Datasheet LT3650-4.1, LT3650-4.2 (Analog Devices) - 9

LT3650-4.1/LT3650-4.2
operaTion OVERVIEW
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. Internal reverse-voltage actively charging, the IC automatically enters a low protection allows direct connection to the input supply current standby mode in which supply bias currents without a blocking diode. are reduced to 85µA. If the battery voltage drops 2.5% The LT3650 incorporates a 2A switch that is driven by from the full charge float voltage, the LT3650 engages a bootstrapped supply to maximize efficiency during an automatic charge cycle restart. The IC also automati- charging cycles. A wide input range allows the operation cally restarts a new charge cycle after a bad-battery fault to full charge from 5V ±5% to 32V. A precision threshold once the failed battery is removed and replaced with shutdown pin allows incorporation of UVLO functionality another battery. using a simple resistor divider. The IC can also be put into The LT3650 contains provisions for a battery temperature a low current shutdown mode, in which the input supply monitoring circuit. This feature monitors battery tem- bias is reduced to only 15µA. perature by using a thermistor during the charging cycle, The LT3650 incorporates several degrees of charge current suspends charging, and signals a fault condition if the control freedom. The overall maximum charge current is battery temperature moves outside a safe charging range. set using an external inductor current sense resistor. A The LT3650 contains two digital open-collector outputs, maximum charge current programming pin allows dynamic which provide charger status and signal fault conditions. manipulation of the battery charge current. The LT3650 These binary coded pins signal battery charging, standby also incorporates a system input supply current limit or shutdown modes, battery temperature faults and bad- control feature that servos the battery charge current to battery faults. accommodate overall system load requirements. The LT3650 automatically enters a battery precondi-
General Operation (See Block Diagram)
tion mode if the sensed battery voltage is very low. In The LT3650 uses average current mode control architec- this mode, the charge current is reduced to 15% of the ture, such that the IC servos directly to average charge programmed maximum, as set by the inductor sense current. The LT3650 senses charger output voltage via resistor, RSENSE. Once the battery voltage climbs above the BAT pin. The difference between the voltage on this the internally set precondition threshold of 2.9V, the IC pin and an internal float voltage reference is integrated by automatically increases the maximum charge current to the voltage error amplifier (V-EA). This amplifier generates the full programmed value. an error voltage on its output (ITH), which corresponds The LT3650 can use a charge current based C/10 termina- to the average current sensed across the inductor cur- tion scheme, which ends a charge cycle when the battery rent sense resistor, RSENSE, which is connected between charge current falls to one-tenth the programmed maxi- the SENSE and BAT pins. The ITH voltage is then divided mum charge current. The LT3650 also contains an internal down by a factor of 10, and imposed on the input of the charge cycle control timer, for timer-based termination. current error amplifier (C-EA). The difference between this When using the internal timer, the IC combines C/10 detec- imposed voltage and the current sense resistor voltage tion with a programmable time constraint, during which is integrated, with the resulting voltage (VC) used as a the charging cycle can continue beyond the C/10 level threshold that is compared against an internally gener- to top-off a battery. The charge cycle terminates when ated ramp. The output of this comparison controls the a specific time elapses, typically three hours. When the charger’s switch. 36504142fc 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