DESIGN FEATURES Very Low Cost Li-Ion Battery Charger Requires Little Area and Few Components by David Laude A Simple, Low Cost The LTC1734 is a precision, low cost, Li-Ion Charger Introduction single-cell, linear Li-Ion battery charger with constant voltage and constant current control. The small quantity and low cost of external components results in a very low overall system cost and the part’s 6-pin SOT23 package allows for a compact design solution. Previous products usually required an external current sensing resistor and blocking diode whose functions are included in the LTC1734. Other features include: ❏ 1% accurate float voltage options of 4.1V or 4.2V ❏ Programmable constant current range of 200mA to 700mA ❏ Charging current monitor output and manual shutdown for use with a microcontroller ❏ Automated shutdown with no battery drain after supply removal ❏ Undervoltage lockout ❏ Self protection for overcurrent and overtemperature Applications include such compact devices as cellular phones, digital cameras and handheld computers. The LTC1734 can also be used as a general purpose current source or for charging nickel-cadmium or nickelmetal-hydride batteries. A battery charger programmed for 300mA in the constant current mode with a charge current monitoring function is shown in Figure 1. The PNP transistor is needed to source the charging current and resistor R1 is used to program the maximum charging current. Note that no external current sense resistor or diode to block current is required. When the supply is opened or shorted to ground, the charger shuts down and no quiescent current flows from the battery to the charger. This feature extends battery life. Capacitor C2 can consist of up to 100µF of bypass caps, which would normally be distributed along the battery line. The supply voltage can range from 4.75V to 8V, but power dissipation of the PNP may become excessive near the higher end. The programming pin (PROG) accomplishes several functions. It is used to set the current in the constant current mode, monitor the charging current and manually shut down the charger. In the constant current mode, the LTC1734 maintains the PROG pin at 1.5V. The PROG pin voltage drops below 1.5V as the constant voltage mode is entered and charge current drops off. The charge current is always one thousand times the current through R1 and is therefore proportional to the PROG pin voltage. At 1.5V the current is the full 300mA, whereas at 0.15V the current is 300mA/10 or 30mA. If the grounded side of R1 is pulled above 2.15V or is allowed to float, the charger enters the manual shutdown mode and charging ceases. These features enable smarter charging by allowing a microcontroller to monitor the charging current and shut down the charger at the appropriate time. The ISENSE and BAT pins are used to monitor charge current and battery voltage, respectively; the DRIVE pin controls the PNP’s base. A Programmable Constant Current Source An example of a programmable current source is shown in Figure 2. To ensure that only the constant current mode is activated, the BAT pin is tied to ground to prevent the constant voltage control loop from engaging. The control inputs either float or are connected to ground. This can be achieved by driving them from the drains of NMOS FETs or from the collectors of NPNs. When both inputs are floating, manual shutdown is continued on page 15 LTC1734 1 3 VIN 5V 2 C1 1µF ISENSE VCC GND LTC1734 DRIVE BAT PROG 6 4 R1 5.1k Q1: ZETEX FMMT549 (631) 543-7100 CHARGE CURRENT MONITOR Figure 1. Simple, low cost charger programmed for 300mA output current 12 1 Q1 5 C2 10µF SINGLE Li-Ion CELL VIN 5V C1 1µF Q1: ZETEX FCX589 3 2 ISENSE VCC GND DRIVE BAT PROG 6 Q1 5 4 LOAD R1 3k (631) 543-7100 R2 7.5k CONTROL 1 CONTROL 2 Figure 2. Programmable current source with output current of 0mA, 200mA, 500mA or 700mA Linear Technology Magazine • February 2001 DESIGN FEATURES capacitance. Figures 3a and 3b show the response of the LTC1773 to a 100mA to 5A load step during Burst Mode and continuous mode operations, respectively. 2.7V ≤ VIN ≤ 6V 47pF 30k 1 220pF 2 0.1µF 3 1.8V/7A Application Figure 4 shows a step-down application from 3.3V to 1.8V at 7A. When operating below 5V, care should be taken to choose the proper sublogiclevel MOSFETs that have relatively low gate charge. For high current (>3A) applications, single P-channel and N-channel MOSFETs should be used instead of complementary MOSFETs in one package. A good figure of merit for MOSFETs is the RDS(ON) gatecharge product. The lower this value is, the higher the application’s efficiency will be. In addition to normal step-down applications, the LTC1773 can also be used in a zeta converter configuration that will do both step-down and step-up conversions, as shown in Figure 5. This application is ideal for battery-powered operation, in which a regulated 3.3V output is maintained during the entire discharge cycle of a Li-Ion battery from 4.7V to 2.5V. Conclusion The LTC1773 offers flexibility, high efficiency and many other popular features in a small MSOP-10 package. For low voltage portable systems that require small footprint and high efficiency, the LTC1773 is the ideal solution. 4 5 100pF LTC1773 ITH 10 SW RUN/SS SENSE SYNC/FCB 8 VIN VFB TG GND BG 99k 1% RSENSE 0.01Ω 9 – M1 7 6 M2 + 100pF 80.6k 1% D2* MBRS340T3 CIN: PANASONIC SPECIAL POLYMER COUT: KEMET T510687K004AS L1: TOKO TYPE D104C 919AS-1RON RSENSE: IRC LR2512-01-R010-J M1: FAIRCHILD FDS6375 M2: SILICONIX Si9804DY (714) 737-7334 (408) 986-0424 (847) 699-3430 (361) 992-7900 (408) 822-2126 (800) 554-5565 Linear Technology Magazine • February 2001 + CIN 150µF 6.3V 4.7µF 6.3V 0.1µF COUT 680µF 4V ×2 *NOTE: D2 IS OPTIONAL. IF REMOVED, EFFICIENCY DROPS BY 1% Figure 4. 3.3V to 1.8V/7A regulator 33pF 2.7V ≤ VIN ≤ 4.2V 30k 200pF 1 2 0.1µF VIN 3 4 5 LTC1773 ITH SW RUN/SS SENSE– SYNC/FCB VIN VFB TG GND BG 10 9 RSENSE 0.025Ω 8 M1 47µF L1 2µH 7 + + M2 80.6k 1% CIN: COUT: L1: RSENSE: M1: M2: 249k 1% SANYO POSCAP 6TPA150M AVX TPSD227M006R0100 COILTRONICS CTX2-4 IRC LR1206-01-R033-F SILICONIX Si9803DY SILICONIX Si9804DY VOUT 3.3V 1A 6 + CIN 150µF 6.3V COUT 220µF 6.3V L1 2µH (714) 373-7334 (207) 282-5111 (561) 752-5000 (361) 992-7900 (800) 554-5565 Figure 5. Single Li-Ion cell to 3.3V/1A synchronous zeta converter Conclusion LTC1734, continued from page 12 entered. Connecting Control 1 to ground causes 500mA of current to flow into the load, whereas Control 2 results in 200mA of current. When both control inputs are grounded the current is 700mA. A voltage DAC, VOUT 1.8V 7A L1 1µH connected to the PROG pin through a resistor, could also be used to control the current. Applications include charging nickel-cadmium or nickelmetal-hydride batteries, driving LEDs or biasing bridge circuits. Low cost, small footprint, reduced component count, precision and versatility make the LTC1734 an excellent solution for implementing compact and inexpensive battery chargers or constant current sources. 15