DESIGN IDEAS 4.5µA Li-Ion Battery Protection Circuit by Albert Lee Li-Ion Battery Undervoltage Lockout Figure 1 shows an ultralow power, precision undervoltage-lockout circuit. The circuit monitors the voltage of a Li-Ion battery and disconnects the load to protect the battery from deep discharge when the battery voltage drops below the lockout threshold. Storing a battery-powered product in a discharged state puts the battery at risk of being completely discharged. In a discharged condition, current to the protection circuitry continuously discharges the battery. If the battery is discharged below the recommended end-of-discharge voltage, overall battery performance degrades, the cycle life is shortened and the battery may die prematurely. In contrast, if the lockout voltage is set too high, maximum battery capacity is not realized. The low-battery mode of operation is indicated when, for instance, a cell phone automatically powers down after the battery-low indicator has been flashing for some time. If the phone is misplaced in this condition and found months later, the protection circuitry shown in Figure 1 will not overdrain and damage the battery because the protection circuitry takes less than 4.5µA of current. At this low current, the time the Li-Ion battery takes to reach the end-of-discharge voltage is significantly extended. For other protection circuitry that typically requires higher current, the rate of discharge is faster, allowing the battery voltage to drop below the safe limit in a shorter time. Note that if the battery is allowed to discharge below the safe limit, unrecoverable capacity loss occurs. The Micropower Voltage Reference and Op Amp The LT1389 is not just another voltage reference. Its very low current consumption makes it the ideal choice for applications that require maximum battery life and excellent precision. It requires only 800nA of current and provides 0.05% initial voltage accuracy and 20ppm/°C maximum temperature drift, equating to 0.19% absolute accuracy over the commercial temperature range and 0.3% over the industrial temperature range. Operating at one-fifteenth the current required by typical references with comparable accuracy, the LT1389 is the lowest power voltage reference available today. The LT1389 precision shunt voltage reference is available in four fixed-voltage versions: 1.25V, 2.5V, 4.096V and 5.0V. It is available in the 8-lead SO package, in commercial and industrial temperature grades. Low power (IS < 1.5µA) and precision specifications make the LT1495 rail-to-rail input/output op amp the perfect companion to the LT1389. The extremely low supply current is combined with excellent amplifier specifications: input offset voltage is 375µV maximum with a typical drift SW1 VBATT R3 2.05M 1% R1 3.57M 0.1% RSW 1M 5% + B TO LOAD U1 1/2 LT1495 A Li-Ion CELL 4.1V R2 3M 0.1% U2 LT1389 1.250V R4 150k 1% – R5 10M 5% SW1: PMOS SPECIFIED FOR MAXIMUM LOAD CURRENT R2, R3: IRC CAR6 SERIES (512) 992-7900 of only 0.4µV/°C, input offset current is 100pA maximum and input bias current is 1nA maximum. The device characteristics change little over the supply range of 2.2V to ±15V. The low bias currents and offset current of the amplifier permit the use of megohmlevel source resistors without introducing significant errors. The LT1495 is available in plastic 8-pin PDIP and SO-8 packages with the standard dual op amp pinout. Consuming virtually no current, the LT1389 and the LT1495 are ideal choices for the UVLO circuit and many other battery applications. Circuit Operation The circuit is set up for a single-cell LiIon battery, where the lockout voltage—the voltage when the protection circuit disconnects the load from the battery— is 3.0V. This voltage, set by the ratio of R1 and R2, is sensed at node A. When the battery voltage drops below 3.0V, node A falls below the threshold at node B, which is defined as: VB = 1.25V + I • R4 = 1.37V where I = (Vt – 1.25V)/(R3 + R4) = 800nA Vt = lockout voltage The output of U1 will then swing high, turning off SW1 and disconnecting the load from the battery. However, once the load is removed, the battery voltage rebounds and will cause node A to rise above the reference voltage. The output of U1 will then switch low, reconnecting the load to the battery and causing the battery voltage to drop below 3.0V again. The cycle repeats itself and oscillation occurs. To avoid this condition, R5 is added to provide some hysteresis around the trip point. When the output of U1 swings high to shut off SW1, node B is bumped up 42mV above node A, preventing oscillation around the trip Figure 1. Undervoltage lockout circuit 36 Linear T echnology Magazine • June 1999 DESIGN IDEAS point. Using the formula below, the amount of hysteresis for the circuit is calculated to be 92mV. Hence, VBATT must climb back above 3.092V before the battery is connected. Hysteresis = VB' • R1/R2 + VB – Vt where VB' = (VOMAX – I • R4) • R4/R5 + VREF + I • R4 Vt = lockout voltage VOMAX = maximum output swing (high) of U1 at VBATT is equal to the lockout voltage ∆92mV The worst-case voltage-monitor accuracy is better than 0.4%. Interestingly, the battery’s longevity and capacity are directly related to the depth of discharge. More cycles can be obtained C2** 22µF S/S LT1512 GND GND 3.092 3 VBATT (V) Figure 2. VBATT vs VA with hysteresis Si4936DY DCIN 3.3V D1 MBRS130LT3 100mA VSW BAT54C L1B* LTC1473L FB R1 47.5k VC IFB R4 24Ω C5 0.1µF R5 1k LOAD CONNECTED LOAD DISCONNECTED L1A* VIN There need not be a trade-off between performance and current consumption. The LT1389 nanopower precision shunt voltage reference and the LT1495 1.5µA precision rail-torail input/output op amp deliver the highest performance with virtually zero current consumption. ∆42mV 1.37 Being Precise SYNC AND/OR SHDN Conclusion VB (V) Consult the battery manufacturer regarding the maximum ESR at maximum recommended discharge current. Multiply the two values to get the minimum hysteresis required. C3 22µF 25V ±155mV, cutting off at either at 2.945V or at 3.255V. At a lockout voltage of 3.255V, maximum capacity is not obtained. In addition, the operating range is reduced, with the fully charged battery voltage being 4.1V. For a 0.4% overall accurate system, the lockout voltage would be at 3.088V or at 3.112V, more than twelve times better accuracy and optimally achieving the highest capacity. Furthermore, the load is kept disconnected with only 4.5µA to the protection circuit. Thus, the protection circuit works by preventing deep discharge of the battery. by partially rather than fully discharging the Li-Ion battery, and, conversely, more use time can be obtained by fully discharging a Li-Ion battery. Cutting off the load at the perfect end-of-discharge voltage would ideally result in the best of both cases. To perform this task requires an accurate overall system. For example, if the optimum lockout voltage is to be set at 3.1V, a 5% overall accurate system would yield C4 0.22µF R3 1Ω C1 22µF 25V R2 12.4k CTIMER 4700pF GA1 IN2 SAB1 DIODE GB1 TIMER + V+ + *L1A, L1B ARE TWO 33µH WINDINGS ON A SINGLE CORE: COILTRONICS CTX33-3 (561) 241-7876 **TOKIN CERAMIC 1E22ZY5U-C203-F (408) 432-8020 IN1 1mH 1µF + 1µF SENSE RSENSE 0.04Ω + SENSE – VGG GA2 SW SAB2 GND GB2 3.3V OR VBAT1 COUT BAT1 4 NiMH 3 Si4936DY Figure 2. Battery-backup circuit with LT1512 battery charger PowerPath, continued from page 31 the external NMOS switches are allowed to be in current limit, and the value of RSENSE determines the inrush current limit, which is set at 2× to 3× of the maximum required output current. When V+ falls below 2.5V, the LTC1473L’s undervoltage lockout circuit turns off both switches. With a Linear T echnology Magazine • June 1999 built-in hysteresis of 100mV, the LTC1473L becomes active again when V+ rises above 2.6V. Therefore, for 3.3V systems, small Schottky diodes are used to power V+ from both DCIN and BAT1 so that the undervoltage lockout circuit will not be falsely tripped. Since the LTC1473L has an IQ of less than 100µA at 3.3V, the drop across the Schottky diode is less than 0.4V, leaving enough room for a typical ±5% supply tolerance. Glitch-free and seamless transition of power is crucial for maintaining normal operation in low voltage electronic equipment. The LTC1473L makes the transition transparent and trouble free. (For systems using supply voltages between 6V and 28V, refer to the LTC1473 data sheet.) 37