Datasheet LT3650-8.2, LT3650-8.4 (Analog Devices) - 9

LT3650-8.2/LT3650-8.4
operaTion OVERVIEW
the timer-based scheme is used, the IC also supports bad- LT3650 is a complete monolithic, mid-power, Li-Ion battery battery detection, which triggers a system fault if a battery charger, addressing high input voltage applications with stays in precondition mode for more than one-eighth of solutions that use a minimum of external components. the total programmed charge cycle time. The IC uses a 1MHz constant-frequency, average current Once charging is terminated and the LT3650 is not mode step-down architecture. actively charging, the IC automatically enters a low current The LT3650 incorporates a 2A switch that is driven by a standby mode in which supply bias currents are reduced bootstrapped supply to maximize efficiency during charging to 85µA. If the battery voltage drops 2.5% from the full cycles. A wide input range allows the operation to full- charge float voltage, the LT3650 engages an automatic charge from 9V to 32V. A precision threshold shutdown charge cycle restart. The IC also automatically restarts a pin allows incorporation of UVLO functionality using a new charge cycle after a bad-battery fault once the failed simple resistor divider. The IC can also be put into a low battery is removed and replaced with another battery. current shutdown mode, in which the input supply bias The LT3650 contains provisions for a battery temperature is reduced to only 15µA. monitoring circuit. This feature monitors battery tem- The LT3650 incorporates several degrees of charge current perature by using a thermistor during the charging cycle, control freedom. The overall maximum charge current is suspends charging, and signals a fault condition if the set using an external inductor current sense resistor. A battery temperature moves outside a safe charging range. maximum charge current programming pin allows dynamic The LT3650 contains two digital open-collector outputs, manipulation of the battery charge current. The LT3650 which provide charger status and signal fault conditions. also incorporates a system input supply current limit These binary coded pins signal battery charging, standby control feature that servos the battery charge current to or shutdown modes, battery temperature faults and bad- accommodate overall system load requirements. battery faults. The LT3650 automatically enters a battery precondition mode if the sensed battery voltage is very low. In this
General Operation (See Block Diagram)
mode, the charge current is reduced to 15% of the The LT3650 uses average current mode control architec- programmed maximum, as set by the inductor sense ture, such that the IC servos directly to average charge resistor, RSENSE. Once the battery voltage climbs current. The LT3650 senses charger output voltage via the above the internally set precondition threshold, the IC BAT pin. The difference between the voltage on this pin automatically increases the maximum charge current to and an internal float voltage reference is integrated by the the full programmed value. voltage error amplifier (V-EA). This amplifier generates an The LT3650 can use a charge current based C/10 termina- error voltage on its output (ITH), which corresponds to the tion scheme, which ends a charge cycle when the battery average current sensed across the inductor current sense charge current falls to one-tenth the programmed maxi- resistor, RSENSE, which is connected between the SENSE mum charge current. The LT3650 also contains an internal and BAT pins. The ITH voltage is then divided down by a charge cycle control timer, for timer-based termination. factor of 10, and imposed on the input of the current er- When using the internal timer, the IC combines C/10 ror amplifier (C-EA). The difference between this imposed detection with a programmable time constraint, during voltage and the current sense resistor voltage is integrated, which the charging cycle can continue beyond the C/10 with the resulting voltage (VC) used as a threshold that level to “top-off” a battery. The charge cycle terminates is compared against an internally generated ramp. The when a specific time elapses, typically three hours. When output of this comparison controls the charger’s switch. 36508284fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts