Li-Ion Charge Termination IC Interfaces with PWM Switchers Design Note 242 Fran Hoffart Rechargeable lithium-ion batteries are rapidly becoming the battery of choice for many battery-powered products. These products include notebook computers, PDAs, video camcorders, digital cameras, cellular phones, portable test equipment and others. Compared to other rechargeable power sources, Li-Ion batteries have higher energy density for both weight and volume and provide longer run time between charges. tery from overtemperature and overcurrent conditions. In addition, battery packs may also contain back-toback MOSFET switches that disconnect the battery if an overcurrent condition exists for either charge or discharge, or if an overvoltage or undervoltage condition exists. Many of these precautions are needed because lithium-ion cells are easily damaged by both overcharge and overdischarge conditions. Charging Li-Ion batteries is a relatively simple process. Apply a current-limited (at a 1C rate) constant voltage (±1% tolerance) for approximately three hours, then stop charging. (C is a battery term used to indicate the Ampere-hour capacity of a battery). A complete charge cycle may also include precharge qualification for battery temperature and precharge qualification for deeply discharged batteries. LT®1510 Battery Charger IC The LT1510 is a high efficiency switching regulator power IC designed specifically for battery charging applications. A step-down current mode 200kHz or 500kHz PWM topology is used. Included on the die is a 2A switch along with programmable current and voltage control circuitry. Available in a 16-pin SO and the tiny MSOP surface mount package, the LT1510 is capable of providing up to 1.5A of charge current in many situations. Although the LT1510 provides many charger functions, it lacks a timer and requires precision resistors to program the charge voltage. This is when the LTC ®1729 should be added. Battery Pack Protection Because of the high energy associated with lithium-ion cells and their sensitivity to abuse, many battery manufacturers require protective devices inside the battery pack for both safety and performance reasons. These devices often consist of poly fuses, thermal fuses and bimetallic breakers, which protect the bat- VIN 12V TO 20V LT1510 1, 4 L1 33μH CTX33-2 2, 3 D1 MBRS140L 1 C2 0.22μF 3 D3 MMBD914 5 2 4 6 7 8 + 10mV – GND GND SW VCC BOOST VCC GND OVP L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. PROG VC SENSE BAT GND GND GND GND 16 15 D2 MBRS140L 14 C1 10μF ( ) CHARGE CURRENT = 2.465 (2000) R1 13 12 11 R2 300Ω C3 1μF R3 1k C5 0.1μF 10 9 R1 4.3k PROGRAM CHARGE CURRENT IBAT R6 100Ω C7 1μF C6 0.1μF LTC1729-8.4 1 2 3 4 OVP BAT VCC VC SENSE STATUS GND NTC 8 7 6 + R4 4.7k C4 22μF D5 LED1 VBAT 8.4V + Li-Ion BATTERY THERMAL CONTACT 5 R5 10k NTC THERMISTOR DN242 F01 Figure 1. Complete 1.3A Battery Charger for Two Li-Ion Cells 10/00/242_conv The Charge Cycle A typical charge cycle of the circuit in Figure 1 is as follows: with the input voltage applied and no battery connected, the charger output is pulled high by an internal 200μA current source in the LT1510 and clamped at 9V by the LTC1729. Connecting a discharged 2-cell battery to the charger will pull the charger output (BAT pin) down to the battery voltage, starting the charge cycle. For temperature qualification, the voltage on the NTC (thermistor) pin must be between 0.405V and 2.79V, indicating that the temperature is between 0°C and 50°C; otherwise the charge cycle is put on hold until the temperature is within this range. For deeply discharged batteries with voltages below 5.2V, a preconditioning 16mA trickle charge begins and continues until the battery voltage exceeds 5.2V. After the qualification and preconditioning is completed, the constant-current portion of the charge cycle begins (see curves in Figure 2). As the battery accepts charge, the battery voltage rises and approaches the programmed voltage of 8.4V, at which time, the constant-voltage portion of the charge cycle begins. With the battery voltage held constant, the charge current will drop exponentially, eventually reaching tens of mA before the Data Sheet Download www.linear.com Linear Technology Corporation The open-drain STATUS pin is pulled low when the battery is installed and the charge current is greater than 100mA. When the charge current drops below 100mA, a 50μA current source pulls this pin low and, after the 3-hour timer has timed out, this pin is open circuit. The 100mA threshold level is programmable by changing R6. Board Layout and Testing Although the LT1510 can provide charge current in excess of 1A, a good thermal layout of the PC board is required. Wide copper traces for the ground pins, feedthrough vias and generous amounts of copper on both sides of the board are all necessary to minimize the IC temperature rise. When testing the charger, use either a battery or a battery simulator for a load (a conventional electronic load presents a high impedance, unlike a battery). A simple battery simulator consists of an adjustable lab power supply with a load resistor across the output. Select a resistor that will result in approximately twice the rated charge current flowing through it. The power supply can now be used in place of the battery for testing purposes. A fully discharged to a fully charged battery can be simulated by varying the power supply voltage. For additional information, please consult the LT1510 and LTC1729 data sheets. TRICKLE CHARGE CONSTANT CURRENT CHARGE CHARGE CYCLE COMPLETE 1.5 CHARGE CURRENT (A) Complete 2-Cell Li-Ion Charger A complete constant-current/constant-voltage Li-Ion 2-cell charger is shown in Figure 1. The LT1510 provides the charge current and the LTC1729 provides the charge termination. R1 allows the charge current to be easily programmed. 3-hour timer expires, thus ending the charge cycle. CONSTANT VOLTAGE CHARGE 1.3A 1.0 0.5 100mA 16mA 0 9 8.4V 8 BATTERY VOLTAGE (V) LTC1729 Li-Ion Charge Termination IC This 8-pin IC interfaces with the LT1510 (as well as other LTC charger products, such as the LT1511, LT1769, LT1505, LT1512 and LT1513) to provide a complete Li-Ion charger solution. The LTC1729 provides a precision voltage divider for programming the charge voltage and includes a preconditioning trickle charge for deeply discharged cells. Battery temperature is monitored using a thermistor; a 3-hour timer ends the charge cycle. Also included is a status output pin that provides a signal when the charge current drops below a programmable threshold level, indicating a near-full-charge condition. This signal can be used to drive an LED, to provide charge indication to other circuitry or to terminate the charge when the charge current drops below the threshold level. STATUS PIN (WEAK PULL-DOWN) 7 STATUS PIN (PULLED LOW) 6 STATUS PIN GOES HIGH (TIMER STOPS CHARGE CYCLE) 5.2V 5 BATTERY INSTALLED 4 0 1 2 TIME (HOURS) 3 DN242 F02 Figure 2. Charge Current and Battery Voltage for a Typical Charge Cycle For applications help, call (408) 432-1900 dn242f_conv LT/TP 1000 370K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2000