designfeature Sam Davis, Editor in Chief Power-Tracking Battery-Charger IC Supports Solar-Power Systems I n non-utility grid applications solar panels produce dc power for emergency roadside telephones, navigation buoys, and other remote loads. Virtually all 12-V-system solar panels comprise a series of photovoltaic cells that have a maximum output power of less than 25 W. In producing this power the solar-panel system uses a battery to provide power when the panel is “dark.” The rechargeable battery can supply power for long periods of time, requiring a charger that can properly operate a solar panel. Meeting this need is Linear Technology’s LT3652 monolithic buck-charger IC, which operates with a single solar panel. The IC uses average-current-mode control-loop architecture to provide constant current/constant voltage (CC/CV) charge characteristics with a programmable charge current. The charger can be programmed to proCMSH1-40MA SOLAR PANEL duce a 14.4-V float voltage. Housed in a SYSTEM LOAD INPUT CMSH3-40MA 3- x 3-mm DFN-12 package, the IC can (<40 OC VOLTAGE) LT3652 charge a variety of battery configurations, SW 530 k CMSH3-40MA VIN including up to three Li-Ion/Polymer 1 µF 10 µH VIN_REG 10 µF cells in series, up to four Lithium Iron BOOST 0.05 SHDN 100 k Phosphate (LiFePO4) cells in series, and SENSE CHRG sealed lead-acid batteries up to 14.4 V. BAT FAULT 542 k Depending on panel characteristics, NTC 10 µF TIMER VFB the LT3652 can produce peak power 459 k in excess of 95%, with panel output voltages from 12.5 V to 18.5 V. Its + 10 k B = 3,380 input-voltage-regulation loop controls the solar-panel output voltage to pro2-CELL LiFePO4 (2 x 3.6 V) duce peak output power while charging BATTERY PACK the associated battery, delivering nearly Fig. 1. A 2-A solar-panel power manager with a 7.2-V LiFePO4 battery and 17-V peak-power tracking. If the panel the same efficiency as more complex voltage drops to zero, the battery supplies power to the load. The 10-µF input capacitor absorbs input switching and expensive Maximum Peak Power ripple current, so it must have an adequate ripple current rating. Tracking (MPPT) techniques. Fig. 1 shows a typical solar panelcharger application circuit with a 7.2-V LiFePO4 battery pack. Connecting a resistor 2.5 divider from VIN (pin 1) to VIN_REG (pin 2) programs the minimum operational input voltage, which in turn programs the peak power voltage for the solar panel. The 2.0 LT3652 controls the maximum charge current required to maintain the programmed operational VIN voltage by maintaining the voltage on VIN_REG at or above 2.7 V. If 1.5 LT3652Fig1_callouts_sept2009 VIN_REG goes below 2.7 V it reduces the maximum charge current and maintains the 1.0 panel at the output voltage corresponding to its peak output power point. Fig. 2 plots the LT3652’s maximum charger-output current vs. input 0.5 Fig. 2. Input voltage regula- voltage from a solar panel. tion described by the maximum The LT3652 accepts inputs from 4.95 V to 32 V, with 0 16.0 16.5 17.0 17.5 18.0 charger output current vs. the a 40-V maximum rating. Its input-voltage-regulation loop VIN (V) input voltage, VIN. also allows optimized charging from poorly regulated sourcCHARGER OUTPUT CURRENT (A) Individual solar-panel systems produce dc power for remote applications while also storing energy in a rechargeable battery supported by a batterycharger IC. 32 Power Electronics Technology | September 2009 www.powerelectronics.com Solar-panelcharger MBRS140 SOLAR PANEL INPUT <40-V OC 10 VOLTAGE 499 k µF 16-V PEAK POWER VOLTAGE MBRS340 es where the input can colLT3652 SW lapse in overcurrent condiVIN BZX84C6V2L 1 µF tions. An integrated 2-A VIN_REG BOOST WURTH 22 µH switch driven by a boot747779122 0.1 1N914 SHDN SYSTEM SENSE strapped supply maximizes 100 k LOAD CHRG BAT efficiency during charging. + FAULT 4.7 µF 100 µF 10 µF NTC The LT3652 can be con10 k TIMER 309 k VFB B = 3,380 figured to charge at average + mvRata 174 k currents to 2 A. The user NCP18XH103 12-V LEAD ACID BATTERY 1M 910 sets the max charge cur1N4148 100 k rent by choosing an inductor sense resistor, so that the max average current through the sense resistor Fig. 3. 1-A solar panel powered three-stage 12-V lead-acid fast/float charger. creates a 100-mV drop. The sense resistor in Fig. 1 is 0.05 Ω. Besides having the appropriate The IC contains provisions for a battery-temperature moniinductance, the selected inductor must have a saturation curtoring circuit using a thermistor during the charging cycle. If rent equal to or exceeding maximum peak current. the battery temperature moves outside the safe charging range (0° to 40°C), the IC suspends charging and signals a fault conCharging the Battery dition until the temperature returns to the safe range. The charge and discharge capacity of a battery is in terms of A shutdown mode can disable all charging functions. This “C,” given as ampere-hours (Ah). The actual battery capacity precision threshold allows the use of the SHDN pin 3 to incordepends on the C-rate and temperature. Most batteries are porate UVLO (undervoltage lockout) functions. Pulling the rated at 1 C. A discharge of 1 C draws a current equal to the SHDN pin below 0.4 V causes the IC to enter a low-current rated capacity. For example, a battery rated at 1,000 mAh shutdown mode where VIN current reduces to 15 μA. Typical LT3652Fig3_callouts_sept2009 provides 1,000 mA for one hour if discharged at a 1-C rate. SHDN input bias current is 10 nA. When not charging, the IC The LT3652 charger IC uses a charge-current-based C/10 draws less than 1 µA from the battery. termination scheme to end a charge cycle when the battery The open-collector CHRG pin 4 and the FAULT pin 5 charge current falls to one-tenth of its programmed maximum usually use a resistor pulled up to a reference voltage. These charge current. It also contains an internal charge-cycle control status pins can be pulled up to voltages as high as VIN when timer for timer-based termination. When using the internal disabled, and can sink currents up to 10 mA when enabled. timer, the IC combines C/10 detection with a programmable During a battery charging cycle, if the required charge time constraint, during which the charging cycle can continue current is greater than 1/10 of the programmed max current beyond the C/10 level to top-off a battery. (C/10), CHRG pin 4 is pulled low. A temperature fault also A capacitor connected from Timer pin 6 to ground sets the causes this pin to be pulled low. After C/10 charge terminaend-of-charge (EOC) time. A 0.68-μF capacitor generates a tion, or if the internal timer is used for termination and charge three-hour timer EOC and a precondition limit time of 22.5 current is less than C/10, the pin stays at high-impedance. min. When using the timer-based scheme, the IC also supFAULT pin 5 indicates fault conditions during a battery ports bad battery detection, which triggers a system fault if a charging cycle. A temperature fault causes this pin to be battery stays in precondition mode for more than one-eighth pulled low. If the internal timer is used for termination, a bad of the total charge-cycle time. battery fault also causes this pin to be pulled low. If no fault If a timer-based termination is not desired, you can disable conditions exist, this pin remains high-impedance. the timer function by connecting the TIMER pin to ground (Fig. 1). With the timer function disabled, charging terminates Sealed Lead-Acid Battery Application when the charge current drops below a C/10 threshold. Fig. 3 shows a sealed lead-acid battery using the LT3652. This After terminating the charge, the LT3652 automatically 1-A circuit fast-charges with CC/CV characteristics up to 14.4 enters a low-current standby mode where supply bias currents V. If the charge current falls to 0.1 A, the charger switches to drop to 85μA. The IC continues to monitor the battery volt13.5-V float-charge mode. age while in standby, and if that voltage falls 2.5% from the The charger re-initiates the 14.4-V fast-charge mode if the full-charge float voltage, the LT3652 engages an automatic battery voltage falls below 13.2 V and trickle charges at 0.15 charge-cycle restart. The IC also automatically restarts a new A if the battery voltage is below 10 V. A resistor divider charge cycle after a bad battery fault once the failed battery is between BAT pin 9 and VFB pin 7 programs the output batremoved and replaced with another battery. tery float voltage. www.powerelectronics.com September 2009 | Power Electronics Technology 33