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LTC4090 Datasheet(PDF) 16 Page - Linear Technology |
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LTC4090 Datasheet(HTML) 16 Page - Linear Technology |
16 / 28 page LTC4090/LTC4090-5 16 4090fa OPERATION where VPROG is the PROG pin voltage and RPROG is the total resistance from the PROG pin to ground. Keep in mind that when the LTC4090/LTC4090-5 are powered from the IN pin, the programmed input current limit takes precedence over the charge current. In such a scenario, the charge current cannot exceed the programmed input current limit. For example, if typical 500mA charge current is required, calculate: R V mA k PROG == 1 500 50 000 100 •, For best stability over temperature and time, 1% metal film resistors are recommended. Under trickle charge conditions, this current is reduced to 10% of the full- scale value. The Charge Timer The programmable charge timer is used to terminate the charge cycle. The timer duration is programmed by an external capacitor at the TIMER pin. The charge time is typically: t hours C R hours µF k TIMER TIMER PROG () •• .• = 3 0 1 100 The timer starts when an input voltage greater than the undervoltage lockout threshold level is applied or when leaving shutdown and the voltage on the battery is less than the recharge threshold. At power-up or exiting shutdown with the battery voltage less than the recharge threshold, the charge time is a full cycle. If the battery is greater than the recharge threshold the timer will not start and charging is prevented. If after power-up the battery voltage drops below the recharge threshold, or if after a charge cycle the battery voltage is still below the recharge threshold, the charge time is set to one-half of a full cycle. The LTC4090/LTC4090-5 have a feature that extends charge time automatically. Charge time is extended if the charge current in constant current mode is reduced due to load current or thermal regulation. This change in charge time is inversely proportional to the change in charge current. As the LTC4090/LTC4090-5 approach constant voltage mode the charge current begins to drop. This change in charge current is due to normal charging operation and does not affect the timer duration. Consider, for example, a USB charge condition where RCLPROG = 2k, RPROG = 100k and CTIMER = 0.1μF. This corresponds to a three hour charge cycle. However, if the HPWR input is set to a logic low, then the input current limit will be reduced from 500mA to 100mA. With no ad- ditional system load, this means the charge current will be reduced to 100mA. Therefore, the termination timer will automatically slow down by a factor of five until the charger reaches constant voltage mode (i.e. VBAT ap- proaches 4.2V) or HPWR is returned to a logic high. The charge cycle is automatically lengthened to account for the reduced charge current. The exact time of the charge cycle will depend on how long the charger remains in constant current mode and/or how long the HPWR pin remains logic low. Once a time-out occurs and the voltage on the battery is greater than the recharge threshold, the charge current stops, and the ⎯C⎯H⎯R⎯G output assumes a high impedance state if it has not already done so. Connecting the TIMER pin to ground disables the battery charger. ⎯C⎯H⎯R⎯G Status Output Pin When the charge cycle starts, the ⎯C⎯H⎯R⎯G pin is pulled to ground by an internal N-channel MOSFET capable of driv- ing an LED. When the charge current drops below 10% of the programmed full charge current while in constant voltage mode, the pin assumes a high impedance state, but charge current continues to flow until the charge time elapses. If this state is not reached before the end of the programmable charge time, the pin will assume a high impedance state when a time-out occurs. The ⎯C⎯H⎯R⎯G current detection threshold can be calculated by the fol- lowing equation: I V R V R DETECT PROG PROG == 01 50 000 5000 . •, |
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