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LTC1734LES6-4.2 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1734LES6-4.2 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1734L 1734lf Monitoring Charge Current The voltage on the PROG pin indicates the charge current as a proportion of the maximum current set by the program resistor. The charge current is equal to 250 • (VPROG/RPROG) amps. This feature allows a microcontrol- ler with an ADC to easily monitor charge current and if desired, manually shut down the charger at the appropri- ate time. The minimum PROG pin current is about 3 µA (IPROGPU). Errors in the charge current monitor voltage on the PROG pin and in the full-scale charge current are inversely proportional to battery current and can be statistically approximated as follows: One Sigma Error(%) ≅ 1 + 0.08/IBAT(A) Dynamic loads on the battery will cause transients to appear on the PROG pin. Should they cause excessive errors in charge current monitoring, a simple RC filter as shown in Figure 2 can be used to filter the transients. The filter will also quiet the PROG pin to help prevent momen- tary entry into the manual shutdown mode. Because the PROG pin is in a closed-loop signal path the pole frequency must be kept high enough to maintain adequate AC stability. This means that the maximum resistance and capacitance presented to the PROG pin must be limited. See the Stability section for more details. Constant Current Source The LTC1734L can be used as a constant current source by disabling the voltage control loop as shown in Figure 3. This is done by pulling the BAT pin below the preset float voltage of 4.2V by grounding the BAT pin. The program resistor will determine the output current. The output current range can be between approximately 10mA and 180mA, depending on the maximum power rating of the external PNP pass transistor. External PNP Transistor The external PNP pass transistor must have adequate beta, low saturation voltage and sufficient power dissipa- tion capability (including any heat sinking, if required). To provide 180mA of charge current with the minimum available base drive of approximately 20mA requires a PNP beta greater than 9. With low supply voltages, the PNP saturation voltage (VCESAT) becomes important. The VCESAT must be less than the minimum supply voltage minus the maximum voltage drop across the internal sense resistor and bond wires (0.3 Ω) and battery float voltage. If the PNP transis- tor can not achieve the low saturation voltage required, base current will dramatically increase. This is to be avoided for a number of reasons: output drive may reach current limit resulting in the charger’s characteristics to go out of specifications, excessive power dissipation may force the IC into thermal shutdown, or the battery could become discharged because some of the current from the DRIVE pin could be pulled from the battery through the forward biased collector base junction. For example, to program a charge current of 100mA with a minimum supply voltage of 4.75V, the minimum operat- ing VCE is: VCE(MIN)(V) = 4.75 – (0.1)(0.3) – 4.2 = 0.52V The actual battery charge current (IBAT) is slightly less than the expected charge current because the charger senses the emitter current and the battery charge current will be reduced by the base current. In terms of β (IC/IB), IBAT can be calculated as follows: IBAT(A) = 250 • IPROG[β/(β + 1)] If β = 50, then IBAT is 2% low. If desired, the 2% loss can be compensated for by increasing IPROG by 2%. Another important factor to consider when choosing the PNP pass transistor is the power handling capability. The transistor’s data sheet will usually give the maximum rated power dissipation at a given ambient temperature with a power derating for elevated temperature operation. The maximum power dissipation of the PNP when charging is: PD(MAX)(W) = IBAT (VDD(MAX) – VBAT(MIN)) VDD(MAX) is the maximum supply voltage and VBAT(MIN) is the minimum battery voltage when discharged. APPLICATIONS INFORMATION |
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