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LTC4053EDD-4.2 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4053EDD-4.2 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LTC4053-4.2 11 4053fa – + – + – + 4053 F04 RNTC 10k RHOT 1% 7/8 VCC 1/2 VCC 3/160 VCC TOO COLD TOO HOT DISABLE NTC LTC4053 VCC NTC Thermistors The LTC4053 NTC trip points were designed to work with thermistors whose resistance-temperature characteris- tics follow Vishay Dale’s “R-T Curve 2”. The Vishay NTHS0603N02N1002J is an example of such a ther- mistor. However, Vishay Dale has many thermistor prod- ucts that follow the “R-T Curve 2” characteristic in a variety of sizes. Futhermore, any thermistor whose ratio of RCOLD to RHOT is about 7.0 will also work (Vishay Dale R-T Curve 2 shows a ratio of RCOLD to RHOT of 2.816/0.4086 = 6.9). NTC Layout Considerations It is important that the NTC thermistor not be in close thermal contact with the LTC4053. Because the LTC4053 package can reach temperatures in excess of the 50 °C trip point, the NTC function can cause a hysteretic oscillation which turns the charge current on and off according to the package temperature rather than the battery temperature. This problem can be eliminated by thermally coupling the NTC thermistor to the battery and not to the LTC4053. Furthermore, it is essential that the VCC connection to RHOT is made according to standard Kelvin sense tech- niques. Since VCC is a high current path into the LTC4053, it is essential to minimize voltage drops between the VCC input pin and the top of RHOT. APPLICATIO S I FOR ATIO Figure 4 Figure 3. Microprocessor Interface 4053 F03 400k 2k 3 8 LTC4053 IN VCC VDD OUT CHRG V+ µPROCESSOR full-scale current (C/10), the N-channel MOSFET is turned off and a 30 µA current source is connected to the CHRG pin. The IN pin will then be pulled high by the 2k pull-up. By forcing the OUT pin to a high impedance state, the current source will pull the pin low through the 400k resistor. When the internal timer has expired, the CHRG pin will assume a high impedance state and the 400k resistor will then pull the pin high to indicate that charging has terminated. NTC Thermistor The battery temperature is measured by placing a negative temperature coefficient (NTC) thermistor close to the battery pack. The NTC circuitry is shown in Figure 4. To use this feature, connect a 10k NTC thermistor between the NTC pin and ground and a resistor (RHOT) from the NTC pin to VCC. RHOT should be a 1% resistor with a value equal to the value of the chosen NTC thermistor at 50 °C (this value is 4.1k for a Vishay NTHS0603N02N1002J thermistor). The LTC4053 goes into hold mode when the resistance of the NTC thermistor drops below 4.1k which should be approximately 50 °C. The hold mode freezes the timer and stops the charge cycle until the thermistor indicates a return to a valid temperature. As the temperature drops, the resistance of the NTC thermistor rises. The LTC4053 is designed to go into hold mode when the value of the NTC thermistor increases to seven times the value of RHOT. For a Vishay NTHS0603N02N1002J thermistor, this value is 28.7k which corresponds to approximately 0 °C. The hot and cold comparators each have approximately 2 °C of hysteresis to prevent oscillation about the trip point. The NTC function can be disabled by grounding the NTC pin. |
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