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LTC4050EMS-4.1 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4050EMS-4.1 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LTC4050 4050f If a PNP transistor is chosen as the pass transistor, a 1000pF capacitor is required from the DRV pin to VCC. This capacitor is needed to help stabilize the voltage loop. A 10 µF capacitor at the BAT pin is also recommended when a battery is not present. VCC Bypass Capacitor Many types of capacitors can be used for input bypassing. However, caution must be exercised when using multi- layer ceramic capacitors. Because of the self resonant and high Q characteristics of some types of ceramic capaci- tors, high voltage transients can be generated under some start-up conditions, such as connecting the charger input to a hot power source. These transients can be minimized by using X5R dielectric capacitors and/or by adding a 1.5 Ω resistor in series with the ceramic input capacitor. For more information, refer to Application Note 88. Thermistor Interface A thermistor connected to the NTC (negative temperature coefficient) pin can be used to sense the battery tempera- ture to determine if the battery is within an acceptable temperature range for charging (between 0 °C and 50°C). A Dale (curve 2) 10k thermistor is recommended although many other types of thermistors can also be used. For example, a BetaCHIP (curve 7) 10k thermistor or other 10k thermistors with a room temperature beta of approxi- mately 3400 will work well. The thermistor is connected from NTC (pin 2) to ground and is biased up by an internal 28.6k trimmed thin film resistor that connects to VCC through a P-channel MOSFET. This MOSFET also biases an internal resistor string to ground, from which voltage thresholds of approximately VCC/2 and VCC/8 are derived. The NTC pin is compared to these thresholds by two comparators that have wired-OR outputs. The thresholds are selected such that an overtemperature condition will occur when the thermistor resistance is less than approxi- mately 4.1k and undertemperature condition will occur when the thermistor resistance is greater than approxi- mately 28.5k. These correspond to thermistor tempera- tures of 50 °C and 0°C for the specific type of thermistor listed above (many others will be close enough for most purposes). The MOSFET is turned off during undervoltage conditions, preventing the dividers that are biased from it from drawing current from the battery when input power is removed. The drop across the MOSFET is common to both resistor dividers and does not cause any loss of accuracy in the circuit. The comparators have approxi- mately 10mV of hysteresis to prevent oscillations around the trip points. When an undertemperature or overtemperature condition is sensed, the current amplifier pulldown is disabled and DRV is pulled high, the timer is placed in a hold condition with the count frozen until the battery temperature is within an acceptable range. The end-of-charge compara- tor is also disabled to prevent a premature end of charge signal due to the lack of battery charging current. APPLICATIONS INFORMATION VCC UV 28.6k 4050 AI 10k DALE CURVE 2 NTC THERMISTOR TBAD (TO CA, EOC, TIMER) 2 NTC NTC Interface Circuitry |
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Similar Description - LTC4050EMS-4.1 |
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