Datasheet LT3651-4.1, LT3651-4.2 (Analog Devices) - 10

LT3651-4.1/LT3651-4.2
OPERATION Overview
cycle after a bad battery fault once the failed battery is The LT3651 is a complete Li-Ion battery charger, addressing removed and replaced with another battery. wide input voltage (up to 32V) and high currents (up to After charging is completed the input bias currents on the 4A). High charging efficiency is produced with a constant pins connecting to the battery are reduced to minimize frequency, average current mode synchronous step-down battery discharge. switcher architecture. The LT3651 contains provisions for a battery temperature The charger includes the necessary circuitry to allow for monitoring circuit. Battery temperature is monitored by programming and control of constant current, constant using a NTC thermistor located with the battery. If the voltage (CC/CV) charging with both current only and timer battery temperature moves outside a safe charging range termination. High charging efficiency is achieved by the of 0°C to 40°C the charging cycle suspends and signals switcher by using a bootstrapped supply for low switch a fault condition. drop for the high side driver and a MOSFET for the low side (synchronous) switch. The LT3651 contains two digital open-collector outputs, which provide charger status and signal fault conditions. Maximum charge current is set with an external sense re- These binary coded pins signal battery charging, standby sistor in series with the inductor and is adjustable through or shutdown modes, battery temperature faults and bad the RNG/SS pin. Total system input current is monitored battery faults. with an input sense resistor and is used to maintain con- stant input current by regulating battery charge current. A precision undervoltage lockout is possible by using a It is adjustable through the I resistor divider on the shutdown pin (SHDN). The input LIM pin. supply current is 17µA when the IC is in shutdown. If the battery voltage is low, charge current is automatically reduced to 15% of the programmed current to provide
General Operation (See Block Diagram)
safe battery preconditioning. Once the battery voltage The LT3651 uses an average current mode control loop climbs above the battery precondition threshold, the IC architecture to control average charge current. The LT3651 automatically increases the maximum charge current to senses charger output voltage via the BAT pin. The dif- the full programmed value. ference between this voltage and the internal float volt- Charge termination can occur when charge current de- age reference is integrated by the voltage error amplifier creases to one-tenth the programmed maximum charge (V-EA). The amplifier output voltage (ITH) corresponds current (C/10 termination). Alternately, termination can to the desired average voltage across the inductor sense be time based through the use of an internal program- resistor, RSENSE, connected between the SENSE and BAT mable charge cycle control timer. When using the timer pins. The ITH voltage is divided down by a factor of 10, termination, charging continues beyond the C/10 level to and provides a voltage offset on the input of the current “top-off” a battery. Charging typically terminates three error amplifier (C-EA). The difference between this im- hours after initiation. When the timer-based scheme is posed voltage and the current sense resistor voltage is used, bad battery detection is also supported. A system integrated by C-EA. The resulting voltage (VC) provides a fault is triggered if a battery stays in precondition mode voltage that is compared against an internally generated for more than one-eighth of the total charge cycle time. ramp and generates the switch duty cycle that controls Once charging is terminated and the LT3651 is not actively the charger’s switches. charging, the IC automatically enters a low current standby The ITH error voltage corresponds linearly to average mode in which supply bias currents are reduced to 100µA. current sensed across the inductor current sense resistor. If the battery voltage drops 2.5% from the full charge float Maximum charge current is controlled by clamping the voltage, the LT3651 engages an automatic charge cycle maximum voltage of ITH to 1V. This limits the maximum restart. The IC also automatically restarts a new charge current sense voltage (voltage across RSENSE) to 95mV 365142ff 10 For more information www.linear.com/LT3651-4.1 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Pin Functions Block Diagram Operation Applications Information Typical Applications Revision History Typical Application Related Parts