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LTC4056-4.2 Datasheet(PDF) 8 Page - Linear Technology |
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LTC4056-4.2 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LTC4056-4.2 8 405642f APPLICATIO S I FOR ATIO Undervoltage Lockout An internal undervoltage lockout (UVLO) circuit monitors the input voltage and keeps the charger in shutdown mode until VCC rises above the UVLO threshold (VUVLOI is typically 4.4V). Approximately 50mV of hysteresis is built in to prevent oscillation around the threshold level. In undervoltage lockout, battery drain current is very low (<1 µA) and supply current is approximately 40µA. Undervoltage Charge Current Limiting The LTC4056 includes undervoltage charge current limit- ing that prevents full charge current until the input supply voltage reaches a threshold value (VUVCL). This feature is particularly useful if the LTC4056 is powered from a supply with long leads (or any relatively high output impedance). For example, USB powered systems tend to have highly variable source impedances (due primarily to cable quality and length). A transient load combined with such an impedance can easily trip the UVLO threshold and turn the charger off unless undervoltage charge current limiting is implemented. Consider a situation where the LTC4056 is operating under normal conditions and the input supply voltage begins to sag (e.g. an external load drags the input supply down). If the input voltage reaches VUVCL (approximately 170mV above the rising undervoltage lockout threshold, VUVLOI), undervoltage charge current limiting will begin to reduce the charge current in an attempt to maintain VUVCL at the VCC input of the IC. The LTC4056 will continue to operate at the reduced charge current until the input supply voltage is increased or voltage mode reduces the charge current further. Trickle Charge and Defective Battery Detection At the beginning of a charge cycle, if the battery voltage is low (below VTRIKL of about 2.8V) the charger goes into trickle charge mode reducing the charge current to ap- proximately 2% of the full-scale current. If the low battery voltage persists for one quarter of the total charge time, the battery is assumed to be defective, the charge cycle is terminated and the CHRG pin output transitions from a strong pull-down to a 12 µA pull-down. To restart the charge cycle, remove the input voltage and reapply it or momentarily force the TIMER/SHDN pin to ground. Programming Charge Current When in constant current mode, the full-scale charge current is programmed using a single external resistor between the PROG pin and ground, RPROG. The current delivered to the ISENSE pin (flowing from VCC through the internal 110m Ω sense resistor) will be 915 times the current in RPROG. Because the LTC4056 provides a virtual 1V source at the PROG pin, the charge current is given by: II V R or R V I CHG PROG PROG PROG CHG = () = = •• • 915 1 915 1 915 Under trickle charge conditions, this current is reduced to approximately 2% of the full-scale value. The actual bat- tery charge current (IBAT) is slightly lower than the ex- pected charge current because the charger forces 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: IA I V R BAT PROG PROG ()= + = + 915 1 915 1 •• β β β β If β = 50, then IBAT is 2% low. If desired, reducing RPROG by 2% can compensate for the 2% drop. For example, if 700mA charge current is required, calcu- late: R V mA k PROG = = 1 700 915 1 3 •. If a low β needs to be compensated for, say β = 50, calculate: R V mA k PROG = + = 915 700 50 50 1 127 •. For best stability over temperature and time, 1% metal- film resistors are recommended. |
Similar Part No. - LTC4056-4.2_15 |
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Similar Description - LTC4056-4.2_15 |
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