LTC4080EMSE-TRPBF

FEATURES Complete Linear Battery Charger with Integrated Buck Converter Battery Charger: ■ Constant-Current/Constant-Voltage Operation with Thermal Feedback to Maximize Charge Rate Without Risk of Overheating ■ Internal 4.5 Hour Safety Timer for Termination ■ Charge Current Programmable Up to 500mA with 5% Accuracy ■ C/10 Charge Current Detection Output ■ 5μA Supply Current in Shutdown Mode Switching Regulator: ■ High Efficiency Synchronous Buck Converter ■ 300mA Output Current ■ 2.7V to 4.5V Input Range (Powered from BAT Pin) ■ 0.8V to V BAT Output Range ■ MODE Pin Selects Fixed (2.25MHz) Constant-Frequency PWM Mode or Low ICC (23μA) Burst Mode® Operation ■ 2μA BAT Current in Shutdown Mode ■ LTC4080 500mA Standalone Li-Ion Charger with Integrated 300mA Synchronous Buck DESCRIPTIO The LTC4080 is a complete constant-current/constantvoltage linear battery charger for a single-cell 4.2V lithium-ion battery with an integrated 300mA synchronous buck converter. The small packages and low external component count make the LTC4080 especially suitable for portable applications. Furthermore, LTC4080 is specifically designed to work within USB power specifications. The ⎯ C⎯ H⎯ R⎯G pin indicates when charge current has dropped to ten percent of its programmed value (C/10). An internal 4.5 hour timer terminates the charge cycle. The full-featured LTC4080 battery charger also includes trickle charge, automatic recharge and soft-start (to limit inrush current). The LTC4080 integrates a synchronous buck converter that is powered from the BAT pin. It has an adjustable output voltage and can deliver up to 300mA of load current. The buck converter also features low-current highefficiency Burst Mode operation that can be selected by the MODE pin. The LTC4080 is available in 10-lead, low profile (0.75 mm) 3mm × 3mm DFN and MSOP Exposed Pad packages. , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 6522118. APPLICATIO S ■ ■ ■ ■ Wireless Headsets Bluetooth Applications Portable MP3 Players Multifunction Wristwatches TYPICAL APPLICATIO VCC (3.75V to 5.5V) CIN 4.7μF VCC LTC4080 EN_CHRG EN_BUCK SW FB BAT Li-Ion Battery Charger with 1.8V Buck Regulator 500mA EFFICIENCY (%) L1, 1OμH CPL 10pF R1 1M CBAT 4.7μF + 4.2V Li-Ion BATTERY MODE GND PROG RPROG 806Ω R2 806k 4080 TA01a VOUT (1.8V/300mA) COUT 4.7μF U 20 U U Buck Efficiency vs Load Current (VOUT = 1.8V) 100 EFFICIENCY (Burst) EFFICIENCY (PWM) 1000 80 100 POWER LOSS (mW) POWER LOSS 10 (PWM) 0 60 40 POWER LOSS (Burst) 0 0.01 VBAT = 3.8V 0.1 VOUT = 1.8V L = 10μH C = 4.7μF 0.01 0.1 1 10 100 1000 LOAD CURRENT (mA) 4080 TA01b 4080fb 1 LTC4080 ABSOLUTE AXI U RATI GS (Note 1) BAT Pin Current ...................................................800mA PROG Pin Current ....................................................2mA Junction Temperature .......................................... .125°C Operating Temperature Range (Note 2) .. – 40°C to 85°C Storage Temperature Range.................. – 65°C to 125°C Lead Temperature (MSE, Soldering, 10 sec) ......... 300°C VCC , t < 1ms and Duty Cycle < 1% .............. – 0.3V to 7V VCC Steady State ......................................... – 0.3V to 6V BAT, ⎯C⎯H⎯R⎯G .................................................. – 0.3V to 6V ⎯E⎯N⎯_⎯C⎯H⎯R⎯G, PROG, ⎯A⎯C⎯P⎯R .................– 0.3V to VCC + 0.3V MODE, EN_BUCK .......................... – 0.3V to VBAT + 0.3V FB ............................................................... – 0.3V to 2V BAT Short-Circuit Duration............................Continuous PIN CONFIGURATION TOP VIEW BAT VCC EN_CHRG PROG ACPR 1 2 3 4 5 11 10 SW 9 EN_BUCK 8 MODE 7 FB 6 CHRG TOP VIEW BAT VCC EN_CHRG PROG ACPR 1 2 3 4 5 10 9 8 7 6 SW EN_BUCK MODE FB CHRG DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN TJMAX = 110°C, θJA = 43°C/W (NOTE 3) EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH LTC4080EDD#PBF LTC4080EMSE#PBF TAPE AND REEL LTC4080EDD#TRPBF LTC4080EMSE#TRPBF PART MARKING LBXD LTCQH PACKAGE DESCRIPTION 10-Lead (3mm × 3mm) Plastic DFN 10-Lead Plastic MSOP TEMPERATURE RANGE –40°C to 85°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2) SYMBOL VCC VBAT ICC ICC_SD PARAMETER Supply Voltage Input Voltage for the Switching Regulator CONDITIONS (Note 4) (Note 5) ● ● ● ● ELECTRICAL CHARACTERISTICS Quiescent Supply Current (Charger On, VBAT = 4.5V (Forces IBAT and IPROG = 0), Switching Regulator Off) VEN_BUCK = 0 Supply Current in Shutdown (Both Battery Charger and Switching Regulator Off) Battery Current in Shutdown (Both Battery Charger and Switching Regulator Off) V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V, VEN_BUCK = 0, VCC > VBAT V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V) V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V, VEN_BUCK = 0, VCC > VBAT V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 4V, VEN_BUCK = 0, VCC (3.5V) < VBAT (4V) IBAT_SD 2 U WW W 11 MSE PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 125°C, θJA = 40°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB MIN 3.75 2.7 TYP 5 3.8 110 5 2 0.6 2 MAX 5.5 4.5 300 10 UNITS V V μA μA μA μA μA 4080fb ● 5 LTC4080 ELECTRICAL CHARACTERISTICS SYMBOL Battery Charger VFLOAT IBAT VUVLO_CHRG VPROG VASD tSS_CHRG ITRKL VTRKL VTRHYS ΔVRECHRG ΔVUVCL1, ΔVUVCL2 tTIMER VBAT Regulated Output Voltage Current Mode Charge Current VCC Undervoltage Lockout Voltage PROG Pin Servo Voltage IBAT = 2mA IBAT = 2mA, 4.3V < VCC < 5.5V RPROG = 4k; Current Mode; VEN_BUCK = 0 RPROG = 0.8k; Current Mode; VEN_BUCK = 0 VCC Rising VCC Falling 0.8k ≤ RPROG ≤ 4k ● ● ● ● ● ● The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2) PARAMETER CONDITIONS MIN 4.179 4.158 90 475 3.5 2.8 0.98 60 15 35 ● TYP 4.2 4.2 100 500 3.6 3.0 1.0 82 32 180 50 2.9 150 100 300 130 4.5 2.25 1.125 0.1 115 750 2 75 MAX 4.221 4.242 110 525 3.7 3.2 1.02 100 45 65 3.05 350 130 UNITS V V mA mA V V V mV mV μs mA V mV mV mV mV Automatic Shutdown Threshold Voltage (VCC – VBAT), VCC Low to High (VCC – VBAT), VCC High to Low Battery Charger Soft-Start Time Trickle Charge Current Trickle Charge Threshold Voltage Trickle Charge Threshold Voltage Hysteresis Recharge Battery Threshold Voltage (VCC – VBAT) Undervoltage Current Limit Threshold Voltage Termination Timer Recharge Time Low-Battery Charge Time VBAT = 2.5V RPROG = 2k (Note 6) VFLOAT – VBAT, 0°C < TA < 85°C IBAT = 0.9 ICHG IBAT = 0.1 ICHG ● ● ● ● VBAT = 2V, RPROG = 0.8k VBAT Rising 2.75 100 70 180 90 3 1.5 0.75 0.085 6 3 1.5 0.115 hrs hrs hrs mA/mA °C mΩ Hz % IC/10 TLIM RON_CHRG fBADBAT DBADBAT Buck Converter VFB IFB fOSC IBAT_NL_CF IBAT_NL_BM IBAT_SLP VUVLO_BUCK End of Charge Indication Current Level Junction Temperature in ConstantTemperature Mode IBAT = 350mA, VCC = 4V Power FET On-Resistance (Between VCC and BAT) ⎯⎯⎯⎯ Defective Battery Detection CHRG Pulse VBAT = 2V Frequency ⎯⎯⎯⎯ Defective Battery Detection CHRG Pulse VBAT = 2V Frequency Duty Ratio FB Servo Voltage FB Pin Input Current Switching Frequency No-Load Battery Current (Continuous Frequency Mode) No-Load Battery Current (Burst Mode Operation) Battery Current in SLEEP Mode Buck Undervoltage Lockout No-Load for Regulator, V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V, L = 10μH, C = 4.7μF No-Load for Regulator, V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V, MODE = VBAT, L = 10μH, C = 4.7μF V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V, MODE = VBAT, VOUT > Regulation Voltage VBAT Rising VBAT Falling ● ● ● ● 0.78 –50 1.8 0.80 2.25 1.9 23 0.82 50 2.75 V nA MHz mA μA VFB = 0.85V ● 10 2.6 2.4 15 2.7 2.5 20 2.8 2.6 μA V V 4080fb 3 LTC4080 ELECTRICAL CHARACTERISTICS SYMBOL RON_P RON_N ILIM_P ILIM_N IZERO_CF IPEAK IZERO_BM tSS_BUCK Logic VIH VIL VOL IIH IIL R⎯E⎯N⎯_⎯C⎯H⎯R⎯G I⎯C⎯H⎯R⎯G I⎯A⎯C⎯P⎯R Input High Voltage Input Low Voltage Output Low Voltage (⎯C⎯H⎯R⎯G, ⎯A⎯C⎯P⎯R) Input Current High Input Current Low ⎯EN⎯_⎯C⎯H⎯R⎯G Pin Input Resistance ⎯ ⎯C⎯H⎯R⎯G Pin Leakage Current ⎯A⎯C⎯P⎯R Pin Leakage Current ⎯E⎯N⎯_⎯C⎯H⎯R⎯G, EN_BUCK, MODE Pin Low to High ⎯ E⎯N⎯_⎯C⎯H⎯R⎯G, EN_BUCK, MODE Pin High to Low ISINK = 5mA EN_BUCK, MODE Pins at 5.5V, VBAT = 5V ⎯E⎯N⎯_⎯C⎯H⎯R⎯G, EN_BUCK, MODE Pins at GND V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 5V VBAT = 4.5V, V⎯C⎯H⎯R⎯G = 5V VCC = 3V, V⎯C⎯H⎯R⎯G = 5V ● ● ● ● ● ● ● The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VCC = 5V, VBAT = 3.8V, V⎯E⎯N⎯_⎯C⎯H⎯R⎯G = 0V, VEN_BUCK = VBAT, VMODE = 0V. (Note 2) PARAMETER PMOS Switch On-Resistance NMOS Switch On-Resistance PMOS Switch Current Limit NMOS Switch Current Limit NMOS Zero Current in Normal Mode Peak Current in Burst Mode Operation Zero Current in Burst Mode Operation Buck Soft-Start Time MODE = VBAT MODE = VBAT From the Rising Edge of EN_BUCK to 90% of Buck Regulated Output 50 20 375 CONDITIONS MIN TYP 0.95 0.85 520 700 15 100 35 400 150 50 700 MAX UNITS Ω Ω mA mA mA mA mA μs 1.2 0.4 60 –1 –1 1 1.7 105 1 1 3.3 1 1 V V mV μA μA MΩ μA μA Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC4080 is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much higher than 43°C/W. Note 4: Although the LTC4080 charger functions properly at 3.75V, full charge current requires an input voltage greater than the desired final battery voltage per ΔVUVCL1 specification. Note 5: The 2.8V maximum buck undervoltage lockout (VUVLO_BUCK) exit threshold must first be exceeded before the minimum VBAT specification applies. Note 6: IC/10 is expressed as a fraction of measured full charge current with indicated PROG resistor. 4080fb 4 LTC4080 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified) Battery Regulation (Float) Voltage vs Charge Current 4.21 4.20 FLOAT VOLTAGE (V) FLOAT VOLTAGE (V) 4.19 4.18 4.17 4.16 4.15 4.14 4.13 0 50 200 150 100 CHARGE CURRENT (mA) 250 4080 G01 Battery Regulation (Float) Voltage vs Temperature 4.210 4.205 CHARGE CURRENT (mA) 4.200 4.195 4.190 4.185 4.180 4.175 4.170 4.165 4.160 – 50 – 30 – 10 50 30 10 TEMPERATURE (°C) 70 90 4080 G02 Charge Current vs Battery Voltage 250 RPROG = 2k VBAT RISING RPROG = 2k 200 150 100 50 TRICKLE CHARGE 0 0 1 3 4 2 BATTERY VOLTAGE (V) 5 4080 G02a Battery Regulation (Float) Voltage vs Supply Voltage 4.25 4.20 CHARGE CURRENT (mA) 4.15 FLOAT VOLTAGE (V) 4.10 4.05 4.00 3.95 3.90 3.85 4 4.5 5.5 INPUT VOLTAGE (V) 5 6 4080 G03 Charge Current vs Temperature with Thermal Regulation (Constant-Current Mode) 250 VCC = 6V VBAT = 3V RPROG = 2k 1.0 PROG Pin Voltage vs Charge Current RPROG = 2k 200 0.8 THERMAL CONTROL LOOP IN OPERATION VPROG (V) 150 0.6 100 0.4 50 0.2 0 –50 –25 0 25 50 75 100 125 4080 G04 0 0 25 TEMPERATURE (°C) 50 75 100 125 150 175 200 CHARGE CURRENT (mA) 4080 G05 Charger FET On-Resistance vs Temperature 0.9 VCC = 4V 0.8 IBAT = 350mA 0.7 0.6 RDS(ON) (Ω) VOLTAGE (V) 0.5 0.4 0.3 0.2 0.1 0 – 50 – 30 –10 30 50 10 TEMPERATURE (°C) 70 90 4080 G06 ⎯E⎯N⎯_⎯C⎯H⎯R⎯G, EN_BUCK and MODE Pin Threshold Voltage vs Temperature 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 –50 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 4080 G07 ⎯E⎯N⎯_⎯C⎯H⎯R⎯G Pin Pulldown Resistance vs Temperature 1.7 1.6 RESISTANCE (MΩ) 1.5 1.4 1.3 1.2 1.1 1.0 –50 RISING FALLING –30 –10 10 30 50 TEMPERATURE (°C) 70 90 4080 G08 4080fb 5 LTC4080 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified) ⎯C⎯H⎯R⎯G and ⎯A⎯C⎯P⎯R Pin Output Low Voltage vs Temperature 80 70 60 VOLTAGE (mV) 50 40 30 20 10 0 –50 –30 –10 10 30 50 TEMPERATURE (°C) 70 90 4080 G09 Normalized Charger Timer Period vs Temperature 1.05 2.28 2.27 FREQUENCY (MHz) –30 –10 10 30 50 70 90 4080 G10 Buck Oscillator Frequency vs Battery Voltage ICHRG, IACPR = 5mA NORMALIZED TIME PERIOD 1.00 2.26 2.25 2.24 2.23 2.22 2.5 0.95 0.90 0.85 0.80 –50 3.0 TEMPERATURE (°C) 3.5 4.0 BATTERY VOLTAGE (V) 4.5 4080 G11 Buck Oscillator Frequency vs Temperature 2.4 VBAT = 3.8V 2.3 FREQUENCY (MHz) 2.2 VBAT = 2.7V 2.1 2.0 1.9 1.8 –60 –40 –20 0 20 40 60 TEMPERATURE (°C) VBAT = 4.5V EFFICIENCY (%) 100 Buck Efficiency vs Load Current (VOUT = 1.8V) 1000 EFFICIENCY (Burst) EFFICIENCY (PWM) 100 Buck Efficiency vs Load Current (VOUT = 1.5V) 1000 EFFICIENCY (Burst) EFFICIENCY (PWM) 80 100 POWER LOSS (mW) EFFICIENCY (%) POWER LOSS 10 (PWM) 80 100 POWER LOSS (mW) POWER LOSS 10 (PWM) 60 60 40 20 80 100 4080 G12 0 0.01 0 POWER LOSS (Burst) VBAT = 3.8V 0.1 VOUT = 1.8V L = 10μH C = 4.7μF 0.01 0.1 1 10 100 1000 LOAD CURRENT (mA) 4080 G13 40 20 0 0.01 0 POWER LOSS (Burst) VBAT = 3.8V 0.1 VOUT = 1.5V L = 10μH C = 4.7μF 0.01 0.1 1 10 100 1000 LOAD CURRENT (mA) 4080 G13a Buck Output Voltage vs Battery Voltage 1.810 1.805 1.800 1.795 1.790 1.785 1.780 2.5 IOUT = 1mA VOUT SET FOR 1.8V 1.810 Burst Mode OPERATION BUCK OUTPUT VOLTAGE (V) PWM MODE 1.805 1.800 1.795 1.790 1.785 Buck Output Voltage vs Temperature IOUT = 1mA VOUT SET FOR 1.8V Burst Mode OPERATION PWM MODE BUCK INPUT CURRENT (μA) 35 30 25 20 15 10 5 No-Load Buck Input Current (Burst Mode Operation) vs Battery Voltage IOUT = 1mA VOUT = 1.8V L = 10μH BUCK OUTPUT VOLTAGE (V) 3.0 3.5 4.0 BATTERY VOLTAGE (V) 4.5 4080 G14 1.780 –50 –30 30 50 –10 10 TEMPERATURE (˚C) 70 90 4080 G15 0 2.5 3.5 3.0 4.0 BATTERY VOLTAGE (V) 4.5 4080 G17 4080fb 6 LTC4080 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise specified) No-Load Buck Input Current (Burst Mode Operation) vs Temperature 35 NO LOAD INPUT CURRENT (μA) 30 25 20 15 10 5 0 –50 –30 L = 10μH C = 4.7μF VOUT = 1.8V 1.2 VBAT = 4.2V 1.0 ON-RESISTANCE (Ω) 0.8 0.6 0.4 0.2 0 2.5 ON-RESISTANCE (Ω) 3.0 4.5 4.0 BATTERY VOLTAGE (V) 3.5 5.0 4080 G19 Buck Main Switch (PMOS) On-Resistance vs Battery Voltage 1.2 1.0 0.8 0.6 0.4 0.2 Buck Main Switch (PMOS) On-Resistance vs Temperature VBAT = 3.8V VBAT = 2.7V 30 50 –10 10 TEMPERATURE (˚C) 70 90 4080 G18 0 –50 –30 30 50 –10 10 TEMPERATURE (°C) 70 90 4080 G20 Buck Synchronous Switch (NMOS) On-Resistance vs Battery Voltage 1.2 1.0 ON-RESISTANCE (Ω) 0.8 0.6 0.4 0.2 0 2.5 ON-RESISTANCE (Ω) 3.0 4.5 4.0 BATTERY VOLTAGE (V) 3.5 5.0 4080 G21 Buck Synchronous Switch (NMOS) On-Resistance vs Temperature 1.2 1.0 0.8 0.6 0.4 0.2 0 –50 –30 30 50 –10 10 TEMPERATURE (°C) 70 90 4080 G22 Maximum Output Current (PWM Mode) 500 MAXIMUM OUTPUT CURRENT (mA) L = 10μH VOUT SET FOR 1.8V 400 MAXIMUM OUTPUT CURRENT (mA) 80 70 60 50 40 30 20 10 Maximum Output Current (Burst Mode Operation) L = 10μH VOUT SET FOR 1.8V 300 200 100 2.7 3 3.3 3.6 3.9 4.2 4.5 0 2.7 3 3.3 3.6 3.9 4.2 4.5 4080X G24 BATTERY VOLTAGE (V) 4080X G23 BATTERY VOLTAGE (V) 4080fb 7 LTC4080 TYPICAL PERFORMANCE CHARACTERISTICS specified) Output Voltage Transient Step Response (PWM Mode) (TA = 25°C, VCC = 5V, VBAT = 3.8V, unless otherwise Output Voltage Waveform when Switching Between Burst and PWM Mode (ILOAD = 10mA) Output Voltage Transient Step Response (Burst Mode) VOUT 20mV/DIV AC COUPLED ILOAD 250mA/DIV I=0 50μs/DIV 4080 G25 VOUT 50mV/DIV AC COUPLED VOUT 20mV/DIV AC COUPLED ILOAD 50mA/DIV I=0 50μs/DIV 4080 G27 VMODE 5V/DIV 0V 50μs/DIV 4080 G26 Buck VOUT Soft-Start (ILOAD = 50mA) Charger VPROG Soft-Start VOUT 1V/DIV 0V VEN_BUCK 5V/DIV 0V 200μs/DIV 4080 G28 VPROG 200mV/DIV V=0 50μs/DIV 4080 G29 4080fb 8 LTC4080 PI FU CTIO S BAT (Pin 1): Charge Current Output and Buck Regulator Input. Provides charge current to the battery and regulates the final float voltage to 4.2V. An internal precision resistor divider from this pin sets the float voltage and is disconnected in charger shutdown mode. This pin should be decoupled with a low ESR capacitor for low-noise buck operation. VCC (Pin 2): Positive Input Supply Voltage. This pin provides power to the battery charger. VCC can range from 3.75V to 5.5V. This pin should be bypassed with at least a 1μF capacitor. When VCC is less than 32mV above the BAT pin voltage, the battery charger enters shutdown mode. ⎯⎯⎯⎯⎯⎯⎯ EN_CHRG (Pin 3): Enable Input Pin for the Battery Charger. Pulling this pin above the manual shutdown threshold (VIH) puts the LTC4080 charger in shutdown mode, thus stopping the charge cycle. In battery charger shutdown mode, the LTC4080 has less than 10μA supply current and less than 5μA battery drain current if the regulator is not running. Enable is the default state, but the pin should be tied to GND if unused. PROG (Pin 4): Charge Current Program and Charge Current Monitor Pin. Connecting a 1% resistor, RPROG, to ground programs the charge current. When charging in constant-current mode, this pin servos to 1V. In all modes, the voltage on this pin can be used to measure the charge current using the following formula: I BAT V = PROG • 40 0 RPROG pin will be pulled to ground; otherwise the pin is high impedance. ⎯C⎯H⎯R⎯G (Pin 6): Open-Drain Charge Status Output. The charge status indicator pin has three states: pulldown, high impedance state, and pulse at 2Hz. This output can be used as a logic interface or as an LED driver. When the battery is being charged, the ⎯C⎯H⎯R⎯G pin is pulled low by an internal N-channel MOSFET. When the charge current drops to 10% of the full-scale current, the ⎯C⎯H⎯R⎯G pin is forced to a high impedance state. When the battery voltage remains below 2.9V for one quarter of the full charge time, the battery is considered defective, and the ⎯C⎯H⎯R⎯G pin pulses at a frequency of 2Hz with 75% duty cycle. FB (Pin 7): Feedback Pin for the Buck Regulator. A resistor divider from the regulator’s output to the FB pin programs the output voltage. Servo value for this pin is 0.8V. MODE (Pin 8): Burst Mode Enable Pin. Tie this pin high to force the LTC4080 regulator into Burst Mode operation for all load conditions. Tie this pin low to force constantfrequency mode operation for all load conditions. Do not float this pin. EN_BUCK (Pin 9): Enable Input Pin for the Switching Regulator. Pull this pin high to enable the regulator, pull low to shut down. Do not float this pin. SW (Pin 10): Switch Pin for the Buck Regulator. Minimize the length of the metal trace connected to this pin. Place the inductor as close to this pin as possible. GND (Pin 11): Ground. This pin is the back of the Exposed Pad package and must be soldered to the PCB for electrical connection and rated thermal performance. ⎯A⎯C⎯P⎯R (Pin 5): Open-Drain Power Supply Status Output. When VCC is greater than the undervoltage lockout threshold (3.6V) and greater than VBAT + 80mV, the ⎯A⎯C⎯P⎯R U U U 4080fb 9 LTC4080 BLOCK DIAGRA W 2 VCC CHARGER SHUTDOWN MP3 X1 D1 PROG 0.1V MP1 X400 D2 D3 TA 3 EN_CHRG 0.82V REN 6 CHRG MP4 1.22V R2 PULSE LOGIC 2.9V 1V 0.1V CHARGER ENABLE C2 BAT 4 RPROG PROG BADBAT + C4 5 ACPR VCC CHARGER OSCILLATOR COUNTER – + C5 3.6V – VBAT + 80mV LINEAR BATTERY CHARGER 9 EN_BUCK 0.82V SYNCHRONOUS BUCK CONVERTER C6 ENABLE BUCK PWM CONTROL AND DRIVE L1 10 SW MN1 CPL 7 FB 0.8V R8 R7 COUT VOUT 8 MODE 0.82V C7 11 GND Figure 1. LTC4080 Block Diagram 10 + 2.25MHz BUCK OSCILLATOR ERROR AMP – – + + – – + C1 + C3 – + 115°C TDIE 1 – CA – MA + R1 VA BAT – – + + – + CHARGE CONTROL LOGIC MP2 4080 BD 4080fb LTC4080 OPERATIO The LTC4080 is a full-featured linear battery charger with an integrated synchronous buck converter designed primarily for handheld applications. The battery charger is capable of charging single-cell 4.2V Li-Ion batteries. The buck converter is powered from the BAT pin and has a programmable output voltage providing a maximum load current of 300mA. The converter and the battery charger can run simultaneously or independently of each other. BATTERY CHARGER OPERATION Featuring an internal P-channel power MOSFET, MP1, the battery charger uses a constant-current/constantvoltage charge algorithm with programmable current. Charge current can be programmed up to 500mA with a final float voltage of 4.2V ±0.5%. The ⎯C⎯H⎯R⎯G open-drain status output indicates when C/10 has been reached. No blocking diode or external sense resistor is required; thus, the basic charger circuit requires only two external components. The ⎯A⎯C⎯P⎯R open-drain output indicates if the VCC input voltage, and the difference between VCC and BAT, are sufficient for charging. An internal termination timer adheres to battery manufacturer safety guidelines. Furthermore, the LTC4080 battery charger is capable of operating from a USB power source. A charge cycle begins when the voltage at the VCC pin rises above 3.6V and approximately 80mV above the BAT pin voltage, a 1% program resistor is connected from the PROG pin to ground, and the ⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin is pulled below the shutdown threshold (VIL). If the battery voltage is less than 2.9V, the battery charger begins trickle charging at 10% of the programmed charge current. When the BAT pin approaches the final float voltage of 4.2V, the battery charger enters constant-voltage mode and the charge current begins to decrease. When the current drops to 10% of the full-scale charge current, an internal comparator turns off the N-channel MOSFET driving the ⎯C⎯H⎯R⎯G pin, and the pin becomes high impedance. An internal thermal limit reduces the programmed charge current if the die temperature attempts to rise above a preset value of approximately 115°C. This feature protects the LTC4080 from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without the risk of damaging the U LTC4080 or external components. Another benefit of the thermal limit is that charge current can be set according to typical, rather than worst-case, ambient temperatures for a given application with the assurance that the battery charger will automatically reduce the current in worst-case conditions. An internal timer sets the total charge time, tTIMER (typically 4.5 hours). When this time elapses, the charge cycle terminates and the ⎯C⎯H⎯R⎯G pin assumes a high impedance state even if C/10 has not yet been reached. To restart the charge cycle, remove the input voltage and reapply it or momentarily force the ⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above VIH. A new charge cycle will automatically restart if the BAT pin voltage falls below VRECHRG (typically 4.1V). Constant-Current / Constant-Voltage / Constant-Temperature The LTC4080 battery charger uses a unique architecture to charge a battery in a constant-current, constant-voltage and constant-temperature fashion. Figure 1 shows a Simplified Block Diagram of the LTC4080. Three of the amplifier feedback loops shown control the constant-current, CA, constant-voltage, VA, and constant-temperature, TA modes. A fourth amplifier feedback loop, MA, is used to increase the output impedance of the current source pair, MP1 and MP3 (note that MP1 is the internal P-channel power MOSFET). It ensures that the drain current of MP1 is exactly 400 times the drain current of MP3. Amplifiers CA and VA are used in separate feedback loops to force the charger into constant-current or constantvoltage mode, respectively. Diodes D1 and D2 provide priority to either the constant-current or constant-voltage loop, whichever is trying to reduce the charge current the most. The output of the other amplifier saturates low which effectively removes its loop from the system. When in constant-current mode, CA servos the voltage at the PROG pin to be precisely 1V. VA servos its non-inverting input to 1.22V when in constant-voltage mode and the internal resistor divider made up of R1 and R2 ensures that the battery voltage is maintained at 4.2V. The PROG pin voltage gives an indication of the charge current anytime in the charge cycle, as discussed in “Programming Charge Current” in the Applications Information section. 4080fb 11 LTC4080 OPERATIO If the die temperature starts to creep up above 115°C due to internal power dissipation, the transconductance amplifier, TA, limits the die temperature to approximately 115°C by reducing the charge current. Diode D3 ensures that TA does not affect the charge current when the die temperature is below 115°C. In thermal regulation, the PROG pin voltage continues to give an indication of the charge current. In typical operation, the charge cycle begins in constantcurrent mode with the current delivered to the battery equal to 400V/RPROG. If the power dissipation of the LTC4080 results in the junction temperature approaching 115°C, the amplifier (TA) will begin decreasing the charge current to limit the die temperature to approximately 115°C. As the battery voltage rises, the LTC4080 either returns to constant-current mode or enters constant-voltage mode straight from constant-temperature mode. Battery Charger Undervoltage Lockout (UVLO) An internal undervoltage lockout circuit monitors the input voltage and keeps the battery charger off until VCC rises above 3.6V and approximately 80mV above the BAT pin voltage. The 3.6V UVLO circuit has a built-in hysteresis of approximately 0.6V, and the 80mV automatic shutdown threshold has a built-in hysteresis of approximately 50mV. During undervoltage lockout conditions, maximum battery drain current is 5μA and maximum supply current is 10μA. Undervoltage Charge Current Limiting (UVCL) The battery charger in the LTC4080 includes undervoltage charge current limiting that prevents full charge current until the input supply voltage reaches approximately 300mV above the battery voltage (ΔVUVCL1). This feature is particularly useful if the LTC4080 is powered from a supply with long leads (or any relatively high output impedance). See Applications Information section for further details. Trickle Charge and Defective Battery Detection At the beginning of a charge cycle, if the battery voltage is below 2.9V, the battery charger goes into trickle charge mode, reducing the charge current to 10% of the programmed current. If the low battery voltage persists for one quarter of the total time (1.125 hr), the battery is assumed to be defective, the charge cycle terminates and 12 U the ⎯C⎯H⎯R⎯G pin output pulses at a frequency of 2Hz with a 75% duty cycle. If, for any reason, the battery voltage rises above 2.9V, the charge cycle will be restarted. To restart the charge cycle (i.e., when the dead battery is replaced with a discharged battery less than 2.9V), the charger must be reset by removing the input voltage and ⎯⎯⎯⎯⎯⎯⎯ reapplying it or temporarily pulling the EN_CHRG pin above the shutdown threshold. Battery Charger Shutdown Mode The LTC4080’s battery charger can be disabled by pulling the ⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above the shutdown threshold (VIH). In shutdown mode, the battery drain current is reduced to less than 2μA and the VCC supply current to about 5μA provided the regulator is off. When the input voltage is not present, the battery charger is in shutdown and the battery drain current is less than 5μA. Power Supply Status Indicator (⎯A⎯C⎯P⎯R) The power supply status output has two states: pulldown and high impedance. The pulldown state indicates that VCC is above the undervoltage lockout threshold and at least 82mV above the BAT voltage (see Undervoltage Lockout). When these conditions are not met, the ⎯A⎯C⎯P⎯R pin is high impedance indicating that the LTC4080 is unable to charge the battery. ⎯C⎯H⎯R⎯G Status Output Pin The charge status indicator pin has three states: pulldown, pulse at 2Hz (see Defective Battery Detection) and high impedance. The pulldown state indicates that the battery charger is in a charge cycle. A high impedance state indicates that the charge current has dropped below 10% of the full-scale current or the battery charger is disabled. When the timer runs out (4.5 hrs), the ⎯C⎯H⎯R⎯G pin is also forced to the high impedance state. If the battery charger is not in constant-voltage mode when the charge current is forced to drop below 10% of the full-scale current by UVCL, ⎯C⎯H⎯R⎯G will stay in the strong pulldown state. Charge Current Soft-Start The LTC4080’s battery charger includes a soft-start circuit to minimize the inrush current at the start of a charge cycle. When a charge cycle is initiated, the charge current ramps 4080fb LTC4080 OPERATIO from zero to full-scale current over a period of approximately 180μs. This has the effect of minimizing the transient current load on the power supply during start-up. Timer and Recharge The LTC4080’s battery charger has an internal termination timer that starts when the input voltage is greater than the undervoltage lockout threshold and at least 80mV above BAT, and the battery charger is leaving shutdown. At power-up or when exiting shutdown, the charge time is set to 4.5 hours. Once the charge cycle terminates, the battery charger continuously monitors the BAT pin voltage using a comparator with a 2ms filter time. When the average battery voltage falls below 4.1V (which corresponds to 80%-90% battery capacity), a new charge cycle is initiated and a 2.25 hour timer begins. This ensures that the battery is kept at, or near, a fully charged condition and eliminates the need for periodic charge cycle initiations. The ⎯C⎯H⎯R⎯G output assumes a strong pulldown state during recharge cycles until C/10 is reached or the recharge cycle terminates. SWITCHING REGULATOR OPERATION: The switching regulator in the LTC4080 can be turned on by pulling the EN_BUCK pin above VIH. It has two userselectable modes of operation: constant-frequency (PWM) mode and Burst Mode Operation. The constant-frequency mode operation offers low noise at the expense of efficiency whereas the Burst Mode operation offers increased efficiency at light loads at the cost of increased noise and output voltage ripple. A detailed description of different operating modes and different aspects of operation follow. Operations can best be understood by referring to the Block Diagram. Constant-Frequency (PWM) Mode Operation The switching regulator operates in constant-frequency (PWM) mode when the MODE pin is pulled below VIL . In this mode, it uses a current mode architecture including an oscillator, an error amplifier, and a PWM comparator for excellent line and load regulation. The main switch MP2 (P-channel MOSFET) turns on to charge the inductor U at the beginning of each clock cycle if the FB pin voltage is less than the 0.8V reference voltage. The current into the inductor (and the load) increases until it reaches the peak current demanded by the error amp. At this point, the main switch turns off and the synchronous switch MN1 (N-channel MOSFET) turns on allowing the inductor current to flow from ground to the load until either the next clock cycle begins or the current reduces to the zero current (IZERO) level. Oscillator: In constant-frequency mode, the switching regulator uses a dedicated oscillator which runs at a fixed frequency of 2.25MHz. This frequency is chosen to minimize possible interference with the AM band. Error Amplifier: The error amplifier is an internally compensated transconductance (gm) amplifier with a gm of 65 μmhos. The internal 0.8V reference voltage is compared to the voltage at the FB pin to generate a current signal at the output of the error amplifier. This current signal is then converted into a voltage signal (ITH), and represents the peak inductor current required to achieve regulation. PWM Comparator: Lossless current sensing converts the PMOS switch current signal to a voltage which is summed with the internal slope compensation signal. The PWM comparator compares this summed signal to ITH and determines when to turn off the main switch. The switch current sensing is blanked for ~12ns at the beginning of each clock cycle to prevent false switch turn-off. Burst Mode Operation Burst Mode operation can be selected by pulling the MODE pin above VIH. In this mode, the internal oscillator is disabled, the error amplifier is converted into a comparator monitoring the FB voltage, and the inductor current swings between a fixed IPEAK (~80mA) and IZERO (35mA) irrespective of the load current as long as the FB pin voltage is less than or equal to the reference voltage of 0.8V. Once VFB is greater than 0.8V, the control logic shuts off both switches along with most of the circuitry and the regulator is said to enter into SLEEP mode. In SLEEP mode, the regulator only draws about 20μA from the BAT pin provided that the battery charger is turned off. When the output voltage droops about 1% from its 4080fb 13 LTC4080 OPERATIO nominal value, the regulator wakes up and the inductor current resumes swinging between IPEAK and IZERO. The output capacitor recharges and causes the regulator to re-enter the SLEEP state if the output load remains light enough. The frequency of this intermittent burst operation depends on the load current. That is, as the load current drops further, the regulator turns on less frequently. Thus Burst Mode operation increases the efficiency at light loads by minimizing the switching and quiescent losses. However, the output voltage ripple increases to about 2%. To minimize ripple in the output voltage, the current limits for both switches in Burst Mode operation are reduced to about 20% of their values in the constant-frequency mode. Also the zero current of the synchronous switch is changed to about 35mA thereby preventing reverse conduction through the inductor. Consequently, the regulator can only deliver approximately 55mA of load current while in Burst Mode operation. Any attempt to draw more load current will cause the output voltage to drop out of regulation. Current Limit To prevent inductor current runaway, there are absolute current limits (ILIM) on both the PMOS main switch and the NMOS synchronous switch. These limits are internally set at 520mA and 700mA respectively for PWM mode. If the peak inductor current demanded by the error amplifier ever exceeds the PMOS ILIM, the error amplifier will be ignored and the inductor current will be limited to PMOS ILIM. In Burst Mode operation, the PMOS current limit is reduced to 80mA to minimize output voltage ripple. Zero Current Comparator The zero or reverse current comparator monitors the inductor current to the output and shuts off the synchronous rectifier when this current reduces to a predetermined value (IZERO). In fixed frequency mode, this is set to negative 15mA meaning that the regulator allows the inductor current to flow in the reverse direction (from the output to ground through the synchronous rectifier) to a maximum value of 15mA. This is done to ensure that the regulator is able to regulate at very light loads without skipping any cycles thereby keeping output voltage ripple and noise low at the cost of efficiency. 14 U However, in Burst Mode operation, IZERO is set to positive 35mA meaning that the synchronous switch is turned off as soon as the current through the inductor to the output decreases to 35mA in the discharge cycle. This preserves the charge on the output capacitor and increases the overall efficiency at light loads. Soft-Start The LTC4080 switching regulator provides soft-start in both modes of operation by slowly charging an internal capacitor. The voltage on this capacitor, in turn, slowly ramps the current limits of both switches from a low value to their respective maximum values over a period of about 400μs. The soft-start capacitor is discharged completely whenever the regulator is disabled. Short-Circuit Protection In the event of a short circuit at the output or during start-up, VOUT will be near zero volts. Since the downward slope of the inductor current is ~VOUT/L, the inductor current may not get a chance to discharge enough to avoid a runaway situation. Because the current sensing is blanked for ~12ns at the beginning of each clock cycle, inductor current can build up to a dangerously high level over a number of cycles even if there is a hard current limit on the main PMOS switch. This is why the switching regulator in the LTC4080 also monitors current through the synchronous NMOS switch and imposes a hard limit on it. If the inductor current through the NMOS switch at the end of a discharge cycle is not below this limit, the regulator skips the next charging cycle thereby preventing inductor current runaway. Switching Regulator Undervoltage Lockout Whenever VBAT is less than 2.7V, an undervoltage lockout circuit keeps the regulator off, preventing unreliable operation. However, if the regulator is already running and the battery voltage is dropping, the undervoltage comparator does not shut down the regulator until VBAT drops below 2.5V. 4080fb LTC4080 OPERATIO Dropout Operation When the BAT pin voltage approaches VOUT, the duty cycle of the switching regulator approaches 100%. When VBAT is approximately equal to VOUT, the regulator is said to be in dropout. In dropout, the main switch (MP2) stays on continuously with the output voltage being equal to the battery voltage minus the voltage drops across the main switch and the inductor. APPLICATIO S I FOR ATIO BATTERY CHARGER Programming Charge Current The battery charge current is programmed using a single resistor from the PROG pin to ground. The charge current is 400 times the current out of the PROG pin. The program resistor and the charge current are calculated using the following equations: RPROG = 400 • 1V IBAT , IBAT = 400 • 1V RPROG The charge current out of the BAT pin can be determined at any time by monitoring the PROG pin voltage and using the following equation: I BAT = VPROG • 400 RPROG Stability Considerations The LTC4080 battery charger contains two control loops: constant-voltage and constant-current. The constantvoltage loop is stable without any compensation when a battery is connected with low impedance leads. Excessive lead length, however, may add enough series inductance to require a bypass capacitor of at least 1μF from BAT to GND. Furthermore, a 4.7μF capacitor with a 0.2Ω to 1Ω series resistor from BAT to GND is required to keep ripple voltage low when the battery is disconnected. In constant-current mode, the PROG pin voltage is in the feedback loop, not the battery voltage. Because of U W U U U Global Thermal Shutdown The LTC4080 includes a global thermal shutdown which shuts off the entire part (both battery charger and switching regulator) if the die temperature exceeds 160°C. The LTC4080 resumes normal operation once the temperature drops approximately 14°C. the additional pole created by PROG pin capacitance, capacitance on this pin must be kept to a minimum. With no additional capacitance on the PROG pin, the battery charger is stable with program resistor values as high as 25k. However, additional capacitance on this node reduces the maximum allowed program resistor. The pole frequency at the PROG pin should be kept above 100kHz. Therefore, if the PROG pin is loaded with a capacitance, CPROG, the following equation should be used to calculate the maximum resistance value for RPROG: RPROG ≤ 2π • 105 1 • CPROG Average, rather than instantaneous, battery current may be of interest to the user. For example, when the switching regulator operating in low-current mode is connected in parallel with the battery, the average current being pulled out of the BAT pin is typically of more interest than the instantaneous current pulses. In such a case, a simple RC filter can be used on the PROG pin to measure the average battery current as shown in Figure 2. A 10k resistor has been added between the PROG pin and the filter capacitor to ensure stability. LTC4080 PROG GND RPROG 4080 F02 10k CFILTER CHARGE CURRENT MONITOR CIRCUITRY Figure 2. Isolating Capacitive Load on PROG Pin and Filtering 4080fb 15 LTC4080 APPLICATIO S I FOR ATIO Undervoltage Charge Current Limiting (UVCL) USB powered systems tend to have highly variable source impedances (due primarily to cable quality and length). A transient load combined with such impedance can easily trip the UVLO threshold and turn the battery charger off unless undervoltage charge current limiting is implemented. Consider a situation where the LTC4080 is operating under normal conditions and the input supply voltage begins to sag (e.g. an external load drags the input supply down). If the input voltage reaches VUVCL (approximately 300mV above the battery voltage, ΔVUVCL), undervoltage charge current limiting will begin to reduce the charge current in an attempt to maintain ΔVUVCL between VCC and BAT. The LTC4080 will continue to operate at the reduced charge current until the input supply voltage is increased or voltage mode reduces the charge current further. Operation from Current Limited Wall Adapter By using a current limited wall adapter as the input supply, the LTC4080 can dissipate significantly less power when programmed for a current higher than the limit of the supply. Consider a situation where an application requires a 200mA charge current for a discharged 800mAh Li-Ion battery. If a typical 5V (non-current limited) input supply is available then the peak power dissipation inside the part can exceed 300mW. Now consider the same scenario, but with a 5V input supply with a 200mA current limit. To take advantage of the supply, it is necessary to program the LTC4080 to charge at a current greater than 200mA. Assume that the LTC4080 charger is programmed for 300mA (i.e., RPROG = 1.33kΩ) to ensure that part tolerances maintain a programmed current higher than 200mA. Since the battery charger will demand a charge current higher than the current limit of the input supply, the supply voltage will collapse to the battery voltage plus 200mA times the on-resistance of the internal PFET. The on-resistance of the battery charger power device is approximately 0.75Ω with a 5V supply. The actual on-resistance will be slightly higher due to the fact that the input supply will have collapsed to less than 5V. The power dissipated during this phase of charging 16 U is approximately 30mW. That is a ten times improvement over the non-current limited supply power dissipation. USB and Wall Adapter Power Although the LTC4080 allows charging from a USB port, a wall adapter can also be used to charge Li-Ion batteries. Figure 3 shows an example of how to combine wall adapter and USB power inputs. A P-channel MOSFET, MP1, is used to prevent back conducting into the USB port when a wall adapter is present and Schottky diode, D1, is used to prevent USB power loss through the 1k pulldown resistor. Typically a wall adapter can supply significantly more current than the current-limited USB port. Therefore, an N-channel MOSFET, MN1, and an extra program resistor can be used to increase the charge current when the wall adapter is present. 5V WALL ADAPTER (300mA) USB POWER (200mA) BAT LTC4080 VCC PROG 4 1 ICHG SYSTEM LOAD D1 2 MP1 W U U + Li-Ion BATTERY MN1 1.33k 1k 2k 4080 F03 Figure 3. Combining Wall Adapter and USB Power Power Dissipation The conditions that cause the LTC4080 battery charger to reduce charge current through thermal feedback can be approximated by considering the total power dissipated in the IC. For high charge currents, the LTC4080 power dissipation is approximately: PD = ( VCC − VBAT ) • IBAT + PD _ BUCK Where PD is the total power dissipated within the IC, VCC is the input supply voltage, VBAT is the battery voltage, IBAT is the charge current and PD_BUCK is the power dissipation due to the regulator. PD_BUCK can be calculated as: ⎛1 ⎞ PD _ BUCK = VOUT • IOUT ⎜ − 1⎟ ⎝η ⎠ 4080fb LTC4080 APPLICATIO S I FOR ATIO Where VOUT is the regulated output of the switching regulator, IOUT is the regulator load and η is the regulator efficiency at that particular load. It is not necessary to perform worst-case power dissipation scenarios because the LTC4080 will automatically reduce the charge current to maintain the die temperature at approximately 115°C. However, the approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 115°C – PDθJA TA = 115°C – (VCC – VBAT) • IBAT • θJA if the regulator is off. Example: Consider the extreme case when an LTC4080 is operating from a 6V supply providing 250mA to a 3V Li-Ion battery and the regulator is off. The ambient temperature above which the LTC4080 will begin to reduce the 250mA charge current is approximately: TA = 115°C – (6V – 3V) • (250mA) • 43°C/W TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C TA = 82.75°C If there is more power dissipation due to the regulator, the thermal regulation will kick in at a somewhat lower temperature than this. In the above circumstances, the LTC4080 can be used above 82.75°C, but the charge current will be reduced from 250mA. The approximate current at a given ambient temperature can be calculated: I BAT = 115 °C − T A ( VCC − VBAT ) • θ JA Using the previous example with an ambient temperature of 85°C, the charge current will be reduced to approximately: 115 °C − 85 °C 30 °C I BAT = = = 2 3 2 . 6mA (6V − 3V ) • 43°C / W 129 °C / A Note: 1V = 1J/C = 1W/A Furthermore, the voltage at the PROG pin will change proportionally with the charge current as discussed in the Programming Charge Current section. U VCC Bypass Capacitor Many types of capacitors can be used for input bypassing; however, caution must be exercised when using multi-layer ceramic capacitors. Because of the self-resonant and high Q characteristics of some types of ceramic capacitors, high voltage transients can be generated under some start-up conditions, such as connecting the battery charger input to a live power source. Adding a 1Ω series resistor in series with an X5R ceramic capacitor will minimize start-up voltage transients. For more information, refer to Application Note 88. SWITCHING REGULATOR Setting the Buck Converter Output Voltage The LTC4080 regulator compares the FB pin voltage with an internal 0.8V reference to generate an error signal at the output of the error amplifier. A voltage divider from VOUT to ground (as shown in the Block Diagram) programs the output voltage via FB using the formula: ⎡ R7 ⎤ VOUT = 0 . 8 V • ⎢1 + ⎥ ⎣ R8 ⎦ Keeping the current low (