Welcome to the training module on MAX1925/26 – Lithium-ion (Li+) battery chargers. This module introduces MAX1925/26 single-cell Li+ battery charges, and their features, application information.
This page gives you an overall features of MAX1925/26, it is lithium ion (Li+) switch-mode battery chargers for use in digital still cameras and PDAs. The MAX1925/MAX1926 use an inexpensive external PMOS pass element step-down configuration for applications where low heat dissipation and small size are critical. Charge current is programmable up to 1A, and an external capacitor sets the maximum charge time. Its features include automatic input power detection (AC adapter detection), logic-controlled enable, and temperature monitoring with an external thermistor. The MAX1925 disables charging for inputs greater than 6.1V, while the MAX1926 charges for inputs between 4.25V and 12V. This device can be used for cradle chargers, Digital Still Cameras, PDA’s, Self charging battery packs.
The devices include battery under voltage /overvoltage fault protection. The MAX1925/MAX1926 use EN and THRM for shutdown, battery detection, and temperature monitoring. The devices provide outputs to indicate charge status (CHG) and presence of input power (ACON). The MAX1925/MAX1926 include two prequalification modes that must be passed before the charger enters the fast-charge state. During fast charge, the charger operates initially in constant-current mode until the battery voltage reaches 4.2V. When the battery voltage has reached 4.2V, the charger operates in constant voltage mode. In constant-current mode, the charger acts as a hysteretic current source, controlling the inductor’s peak and valley currents. In constant-voltage mode, the charger regulates the peak and valley of the output ripple.
This diagram shows charging behaviour of a typical Li+ cell. The MAX1925/MAX1926 remains in fast-charge mode until the battery voltage reaches regulation and the charge current drops below 1/8th of I FASTCHG . The charger then enters full topoff mode and the CHG LED is turned off. In full topoff mode, the controller continues to operate as in fast-charge mode, except that it remains in constant-voltage mode (CVM) unless the battery voltage falls. the charger then enters the Kelvin state where charge current is interrupted so that the battery voltage can be accurately measured. The MAX1925/MAX1926 do not enter done mode until t FULLCHG has been reached. If the battery is removed and a new battery is connected during either fast-charge or full topoff modes, the charger begins with full charge current without prequalification unless the part is reset. Detect battery insertion by connecting THRM to a thermistor on the battery, or to a 10kΩ resistor linked to a battery door mechanism.
Here we show characteristics of NiCd battery charging, The charger applies a constant current while monitoring battery voltage and other variables to determine when to terminate the charge. Fast-charge rates in excess of 2C are possible, but the most common rate is about C/2. Because charging efficiency is somewhat less than 100%, a full charge at the C/2 rate requires slightly more than two hours. At rates exceeding C/2 (resulting in a charge time of no more than two hours), the charger also monitors the cell's temperature and voltage. Because cell temperature rises rapidly when a cell reaches full charge, the temperature monitor enables another termination technique. Termination on this positive temperature slope is called DT termination.
This is an generic charging Block diagram shown here, the bulk power source provides raw DC power, usually from a wall cube or brick. The current and voltage controls regulate current and voltage applied to the battery. For less-expensive chargers, the regulator is usually a power transistor or other linear-pass element that dissipates power as heat. It can also be a buck switching supply that includes a standard freewheeling diode for average efficiency or a synchronous rectifier for highest efficiency. It has various measurement and control functions. An analog current-control loop limits the maximum current delivered to the battery. This block provides intelligence for the system and implements the state machine previously described. It knows how and when to terminate a fast charge. Intelligence is internal to the chip in stand-alone charger ICs.
This page gives information about charging current and its timing diagram as shown. When the cell voltage is less than 2V, the cell is charged from an internal linear 4mA current source (PREQUAL1). When the cell voltage exceeds 2V, the cell is charged with 10% of the programmed fast-charge current (IFASTCHG) until it reaches 3V. When the cell voltage is above 3V, fast charging occurs at the full set current. If the cell fails to reach the next prequalification threshold before a set time, charging stops, a fault alarm is set, and the CHG output blinks. The MAX1925/MAX1926 feature two precondition levels to restore near-dead cells. The devices source 4mA to a cell that is below 2V while sourcing C/10 to a cell between 2V and 3V. Full charge current is then applied above 3V. An output drives a LED to indicate charging (LED on) and fault conditions (LED blinking).
This page gives you application information like selection of inductor, output capacitor and MOSFET selection.
This page shows a typical operating circuit that uses an external PMOS pass element step-down configuration for high efficiency. Additional features include automatic input power detection, logic-controlled enable, and temperature monitoring with an external thermistor. The MAX1925 disables charging for inputs greater than 6.1V, while the MAX1926 charges for inputs between 4.25V and 12V.
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