LINEAR TECHNOLOGY JUNE 2006 IN THIS ISSUE… COVER ARTICLE Reduce Charge Time for High Capacity Li-Ion Batteries with 2A Continuous Charging .............1 Tom Hack Issue Highlights ..................................2 Linear Technology in the News….........2 DESIGN FEATURES 650MHz Selectable-Gain Amplifier/ Differential ADC Driver Has Small Form but Many Functions ...................6 Cheng-Wei Pei Dual Step-Up Converter Drives White LEDs with 1000:1 PWM Dimming ......10 Keith Szolusha Hot Swap™ Controller Monitors and Reports Power Supply Status ............12 Josh Simonson Efficient Buck-Boost Converter Ideal for Power Saving Modes and Wide Input Voltage Ranges ..................................16 Kevin Ohlson Dual/Triple Power Supply Monitor for Undervoltage and Overvoltage on Positive and Negative Supplies .....19 Andrew Thomas High Speed Low Power RS485 Transceivers with Integrated Switchable Termination ....................27 Ray Schuler and Steven Tanghe VOLUME XVI NUMBER 2 Reduce Charge Time for High Capacity Li-Ion Batteries with 2A Continuous Charging Introduction The latest high capacity Li-Ion batteries meet the needs of power hungry portable devices, but they also increase the demands placed on battery chargers—demands that can be too much for a standard linear charger. For instance, a linear charger, operating at 1A charging current, charges a 1Ahr battery to 70% capacity within one hour, and fully charges it within three hours. Newer 2Ahr batteries need twice that current in order to be fully charged in the same amount of time. The problem is that a linear charger operating at 2A produces too much heat for continuous charging—it’s just too inefficient. The LTC4001 solves this problem by incorporating a high 1.5A VLDO™ Operates Down to 0.4V Output and Maintains 100mV Dropout .........................................................30 Bill Walter DESIGN IDEAS ....................................................33–46 (complete list on page 33) New Device Cameos ...........................46 Design Tools ......................................47 Sales Offices .....................................48 Figure 1. A typical LTC4001-based Li-Ion battery charger occupies minimal board real estate. by Tom Hack efficiency PWM charger to perform continuous 2A battery charging. It works with both standard and currentlimited wall adapters—where the latter lowers battery charger dissipation and operating temperature. Big Features; Small Footprint A full-featured battery charger based on the LTC4001 requires an area not much larger than a dime (Figure 1). Fully programmable timer and charge rate terminations are included. Automatic battery “topping off” is also included. Filtering prevents accidental recharge from occurring in noisy environments (such as found in GPRS cellular phones). The LTC4001 works readily with NTC thermistors for battery temperature sensing. Remote battery sensing is included. Soft-start is fully programmable. The LTC4001 also drives charge status LEDs and provides logic signals for microprocessor-based designs. The LTC4001 is tiny, fitting into a 4mm × 4mm package, but other factors also contribute to the charger’s small footprint. High operating frequency (1.5MHz) reduces the size of the inductors and capacitors. Input short circuit blocking is built in so no external diode is required. Current continued on page 3 L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES L A Bare Bones Charger LTC4001, continued from page 1 sensing is internal, so there is no need for an expensive milliohm-sized current sense resistor. Inside the LTC4001 The LTC4001 is the basis for a complete 2A Li-Ion battery charger (Figure 2). A 50mA linear charger provides cell conditioning while a synchronous buck charger provides constant-current/constant-voltage high rate charging (up to 2A). Protection and lockouts guard against a variety of events including: shorts at the battery and wall adapter inputs; improper programming of the charge current; open battery and/or overvoltage battery; defective battery; insufficient wall adapter voltage; chip over-temperature; battery over- or under- temperature. 1.5µH WALL ADAPTER 4.5V TO 5.5V + BATSENS BAT 10µF + 4.2V Li-Ion LTC4001 CHRG EN NTC SS GNDSENS PROG IDET TIMER 274Ω 0.1µF Figure 3. A bare bones battery charger 3 5 50mA SENSE CURRENT REVERSAL COMPARATOR + Q 1 2 PGND SW – RAMP SENSE FAULT DRIVER S PWM COMPARATOR SW 10µF MICROPROCESSOR INTERFACE PVIN CLK VINSENSE PVIN PGND 8 OSCILLATOR indicator lights, battery temperature monitoring, and a timer (which may be provided by a microprocessor). In place of a timer, charge terminates when charge current drops below onetenth the high rate charge current (an Figure 3 shows a bare bones 2A battery charger. With only five additional components, this charger offers a high efficiency, high power solution. This implementation leaves out status RD 9 BAT 16 VINSENSE BATSENS OVERCURRENT COMPARATOR SHUTDOWN COMPARATOR + – – + – SS SHUTDOWN LOW BATTERY OVERCURRENT PWM ON TRICKLE ON LOGIC FAULT FAULT TIMER TIMER NTC NTC COMPARATOR CHIP OVERTEMP COMPARATOR CHARGE CURRENT ERROR AMP – + – DISCHARGE SS PROG SHORTED 11 PROG ERROR AMP + 1.2V CHRG TFAULT PROG SHORT COMPARATOR FLOAT VOLTAGE ERROR AMP SOFT-START COPMPARATOR 1.1V + – 15 RECHARGE COMPARATOR LOW CURRENT VIN GOOD RECHARGE + – 10 EN CHRG + – – + + – 7 EN UNDERVOLTAGE COMPARATOR + – 6 IDET COMPARATOR + – 14 LOW-BATTERY COMPARATOR SS SS LOW 150mV OVERVOLTAGE CHIP OVER TEMP BATTERY OVERVOLTAGE COMPARATOR + – CONNECT VOLTAGE REFERENCE GND 17 IDET 13 1.2V PROG 12 GNDSENS 4 Figure 2. Simplified block diagram of the LTC4001 Li-Ion battery charger Linear Technology Magazine • June 2006 3 L DESIGN FEATURES The LED also indicates when the battery is nearly full charged. As the battery approaches the float voltage and charge current drops below the IDET threshold the LED is dimly lit. This is difficult to see, so a better approach uses two LEDs to indicate all charger states (Figure 5). VIN D1 CHRG R1 1k LTC4001 CHRG Interfacing with Microprocessors Figure 4. A simple status indicator VIN R1 27k Q1 2N3906 Q2 TP0610 D1 GRN C/X D2 AMB CHRG R2 1k R3 1k LTC4001 CHRG Q3 2N7002 Figure 5. Full featured status indication IDET threshold equal to 200mA in this case). Internal charge termination may be completely defeated by connecting the timer pin to the IDET pin instead of ground (allowing a microprocessor complete control of charge termination). Adding Status Lights The CHRG pin indicates a variety of charger states (Table 1). Adding a resistor and LED in series with this pin to VIN (Figure 4) indicates charger off (LED off), high rate charging or cell conditioning (LED on continuously at high brightness), and battery temperature out of range/NTC fault (LED blinking). The interface in Figure 6 can distinguish between all states available on the CHRG pin. To detect cell conditioning or high rate charging, force the digital output pin, OUT, high and measure the voltage on the CHRG pin. The N-channel mosfet pulls CHRG low even with a 2k pull-up resistor. Near end of charge, the NMOS turns off, and CHRG sinks only 30µA. The IN pin is pulled high by the 2k resistor connected to OUT. If OUT is placed into a high impedance state, the 30µA sink current from the CHRG pin pulls IN low. When charging stops, CHRG opens and OUT stays high, even with a 390k pull up resistor. If a battery temperature fault occurs during high rate charging, the CHRG pin blinks using a serrated pulse pattern. Nominal timing of this pattern is given in Figure 7. The extra edges provide rapid indication to a microprocessor and may be used to drive a microprocessor interrupt line for low processor overhead, but still provide for a visible fault indication when using LEDs. Operation with Conventional and Current Limited Wall Adapters Wall adapters with or without current limiting may be used with the LTC4001, but the lowest power dissipation battery charging occurs with a current limited wall adapter. To use this feature, program the LTC4001 above the wall adapter current limit. For example, if the wall adapter current limit is 2A, set the LTC4001 charge current slightly higher than 2A (allowing for tolerances). To understand operation with a current limited wall adapter, assume battery voltage VBAT is initially below VTRIKL, the trickle charge threshold (Figure 8). Battery charging begins at approximately 50mA, well below the wall adapter current limit so the voltage into the LTC4001 (VIN) is the wall adapter’s rated output voltage (VADAPTER). Battery voltage rises eventually reaching VTRIKL. The linear charger shuts off and the PWM (high rate) charger turns on using soft start. Battery charging current rises during the soft-start cycle causing a corresponding increase in wall adapter load current. When the wall adapter reaches current limit, the wall adapter output voltage collapses, and VIN VDD Battery Temperature Sensing By adding one resistor and one thermistor, battery temperature sensing may be included. The LTC4001 is designed for Vishay Dale’s “R-T Curve 2” therm- LTC4001 CHRG R1 390k R2 2k µPROCESSOR OUT IN Figure 6. A microprocessor interface Table 1. CHRG Behavior 4 istors, but any thermistor with an RCOLD-to-RHOT ratio of about 7 will also work. If battery sensing is not needed, the NTC pin is grounded. Charger State CHRG Behavior Not charging Open High rate Charging and IBAT>IDET Or cell conditioning NMOS turned on pulling pin low High Rate Charging and IBAT<IDET 30µA pull down current NTC temperature fault while charging at IBAT>IDET Blink 20µs 667ms Figure 7. CHRG temperature fault waveform Linear Technology Magazine • June 2006 DESIGN FEATURES L LINEAR CHARGING VADAPTER WALL ADAPTER IN CURRENT LIMIT So how does LTC4001 dissipation stack up against a 2A linear charger? Most of a linear charger’s dissipation occurs in the series pass element so the dissipation is approximately equal to the voltage drop in the pass element times the charge current. Worst case dissipation occurs at the lowest battery voltage where high rate charging occurs (to make a valid comparison to the LTC4001 this would be 2.85V). For a 5.0V input, this translates into a dissipation of 4.3W! Higher input voltage makes the situation even worse. PWM CHARGING VBAT + VDROP VIN ILIMIT IBAT ITRICKLE VTRIKL VFLOAT VBAT Figure 8. Idealized charging behavior Low Dissipation Trickle charging uses a linear charger but low charge current produces low power dissipation, typically 256mW (VIN = 5V, VBAT = 0). High rate charging uses a high efficiency buck switcher and total charger dissipation is approximately 1.2W at 2A (Figure 9). High rate charging with a current limited wall adapter produces even lower charger dissipation (537mW at VBAT = 4.2V with a 2A current limited wall adapter) because there is very little voltage drop for the battery charging path inside the LTC4001. 1.25 TOTAL APPLICATION CIRCUIT POWER DISSIPATION (W) the LTC4001 PWM charger duty cycle ramps up to 100% (the top-side PMOS switch in the LTC4001 buck regulator stays on continuously.) As the battery voltage approaches VFLOAT, the float voltage error amplifier commands the PWM charger to deliver less than ILIMIT. The wall adapter exits current limit and VIN jumps back up to VADAPTER. Battery charging current continues to drop as VBAT rises, dropping to zero at VFLOAT. Because the voltage drop in the LTC4001 is very low when charge current is highest, power dissipation is also very low. R12 27k Q2 TP0610 D1 GRN C/X D2 AMB CHRG R1 1k Q3 2N7002 R2 1k A full featured battery charger is shown in Figure 10. It includes a three hour timer, battery temperature monitoring, programmable charge and IDET currents, remote sensing, and status lights. A fault light has been included that indicates when a shorted battery is detected or when the battery is out of normal temperature range. VIN = 5V VBAT = 4V 1.00 Conclusion The LTC4001 sets a new standard for small, low parts count, full-featured, high efficiency Li-Ion battery chargers. Low power dissipation makes continuous 2A battery charging practical, cutting dissipation to approximately one fifth the dissipation of a straight linear charger. L 0.75 0.50 0.25 0 500 1500 1000 2000 IBAT (mA) Figure 9. High rate charger power dissipation VIN 2A CURRENT LIMITED WALL ADAPTER 4.5V TO 5.5V Q1 2N3906 A Charger with All the Bells and Whistles R8 10k L1 1.5µH 2A SW VINSENSE PVIN C1 10µF 10V SENSE BATSENS BAT 10µF 10V PGND + 2Ahr 4.2V Li-Ion LTC4001 CHRG GNDSENS EN GND NTC FAULT EXT NTC PROG R9 1.33k R10 10k AT 25°C D3 RED FAULT R3 1k R4 2A 549Ω C3 0.22µF R5 1A 1.10k TIMER SS IDET C4 0.1µF R7 0.1A 1.10k R6 0.2A 549Ω L1: VISHAY DALE IHLP-2525AH-01 R10: NTC VISHAY DALE NTHS0603N02N1002J Figure 10. A full featured battery charger Linear Technology Magazine • June 2006 5