DESCRIPTION

The LTC4121 is a 400mA constant-current/constantvoltage (CC/CV) synchronous step-down battery charger. In addition to CC/CV operation, the LTC4121 regulates its input voltage to a programmable percentage of the input open-circuit voltage. This technique maintains maximum power transfer with high impedance input sources such as solar panels.

An external resistor programs the charge current up to 400mA. The LTC4121-4.2 is suitable for charging Li-Ion/ Polymer batteries, while the programmable float voltage of the LTC4121 is suitable for several battery chemistries.

The LTC4121 and LTC4121-4.2 include an accurate RUN pin threshold, low voltage battery preconditioning and bad battery fault detection, timer termination, auto-recharge, and NTC temperature qualified charging. The FAULT pin provides an indication of bad battery or temperature faults.

Once charging is terminated, the LTC4121 signals end-ofcharge via the CHRG pin, and enters a low current SLEEP mode. An auto-restart feature starts a new charging cycle if the battery voltage drops by 2.2%.

 

CARACTÉRISTIQUES

Wide Input Voltage Range: 4.4V to 40V

Temperature Compensated Input Voltage Regulation for Maximum Power Point Tracking (MPPT)

Adjustable Float Voltage 3.5V to 18V (LTC4121)

Fixed 4.2V Float Voltage Option (LTC4121-4.2)

Haute efficacité : Jusqu'à 95%

50mA to 400mA Programmable Charge Current

±1% Feedback Voltage Accuracy

Programmable 5% Accurate Charge Current

Thermally Enhanced, Low Profile (0.75mm) 16-Lead (3mm × 3mm) QFN Package

 

CANDIDATURES

Instruments portatifs

Solar Powered Devices

Industrial/Military Sensors and Devices

 

FONCTIONNEMENT

Vue d'ensemble

The LTC4121 is a synchronous step-down (buck) monolithic battery charger with maximum power-point tracking (MPPT) control of the source voltage. The LTC4121/LTC4121-4.2 serves as a constant-current/ constant-voltage battery charger with the following built-in charger functions: programmable charge current, battery precondition with ½ hour timeout, precision shutdown/ run control, NTC thermal protection, a 2-hour safety termination timer, and automatic recharge. The LTC4121/ LTC4121-4.2 also provides output pins to indicate state of charge and fault status.

Maximum Power Point Tracking

The LTC4121 employs an MPPT algorithm that compares a stored open-circuit input voltage measurement against the instantaneous input voltage while charging. The LTC4121 automatically reduces the charge current if the input voltage falls below the user defined percentage of the open-circuit voltage. This algorithm lets the LTC4121 optimize power transfer for a variety of different input sources including first order temperature compensation of a solar panel.

The LTC4121 periodically pauses charging to measure the open-circuit voltage allowing the LTC4121 to track fluctuations in the available power. About once every 30 seconds the LTC4121 pauses charging and waits about 36ms (PWMP) for the input voltage to recover to its open-circuit potential. At the end of this recovery time, the LTC4121 samples the input voltage divided by 10 (1/ KF), and stores this value on a digital to analog converter (DAC). When charging resumes, the DAC voltage is compared against the MPPT pin voltage that is programmed with a resistive divider. If the MPPT voltage falls below the DAC voltage, the charge current is reduced to regulate the input voltage at that level. This regulation loop serves to maintain the input voltage at or above a user defined level that corresponds to the peak power available from the applied source.

The charge current drops to zero and the LTC4121 waits PWMP and then samples the open-circuit voltage. When charging resumes the input voltage collapses if the source cannot support the demanded charge current. When the input voltage drops to VMP, the charge current is reduced so as to maintain VIN at VMP.

End-of-Charge Indication and Safety Timeout

The LTC4121 uses a safety timer to terminate charging. Whenever the LTC4121 is in constant current mode the timer is paused, and when FB rises or falls through the VRCHG threshold the timer is reset. When the battery voltage reaches the float voltage, the safety timer begins counting down a 2-hour timeout. If charge current falls below one tenth of the programmed maximum charge current (hC/10), the CHRG status pin rises, but top-off charge current continues to flow until the timer finishes. After the timeout, the LTC4121 enters a low-power sleep mode.