exceeded, the comparator output sets the Over Temperature Latch and the charger is switched to trickle mode. Once the latch is set, the charger cannot be returned to fast charge, even after the temperature falls below the limit. This feature prevents the battery pack from being continuously temperature cycled and overcharged. The latch can be reset by removing and reconnecting the battery pack or by cycling the power supply voltage. If the charger does not require either the time or temperature backup features, they can both be easily disabled. This is accomplished by biasing the t3/Tref Low input to a voltage greater than t2/Tsen, and by grounding the t1/Tref High input. Under these conditions, the Time/Temp Select comparator output is low, indicating that the temperature mode is selected, and that the t2/Tsen input is biased within the limits of an artificial temperature window. Charging of battery packs that are used in portable power tool applications typically use temperature as the only means for fast charge termination. The MC33340/342 can be configured in this manner by constantly resetting the – ∆V detection logic. This is accomplished by biasing the Vsen input to ≈1.5 V from a two resistor divider that is connected between the positive battery pack terminal and ground. The Vsen Gate output is also connected to the Vsen input. Now, each time that the Sample Timer causes the Vsen output to go low, the Vsen input will be pulled below the undervoltage threshold of 1.0 V. This causes a reset of the – ∆V logic every 1.38 seconds, thus disabling detection. Operating Logic The order of events in the charging process is controlled by the logic circuitry. Each event is dependent upon the input conditions and the chosen method of charge termination. A table summary containing all of the possible operating modes is shown in Figure 11. Figure 10. Fast Charge Backup Termination Time/Temperature Limit Backup Programming Inputs Time Limit Backup Termination Mode t3/Tref Low (Pin 5) t2/Tsen (Pin 6) t1/Tref High (Pin 7) Time Limit Fast Charge (Minutes) Time Open Open Open 283 Time Open Open Gnd 247 Time Open Gnd Open 212 Time Open Gnd Gnd 177 Time Gnd Open Open 141 Time Gnd Open Gnd 106 Time Gnd Gnd Open 71 Temperature 0 V to VCC – 0.7 V 0 V to VCC – 0.7 V 0 V to VCC – 0.7 V Timer Disabled |
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