LTC3550-1 Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Internal thermal feedback regulates the battery charge current to maintain a constant die temperature during high power operation or high ambient temperature conditions. The float voltage is fixed at 4.2V and the charge currents are programmed with external resistors. The LTC3550-1 terminates the charge cycle when the charge current drops below the programmed termination threshold after the final float voltage is reached. With power applied to both inputs, the LTC3550-1 can be put into shutdown mode reducing the DCIN supply current to 20μA, the USBIN supply current to 10μA, and the battery drain current to less than 2μA. The synchronous buck converter generates a fixed output voltage of 1.875V. The switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors. U APPLICATIO S ■ , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. *Protected by U.S. patents, includng 6522118, 6700364, 6580258, 5481178, 6304066, 6127815, 6498466, 6611131 Cellular Telephones U TYPICAL APPLICATIO Complete Charge Cycle (1100mA Battery) Dual Input Battery Charger and DC/DC Converter LTC3550-1 SW RUN WALL ADAPTER 1µF 2k 1% 1µF USBIN VCC IUSB BAT IDC 1.24k 1% COUT 10µF CER VOUT DCIN USB PORT 2.2µH ITERM GND 800mA (WALL) 500mA (USB) VOUT 1.875V 600mA 4.7µF 2k 1% + 4.2V SINGLE-CELL Li-Ion BATTERY 3550-1 TA01 BATTERY CHARGE VOLTAGE (V) CURRENT (mA) ■ The LTC®3550-1 is a standalone linear charger with a 600mA monolithic synchronous buck converter. It is capable of charging a single-cell Li-Ion battery from both wall adapter and USB inputs. The charger automatically selects the appropriate power source for charging. Charges Single-Cell Li-Ion Batteries from Wall Adapter and USB Inputs Automatic Input Power Detection and Selection Charge Current Programmable Up to 950mA from Wall Adapter Input High Efficiency 600mA Synchronous DC/DC Converter No External MOSFET, Sense Resistor or Blocking Diode Needed Thermal Regulation Maximizes Charge Rate Without Risk of Overheating* Preset Charge Voltage with ±0.6% Accuracy Programmable Charge Current Termination 1.5MHz Constant Frequency Operation (Step-Down Converter) 18μA USB Suspend Current in Shutdown Independent “Power Present” Status Outputs Charge Status Output Automatic Recharge Available in a Thermally Enhanced, Low Profile (0.75mm) 16-Lead (5mm x 3mm) DFN Package 1000 800 600 400 200 0 4.2 4.0 3.8 3.6 3.4 DCIN VOLTAGE (V) ■ 5.0 CONSTANT VOLTAGE USBIN = 5V TA = 25°C RIDC = 1.25k RIUSB = 2k 2.5 0 –2.5 0 0.5 1.0 2.0 1.5 TIME (HR) 2.5 3.0 3550-1 TA02 35501f 1 LTC3550-1 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW DCIN, USBIN .............................................. –0.3V to 10V ENABLE, ⎯C⎯H⎯R⎯G, ⎯P⎯W⎯R, USBPWR ............... –0.3V to 10V BAT, IDC, IUSB, ITERM ................................ –0.3V to 7V VCC ............................................................... –0.3V to 6V RUN, VOUT ..................................................–0.3V to VCC SW (DC)........................................–0.3V to (VCC + 0.3V) DCIN Pin Current (Note 2) ..........................................1A USBIN Pin Current (Note 2) .................................700mA BAT Pin Current (Note 2) ............................................1A P-Channel SW Source Current (DC).....................800mA N-Channel SW Source Current (DC) ....................800mA Peak SW Sink and Source Current ...........................1.3A Operating Temperature Range (Note 3) ... –40°C to 85°C Maximum Junction Temperature .......................... 125°C Storage Temperature Range................... –65°C to 125°C USBIN 1 16 DCIN IUSB 2 15 BAT ITERM 3 14 IDC PWR 4 CHRG 5 12 ENABLE VOUT 6 11 RUN VCC 7 10 SW GND 8 9 17 13 USBPWR GND DHC PACKAGE 16-LEAD (5mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 40°C (Note 4) EXPOSED PAD IS GROUND (PIN 17) MUST BE SOLDERED TO PCB ORDER PART NUMBER PART MARKING LTC3550EDHC-1 35501 Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VDCIN Wall Adapter Input Supply Voltage ● 4.3 8 V VUSBIN USB Port Input Supply Voltage ● 4.3 8 V VCC Buck Regulator Input Supply Voltage ● 2.5 5.5 V VENABLE ENABLE Input Threshold Voltage 0.4 0.7 1.0 V RENABLE ENABLE Pulldown Resistance 1.2 2 5 VRUN RUN Threshold Voltage ● 0.3 1 1.5 V IRUN RUN Leakage Current ● ±0.01 ±1 µA V⎯C⎯H⎯R⎯G ⎯C⎯H⎯R⎯G Output Low Voltage I⎯C⎯H⎯R⎯G = 5mA 0.35 0.6 V V⎯P⎯W⎯R ⎯P⎯W⎯R Output Low Voltage I⎯P⎯W⎯R = 5mA 0.35 0.6 V VUSBPWR USBPWR Output Low Voltage IUSBPWR = 200µA 0.35 0.6 V VUVDC DCIN Undervoltage Lockout Voltage From Low to High Hysteresis 4.0 4.15 200 4.3 V mV VUVUSB USBIN Undervoltage Lockout Voltage From Low to High Hysteresis 3.8 3.95 200 4.1 V mV MΩ 35501f 2 LTC3550-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted. SYMBOL PARAMETER MIN TYP MAX UNITS VASD-DC VDCIN – VBAT Lockout Threshold Voltage VDCIN from Low to High, VBAT = 4.2V VDCIN from High to Low, VBAT = 4.2V CONDITIONS 140 20 180 50 220 80 mV mV VASD-USB VUSBIN – VBAT Lockout Threshold Voltage VUSBIN from Low to High, VBAT = 4.2V VUSBIN from High to Low, VBAT = 4.2V 140 20 180 50 220 80 mV mV DCIN Supply Current Charge Mode (Note 5) Standby Mode Shutdown Mode RIDC = 10k Charge Terminated ENABLE = 5V ● ● 250 50 20 800 100 40 µA µA µA USBIN Supply Current Charge Mode (Note 6) Standby Mode Shutdown Mode Shutdown Mode RIUSB = 10k, VDCIN = 0V Charge Terminated VDCIN = 0V, ENABLE = 0V VDCIN > VUSBIN ● ● 250 50 18 10 800 100 36 20 µA µA µA µA 4.175 4.158 4.2 4.2 4.225 4.242 V V 760 450 93 800 476 100 –3 –1 ±1 840 500 107 –6 –2 ±2 mA mA mA µA µA µA 0.95 1.0 1.05 V 0.95 1.0 1.05 V 90 45 8.5 4 100 50 10 5 110 55 11.5 6 mA mA mA mA Battery Charger IDCIN IUSBIN VFLOAT Regulated Output (Float) Voltage IBAT BAT Pin Current Constant-Current Mode Constant-Current Mode Constant-Current Mode Standby Mode Shutdown Mode Sleep Mode RIDC = 1.25k RIUSB = 2.1k RIDC = 10k or RIUSB = 10k Charge Terminated Charger Disabled DCIN = 0V, USBIN = 0V VIDC IDC Pin Regulated Voltage Constant-Current Mode VIUSB IUSB Pin Regulated Voltage Constant-Current Mode IBAT = 1mA IBAT = 1mA, 0°C < TA < 85°C RITERM = 1k RITERM = 2k RITERM = 10k RITERM = 20k ● ● ● ● ● ● ● ITERMINATE Charge Current Termination Threshold ΔVRECHRG Recharge Battery Threshold Voltage VFLOAT – VRECHRG, 0°C < TA < 85°C 65 100 135 mV tRECHRG Recharge Comparator Filter Time VBAT from High to Low 3 6 9 ms tTERMINATE Termination Comparator Filter Time IBAT Drops Below Termination Threshold 0.8 1.5 2.2 ms tSS Soft-Start Time IBAT = 10% to 90% Full-Scale 175 250 325 µs RON-DC Power FET On-Resistance (Between DCIN and BAT) 400 mΩ RON-USB Power FET On-Resistance (Between USBIN and BAT) 550 mΩ TLIM Junction Temperature in ConstantTemperature Mode 105 °C Switching Regulator VOUT Regulated Output Voltage ΔVOUT Output Voltage Line Regulation IPK Peak Inductor Current VLOADREG Output Voltage Load Regulation IS Input DC Bias Current Active Mode Sleep Mode Shutdown IOUT = 100mA ● 1.819 ● VCC = 3V, VOUT = 1.7V 0.75 1.875 1.931 0.04 0.4 1 1.25 0.5 (Note 7) VOUT = 1.7V, ILOAD = 0A VOUT = 1.94V, ILOAD = 0A VRUN = 0V, VCC = 5.5V 300 20 0.1 V %/V A % 400 35 1 µA µA µA 35501f 3 LTC3550-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX fOSC Oscillator Frequency VOUT = 100% VOUT = 0V 1.2 1.5 210 1.8 RPFET RDS(ON) of P-Channel FET 0.4 Ω RNFET RDS(ON) of N-Channel FET 0.35 Ω ILSW SW Leakage Current 0.01 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: Guaranteed by long term current density limitations. Note 3: The LTC3550E-1 is guaranteed to meet the performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 4: Failure to solder the exposed backside of the package to the PC board will result in a thermal resistance much higher than 40°C/W. See Thermal Considerations. UNITS MHz kHz ±1 µA Note 5: Supply Current includes IDC and ITERM pin current (approximately 100μA each) but does not include any current delivered to the battery through the BAT pin (approximately 100mA). Note 6: Supply Current includes IUSB and ITERM pin current (approximately 100μA each) but does not include any current delivered to the battery through the BAT pin (approximately 100mA). Note 7: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. 35501f 4 LTC3550-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Regulated Charger Output (Float) Voltage vs Charge Current 1.008 4.215 1.006 4.22 4.210 1.004 4.20 4.205 1.002 4.18 VIDC (V) 4.24 4.16 4.200 0.998 4.14 4.190 0.996 4.12 4.185 0.994 RIDC = 1.25k 0 100 200 300 400 500 600 700 800 CHARGE CURRENT (mA) 4.180 –50 75 0 25 50 TEMPERATURE (°C) –25 IUSB Pin Voltage vs Temperature (Constant-Current Mode) 0.992 –50 1.008 900 1.006 800 900 VDCIN = 5V 400 –25 0 25 50 TEMPERATURE (°C) 75 0 0 0.2 0.4 0.6 0.8 VIDC (V) 3550-1 G04 1.0 1.2 0 ⎯C⎯H⎯R⎯G Pin I-V Curve VDCIN = VUSBIN = 5V TA = –40°C 30 TA = 25°C 25 ICHRG (mA) TA = 90°C 20 15 5 5 5 TA = 25°C TA = 90°C 0 1 2 4 3 VPWR (V) 5 6 7 3550-1 G07 4 TA = 90°C 3 2 1 0 0 1.2 TA = – 40°C TA = 25°C 15 10 VDCIN = 5V VUSBIN = 0V TA = –40°C 20 10 1.0 USBPWR Pin I-V Curve 6 IUSBPWR (mA) 35 25 0.6 0.8 VIUSB (V) 3550-1 G06 35 30 0.4 0.2 3550-1 G05 ⎯P⎯W⎯R Pin I-V Curve VDCIN = VUSBIN = 5V RIUSB = 10k 100 0 100 400 200 RIDC = 10k 100 0.992 –50 RIUSB = 2k 500 300 200 0.994 RIUSB = 1.25k 600 RIDC = 2k 500 300 0.996 100 700 IBAT (mA) IBAT (mA) VUSBIN = 4.3V 0.998 75 VUSBIN = 5V 800 RIDC = 1.25k 600 1.000 0 25 50 TEMPERATURE (°C) Charge Current vs IUSB Pin Voltage 700 1.004 VUSBIN = 8V –25 3550-1 G03 Charge Current vs IDC Pin Voltage 1.002 VDCIN = 4.3V 3550-1 G02 3550-1 G01 VIUSB (V) 100 VDCIN = 8V 1.000 4.195 RIDC = RIUSB = 2k 4.10 IPWR (mA) IDC Pin Voltage vs Temperature (Constant-Current Mode) VDCIN = VUSBIN = 5V VDCIN = VUSBIN = 5V VFLOAT (V) VFLOAT (V) Regulated Charger Output (Float) Voltage vs Temperature 4.220 4.26 TA = 25°C, unless otherwise noted. 0 1 2 4 3 VCHRG (V) 5 6 7 3550-1 G08 0 0 1 2 4 3 5 VUSBPWR (V) 6 7 3550-1 G09 35501f 5 LTC3550-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Charge Current vs Ambient Temperature TA = 25°C, unless otherwise noted. Charge Current vs Supply Voltage 1000 900 ONSET OF THERMAL REGULATION Charge Current vs Battery Voltage 1000 ONSET OF THERMAL REGULATION 800 800 800 RIDC = 1.25k RIDC = RIUSB = 2k 400 IBAT (mA) IBAT (mA) IBAT (mA) 700 600 600 600 400 500 200 VDCIN = VUSBIN = 5V VBAT = 4V θJA = 40°C/W 0 –50 –25 400 50 25 75 0 TEMPERATURE (°C) 100 300 4.0 4.5 125 200 RIDC = 1.25k VBAT = 4V θJA = 35°C/W 7.5 7.0 5.5 6.0 6.5 VDCIN (V) 5.0 0 8.0 2.4 2.7 3.0 3.3 3.6 VBAT (V) 3.9 3550-1 G11 3550-1 G10 DCIN Power FET On-Resistance vs Temperature 550 VDCIN = VUSBIN = 5V θJA = 40°C/W RIDC = 1.25k 800 750 500 4.5 3550-1 G12 USBIN Power On-Resistance vs Temperature VBAT = 4V IBAT = 200mA 4.2 ENABLE Pin Threshold Voltage (On-to-Off) vs Temperature 900 VBAT = 4V IBAT = 200mA VDCIN = VUSBIN = 5V 850 400 350 650 VENABLE (mV) RDS(ON) (mΩ) RDS(ON) (mΩ) 700 450 600 550 800 750 700 500 450 300 650 400 250 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 350 –50 –25 125 50 25 75 0 TEMPERATURE (°C) 3550-1 G13 40 2.6 VUSBIN = 8V RENABLE (MΩ) VDCIN = 5V 25 20 VUSBIN = 5V 15 VDCIN = 4.3V 5 0 –50 IUSBIN (µA) IDCIN (µA) 10 10 ENABLE = 5V –25 100 35 VDCIN = 8V 30 15 75 2.8 30 20 50 25 0 TEMPERATURE (°C) ENABLE Pin Pulldown Resistance vs Temperature 45 45 25 –25 3550-1 G15 USBIN Shutdown Current vs Temperature 50 35 600 –50 125 3550-1 G14 DCIN Shutdown Current vs Temperature 40 100 50 25 0 TEMPERATURE (°C) 75 100 3550-1 G16 VUSBIN = 4.3V 5 0 –50 2.4 2.2 2.0 1.8 ENABLE = 0V –25 50 25 0 TEMPERATURE (°C) 75 100 3550-1 G17 1.6 –50 –25 50 25 0 TEMPERATURE (°C) 75 100 3550-1 G18 35501f 6 LTC3550-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Undervoltage Lockout Threshold vs Temperature TA = 25°C, unless otherwise noted. Recharge Threshold Voltage vs Temperature 4.25 4.20 DCIN UVLO Battery Drain Current vs Temperature 4.16 5 4.14 4 4.12 3 VBAT = 4.2V VDCIN, VUSBIN (NOT CONNECTED) VRECHRG (V) VUV (V) 4.10 4.05 4.00 USBIN UVLO IBAT (µA) 4.15 VDCIN = VUSBIN = 4.3V 4.10 VDCIN = VUSBIN = 8V 2 4.08 1 4.06 0 3.95 3.90 3.85 –50 –25 0 25 50 TEMPERATURE (°C) 75 4.04 –50 100 –25 75 0 25 50 TEMPERATURE (°C) 3550-1 G19 –1 –50 100 75 0 25 50 TEMPERATURE (°C) –25 3550-1 G21 3550-1 G20 Charge Current During Turn-On and Turn-Off 100 Buck Regulator Efficiency vs Output Current Buck Regulator Efficiency vs VCC 100 100 VCC = 2.7V 95 ENABLE 5V/DIV IOUT = 100mA 90 90 EFFICIENCY (%) EFFICIENCY (%) IBAT 500mA/DIV IOUT = 10mA 85 80 IOUT = 600mA 75 2 100µs/DIV 3 4 VCC (V) Buck Regulator Output Voltage vs Temperature 50 0.1 1 10 ILOAD (mA) 100 1000 35501 G24 Oscillator Frequency vs VCC 1.70 1.8 VCC = 3.6V 1.65 1.7 1.60 FREQUENCY (MHz) 1.89 VOUT (V) 6 Oscillator Frequency vs Temperature VCC = 3.6V IOUT = 100mA 1.88 1.87 1.86 1.55 1.50 1.45 1.6 1.5 1.4 1.40 1.85 1.84 –50 –25 5 FREQUENCY (MHz) 1.90 VCC = 3.6V 35501 G23 3550-1 G22 1.91 70 60 70 VDCIN = 5V RIDC = 1.25k VCC = 4.2V 80 1.3 1.35 50 25 75 0 TEMPERATURE (°C) 100 125 3550-1 G25 1.30 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3550-1 G26 1.2 2 3 4 VCC (V) 5 6 3550-1 G27 35501f 7 LTC3550-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Buck Regulator Output Voltage vs Load Current 1.90 TA = 25°C, unless otherwise noted. Buck Regulator Switches RDS(ON) vs Temperature RDS(ON) vs VCC 0.7 VCC = 3.6V 0.7 TA = 25°C VCC = 2.7V 0.6 0.6 1.89 VCC = 4.2V 1.87 1.86 0.5 MAIN SWITCH 0.4 0.3 RDS(ON) (Ω) RDS(ON) (Ω) VOUT (V) 0.5 1.88 SYNCHRONOUS SWITCH 0.1 0.1 0 100 200 300 400 500 600 700 800 900 ILOAD (mA) 1 2 4 3 VCC (V) 5 35501 G28 6 7 45 SUPPLY CURRENT (µA) 40 35 30 25 20 15 10 300 VCC = 3.6V VOUT = 1.875V ILOAD = 0A VCC = 5.5V RUN = 0V 250 35 30 25 20 15 10 5 5 0 –50 –25 4 VCC (V) 3 6 5 50 25 0 75 TEMPERATURE (°C) MAIN SWITCH 100 125 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3550-1 G33 Burst Mode Operation RUN = 0V SW 5V/DIV 100 SWITCH LEAKAGE (pA) 100 3550-1 G32 Switch Leakage Current vs VCC SYNCHRONOUS SWITCH 80 VOUT 20mV/DIV 60 MAIN SWITCH IL 200mA/DIV 40 20 0 150 SYNCHRONOUS SWITCH 3550-1 G31 120 200 50 0 125 Switch Leakage Current vs Temperature SWITCH LEAKAGE (nA) 50 40 2 100 3550-1 G30 Buck Regulator Supply Current vs Temperature VOUT = 1.875V ILOAD = 0A 45 SUPPLY CURRENT (µA) MAIN SWITCH SYNCHRONOUS SWITCH 0 50 –50 –25 25 75 0 TEMPERATURE (°C) 3550-1 G29 Buck Regulator Supply Current vs VCC 50 0.3 0.2 0 0 0.4 0.2 1.85 VCC = 3.6V VCC = 3.6V ILOAD = 10mA 0 1 2 3 VCC (V) 4 5 4µs/DIV 35501 G35 6 3550-1 G34 35501f 8 LTC3550-1 U W TYPICAL PERFOR A CE CHARACTERISTICS Start-Up from Shutdown TA = 25°C, unless otherwise noted. Load Step Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED RUN 2V/DIV VOUT 1V/DIV IL 500mA/DIV IL 500mA/DIV IL 500mA/DIV VCC = 3.6V ILOAD = 600mA 40µs/DIV 35501 G36 VCC = 3.6V 20µs/DIV ILOAD = 0mA TO 600mA Load Step 35501 G37 VCC = 3.6V 20µs/DIV ILOAD = 50mA TO 600mA 35501 G38 Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 500mA/DIV IL 500mA/DIV VCC = 3.6V 20µs/DIV ILOAD = 100mA TO 600mA 35501 G39 VCC = 3.6V 20µs/DIV ILOAD = 200mA TO 600mA 35501 G40 35501f 9 LTC3550-1 U U U PI FU CTIO S USBIN (Pin 1): USB Input Supply Pin. Provides power to the battery charger. The maximum supply current is 650mA. This should be bypassed with a 1µF capacitor. VCC (Pin 7): Buck Regulator Input Supply Pin. Must be closely decoupled to GND (Pins 8, 9) with a 2.2µF or greater ceramic capacitor. IUSB (Pin 2): USB Charge Current Program and Monitor Pin. The charge current can be set by connecting a resistor, RIUSB, to ground. When charging in constant-current mode, this pin servos to 1V. The voltage on this pin can be used to measure the charge current delivered from the USB input using the following formula: V IBAT = IUSB • 1000 RIUSB GND (Pins 8, 9): Ground. ITERM (Pin 3): Termination Current Threshold Program Pin. The current termination threshold, ITERMINATE, can be set by connecting a resistor, RITERM, to ground. ITERMINATE is set by the following formula: 100 V ITERMINATE = RITERM When the charge current, IBAT, falls below the termination threshold, charging stops and the ⎯C⎯H⎯R⎯G output becomes high impedance. This pin is internally clamped to approximately 1.5V. Driving this pin to voltages beyond the clamp voltage should be avoided. ⎯P⎯W⎯R (Pin 4): Open-Drain Power Supply Status Output. When the DCIN or USBIN pin voltage is sufficient to begin charging (i.e., when the supply is greater than the undervoltage lockout threshold and at least 180mV above the battery terminal), the ⎯P⎯W⎯R pin is pulled low by an internal N-channel MOSFET. Otherwise, ⎯P⎯W⎯R is high impedance. The output is capable of sinking up to 10mA, making it suitable for driving an LED. ⎯C⎯H⎯R⎯G (Pin 5): Open-Drain Charge Status Output. When the LTC3550-1 is charging, the ⎯C⎯H⎯R⎯G pin is pulled low by an internal N-channel MOSFET. When the charge cycle is completed, ⎯C⎯H⎯R⎯G becomes high impedance. This output is capable of sinking up to 10mA, making it suitable for driving an LED. VOUT (Pin 6): Output Voltage Feedback Pin. Receives the feedback voltage from the buck regulator output. SW (Pin 10): Switch Node Connection to Inductor. This pin connects to the drains of the internal main (top) and synchronous (bottom) power MOSFET switches. RUN (Pin 11): Buck Regulator Run Control Input. Forcing this pin above 1.5V enables the regulator. Forcing this pin below 0.3V shuts it down. In shutdown, all buck regulator functions are disabled drawing <1µA supply current from VCC. Do not leave RUN floating. ENABLE (Pin 12): Charger Enable Input. When the LTC3550-1 is charging from the DCIN source, a logic low on this pin enables the charger. When the LTC3550-1 is charging from the USBIN source, a logic high on this pin enables the charger. If this input is left floating, an internal 2MΩ pulldown resistor defaults the LTC3550-1 to charge when a wall adapter is applied and to shut down if only the USB source is applied. USBPWR (Pin 13): Open-Drain USB Power Status Output. When the voltage on the USBIN pin is sufficient to begin charging and there is insufficient power at DCIN, the USBPWR pin is high impedance. In all other cases, this pin is pulled low by an internal N-channel MOSFET, provided that there is power present at DCIN, USBIN, or BAT inputs. This output is capable of sinking up to 1mA, making it suitable for driving high impedance logic inputs. IDC (Pin 14): Wall Adapter Charge Current Program and Monitor Pin. The charge current is set by connecting a resistor, RIDC, to ground. When charging in constantcurrent mode, this pin servos to 1V. The voltage on this pin can be used to measure the charge current using the following formula: V IBAT = IDC • 1000 RIDC BAT (Pin 15): Charger Output. This pin provides charge current to the battery and regulates the final float voltage to 4.2V. 35501f 10 LTC3550-1 U U U PI FU CTIO S DCIN (Pin 16): Wall Adapter Input Supply Pin. Provides power to the battery charger. The maximum supply current is 950mA. This should be bypassed with a 1µF capacitor. Exposed Pad (Pin 17): GND. The exposed backside of the package is ground and must be soldered to the PCB ground for electrical connection and maximum heat transfer. W BLOCK DIAGRA DCIN BAT USBIN 16 15 1 CC/CV REGULATOR CC/CV REGULATOR FREQ SHIFT OSC 4.15V PWR 4 + DC SOFTSTART – USB SOFTSTART – DCIN UVLO 10mA MAX R1 ITH 3.95V 0.6V EA BURST CLAMP USBIN UVLO + + – – 6 VOUT – + 1mA MAX + USBPWR 13 SLOPE COMP R2 7 VCC BAT CHRG 5 10mA MAX BAT – 5Ω + ICOMP 4.1V RECHARGE – BAT S Q R Q RS LATCH LOGIC DC_ENABLE USB_ENABLE + TERM TDIE THERMAL REGULATION CHARGER CONTROL – ENABLE 12 105°C SWITCHING LOGIC AND BLANKING CIRCUIT ANTISHOOTTHRU 10 SW + + RECHRG IRCMP – RENABLE + 100mV IBAT /1000 TERMINATION IBAT /1000 IBAT /1000 3 ITERM RITERM 14 IDC RIDC 2 IUSB RIUSB 11 3550-1 BD – RUN 8, 9, 17 GND 35501f 11 LTC3550-1 U OPERATIO The LTC3550-1 consists of two main blocks: a lithium-ion battery charger and a high-efficiency buck converter that can be powered from the battery. The charger is designed to efficiently manage charging of a single-cell lithium-ion battery from two separate power sources: a wall adapter and USB power bus. The internal P-channel MOSFETs can supply up to 950mA from the wall adapter source and 500mA from the USB power source. The final float voltage accuracy is ±0.6%. The buck converter uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches for the buck converter are internal. The LTC3550-1 requires no external diodes or sense resistors. Lithium-Ion Battery Charger A charge cycle begins when the voltage at either the DCIN pin or USBIN pin rises above the UVLO threshold level and the charger is enabled through the ENABLE pin. The “on” state of this pin depends on which source is supplying power. When the wall adapter input (DCIN) is supplying power, logic low enables the charger and logic high disables it (a 2MΩ pulldown defaults the charger to the charging state). The opposite is true when the USB input (USBIN) is supplying power; logic low disables the charger and logic high enables it (the default is the shutdown state). The DCIN input draws 20µA when the charger is in shutdown. The USBIN input draws 18µA during shutdown if no power is applied to DCIN, but draws only 10µA when VDCIN > VUSBIN. Once the charger is enabled, it enters constant-current mode, where the programmed charge current is supplied to the battery. When the BAT pin approaches the final float voltage (4.2V), the charger enters constant-voltage mode and the charge current begins to decrease. Once the charge current drops below the programmed termination threshold (set by the external resistor RITERM), the internal P-channel MOSFET is shut off and the charger enters standby mode. In standby mode, the charger sits idle and monitors the battery voltage using a comparator with a 6ms filter time (tRECHRG). A charge cycle automatically restarts when the battery voltage falls below 4.1V (which corresponds to approximately 80% to 90% battery capacity). This ensures that the battery is kept near a fully charged condition and eliminates the need for periodic charge cycle initiations. Figure 1 uses a state diagram to describe the behavior of the LTC3550-1 battery charger. STARTUP DCIN POWER APPLIED ONLY USB POWER APPLIED POWER SELECTION DCIN POWER REMOVED CHARGE MODE FULL CURRENT USBIN POWER REMOVED OR DCIN POWER APPLIED CHRG STATE: PULLDOWN CHARGE MODE FULL CURRENT CHRG STATE: PULLDOWN IBAT < ITERMINATE IN VOLTAGE MODE BAT < 4.1V ENABLE DRIVEN LOW IBAT < ITERMINATE IN VOLTAGE MODE STANDBY MODE STANDBY MODE NO CHARGE CURRENT NO CHARGE CURRENT CHRG STATE: Hi-Z CHRG STATE: Hi-Z ENABLE DRIVEN HIGH SHUTDOWN MODE ENABLE DRIVEN LOW IDCIN DROPS TO 20µA CHRG STATE: Hi-Z SHUTDOWN MODE BAT < 4.1V ENABLE DRIVEN HIGH IUSBIN DROPS TO 18µA DCIN POWER REMOVED USBIN POWER REMOVED OR DCIN POWER APPLIED CHRG STATE: Hi-Z 3550-1 F01 Figure 1. LTC3550-1 State Diagram of a Charge Cycle 35501f 12 LTC3550-1 U OPERATIO 600mA Step-Down Regulator The LTC3550-1 regulator uses a constant frequency, current mode step-down architecture. Both the top (P-channel MOSFET) and bottom (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and is turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the output voltage (VOUT), relative to the internal reference, which in turn causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. ® Burst Mode Operation The LTC3550-1 buck regulator is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load current demand. In Burst Mode operation, the peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 20µA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier’s output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3550-1 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Battery Charger Power Source Selection The LTC3550-1 can charge a battery from either the wall adapter input or the USB port input. The charger automatically senses the presence of voltage at each input. If both power sources are present, the charger defaults to the wall adapter source provided sufficient power is present at the DCIN input. “Sufficient power” is defined as: • Supply voltage is greater than the UVLO threshold. • Supply voltage is greater than the battery voltage by 50mV (180mV rising, 50mV falling). The open drain power status outputs (⎯P⎯W⎯R and USBPWR) indicate which power source has been selected. Table 1 describes the behavior of these status outputs. Table 1. Power Source Selection VDCIN > 4.15V and VDCIN > BAT + 50mV VDCIN < 4.15V or VDCIN < BAT + 50mV VUSBIN > 3.95V and VUSBIN > BAT + 50mV VUSBIN < 3.95V or VUSBIN < BAT + 50mV Device Powered from Wall Adapter Source; USBIN Current < 25µA ⎯P⎯W⎯R: LOW USBPWR: LOW Device Powered from Wall Adapter Source Device Powered from USB Source; ⎯P⎯W⎯R: LOW USBPWR: Hi-Z No Charging ⎯P⎯W⎯R: LOW USBPWR: LOW ⎯P⎯W⎯R: Hi-Z USBPWR: LOW Status Indicators Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will Burst Mode is a registered trademark of Linear Technology Corporation. The charge status output (⎯C⎯H⎯R⎯G) has two states: pulldown and high impedance. The pull-down state indicates that the LTC3550-1 is in a charge cycle. Once the charge cycle has terminated or the LTC3550-1 is disabled, the pin state becomes high impedance. The pull-down state is strong enough to drive an LED and is capable of sinking up to 10mA. 35501f 13 LTC3550-1 U OPERATIO ⎯ W ⎯ R ⎯ ) has two states: pullThe power supply status output (P down and high impedance. The pull-down state indicates that power is present at either DCIN or USBIN. If no power is applied at either pin, the ⎯P⎯W⎯R pin is high impedance, indicating that the LTC3550-1 lacks sufficient power to charge the battery. The pull-down state is strong enough to drive an LED and is capable of sinking up to 10mA. The USB power status output (USBPWR) has two states: pull-down and high impedance. The high impedance state indicates that the LTC3550-1 is being powered from the USBIN input. The pull-down state indicates that the charger is either powered from DCIN or is in a UVLO condition (see Table 1). The pull-down state is capable of sinking up to 1mA. assurance that the charger will automatically reduce the current in worst case conditions. DFN package power considerations are discussed further in the Applications Information section. Charge Current Soft-Start and Soft-Stop The 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 from zero to full-scale current over a period of 250µs. Likewise, internal circuitry slowly ramps the charge current from full-scale to zero in approximately 30µs when the charger shuts down or self terminates. This minimizes the transient current load on the power supply during start-up and shut-off. Thermal Limiting An internal thermal feedback loop reduces the programmed charge current if the die temperature attempts to rise above a preset value of approximately 105°C. This feature protects the LTC3550-1 from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without risk of damaging the device. The charge current can be set according to typical (not worst-case) ambient temperature with the Short-Circuit Protection When the regulator output (VOUT) is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, one seventh the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 1.5MHz when VOUT rises above 0V. 35501f 14 LTC3550-1 U U W U APPLICATIO S I FOR ATIO Figure 2 shows the basic LTC3550-1 application circuit. External component selection is driven by the charging requirements and the buck regulator load requirements. LTC3550-1 SW RUN WALL ADAPTER C2 RIUSB USBIN VCC IUSB BAT IDC C1 COUT VOUT DCIN USB POWER L1 ITERM GND RIDC CIN + RITERM VOUT 1.875V 600mA 4.2V SINGLE CELL Li-Ion BATTERY 3550-1 F02 Figure 2. LTC3550-1 Basic Circuit Programming and Monitoring Charge Current The charge current delivered to the battery from the wall adapter supply is programmed using a single resistor from the IDC pin to ground. Likewise, the charge current from the USB supply is programmed using a single resistor from the IUSB pin to ground. The program resistor and the charge current (ICHRG) are calculated using the following equations: RIDC = 1000 V ICHRG(DC) RIUSB = , ICHRG(DC) = 1000 V ICHRG(USB) 1000 V RIDC , ICHRG(USB) = 1000 V RIUSB Charge current out of the BAT pin can be determined at any time by monitoring the IDC or IUSB pin voltage and using the following equations: VIDC • 1000 (ch arg ing from wall adapter ) RIDC V = IUSB • 1000 (ch arg ing from USB sup ply ) RIUSB IBAT = IBAT Programming Charge Termination The charge cycle terminates when the charge current falls below the programmed termination threshold during constant-voltage mode. This threshold is set by connecting an external resistor, RITERM, from the ITERM pin to ground. The charge termination current threshold (ITERMINATE) is set by the following equation: 100 V 100 V RITERM = , ITERMINATE = ITERMINATE RITERM The termination condition is detected by using an internal filtered comparator to monitor the ITERM pin. When the ITERM pin voltage drops below 100mV* for longer than tTERMINATE (typically 1.5ms), charging is terminated. The charge current is latched off and the LTC3550-1 enters standby mode. When charging, transient loads on the BAT pin can cause the ITERM pin to fall below 100mV for short periods of time before the DC charge current has dropped below the programmed termination current. The 1.5ms filter time (tTERMINATE) on the termination comparator ensures that transient loads of this nature do not result in premature charge cycle termination. Once the average charge current drops below the programmed termination threshold, the LTC3550-1 terminates the charge cycle and stops providing any current out of the BAT pin. In this state, any load on the BAT pin must be supplied by the battery. Buck Regulator Inductor Selection For most applications, the value of the inductor will fall in the range of 1µH to 4.7µH. Its value is chosen based on the desired inductor ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VCC or VOUT also increases the ripple current as shown in Equation 1. A reasonable starting point for setting ripple current is ΔIL = 240mA (40% of 600mA). ⎞ ⎛ V V ∆ IL = OUT • ⎜ 1 − OUT ⎟ fO • L ⎝ VCC ⎠ (1) * Any external sources that hold the ITERM pin above 100mV will prevent the LTC3550-1 from terminating a charge cycle. 35501f 15 LTC3550-1 U W U U APPLICATIO S I FOR ATIO The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA + 120mA). For best efficiency, choose a low DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher ΔIL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3550-1 requires to operate. Table 2 shows some typical surface mount inductors that work well in LTC3550-1 applications. Table 2. Representative Surface Mount Inductors PART NUMBER VALUE (µH) DCR (Ω MAX) MAX DC CURRENT (A) SIZE W × L × H (mm) Sumida CDRH3D16 1.5 2.2 3.3 4.7 0.043 0.075 0.110 0.162 1.55 1.20 1.10 0.90 3.8 × 3.8 × 1.8 Sumida CMD4D06 2.2 3.3 4.7 0.116 0.174 0.216 0.950 0.770 0.750 3.5 × 4.3 × 0.8 Panasonic ELT5KT 3.3 4.7 0.17 0.20 1.00 0.95 4.5 × 5.4 × 1.2 Murata LQH32CN 1.0 2.2 4.7 0.060 0.097 0.150 1.00 0.79 0.65 2.5 × 3.2 × 2.0 CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VCC. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN required IRMS ≅ IOMAX VOUT ( VCC − VOUT ) VCC (2) This formula has a maximum at VCC = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple ΔVOUT is determined by: ⎛ 1 ⎞ ∆ VOUT ≅ ∆ IL ⎜ ESR + 8 fCOUT ⎟⎠ ⎝ (3) where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. For a fixed output voltage, the output ripple voltage is highest at maximum input voltage since ΔIL increases with input voltage. Aluminum electrolytic and solid tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. 35501f 16 LTC3550-1 U W U U APPLICATIO S I FOR ATIO Using Ceramic Input and Output Capacitors Higher capacitance values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3550-1’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – (L1 + L2 + L3 + ...) than the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VCC and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is “chopped” between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. where L1, L2, etc. are the individual losses as a percentage of input power. 1. The VCC quiescent current is due to two components: the DC bias current as given in the Electrical Characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VCC to ground. The resulting dQ/dt is the current out of VCC that is typically larger 0.1 POWER LOSS (W) Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3550-1 circuits: VCC quiescent current and I2R losses. The VCC quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 3. 1 0.01 0.001 0.0001 0.00001 0.1 1 10 100 LOAD CURRENT (mA) 1000 3550-1 F03 Figure 3. Power Lost vs Load Current 35501f 17 LTC3550-1 U W U U APPLICATIO S I FOR ATIO Thermal Considerations The battery charger’s thermal regulation feature and the buck regulator’s high efficiency make it unlikely that enough power is dissipated to exceed the LTC3550-1 maximum junction temperature. Nevertheless, it is a good idea to do some thermal analysis for worst-case conditions. The junction temperature, TJ, is given by: TJ = TA + TRISE where TA is the ambient temperature. The temperature rise is given by: TRISE = PD • θJA where PD is the power dissipated and θJA is the thermal resistance from the junction of the die to the ambient temperature. In most applications the buck regulator does not dissipate much heat due to its high efficiency. The majority of the LTC3550-1 power dissipation occurs when charging a battery. Fortunately, the LTC3550-1 automatically reduces the charge current during high power conditions using a patented thermal regulation circuit. Thus, there is no need to design for worst-case power dissipation scenarios because the LTC3550-1 ensures that the battery charger power dissipation never raises the junction temperature above a preset value of 105°C. In the unlikely case that the junction temperature is forced above 105°C (due to abnormally high ambient temperatures or excessive buck regulator power dissipation), the battery charge current will be reduced to zero and thus dissipate no heat. As an added measure of protection, even if the junction temperature reaches approximately 150°C, the buck regulator’s power switches will be turned off and the SW node will become high impedance. The conditions that cause the LTC3550-1 to reduce charge current through thermal feedback can be approximated by considering the power dissipated in the IC. The approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 105°C – TRISE PD(CHARGER) = (VIN – VBAT) • IBAT (5) VIN is the charger supply voltage (either DCIN or USBIN), VBAT is the battery voltage and IBAT is the charge current. Example: An LTC3550-1 operating from a 5V wall adapter (on the DCIN input) is programmed to supply 650mA full-scale current to a discharged Li-Ion battery with a voltage of 2.7V. The charger power dissipation is calculated to be: PD(CHARGER) = (5V – 2.7V) • 650mA = 1.495W For simplicity, assume the buck regulator is disabled and dissipates no power (PD(BUCK) = 0). For a properly soldered DHC16 package, the thermal resistance (θJA) is 40°C/W. Thus, the ambient temperature at which the LTC3550-1 charger will begin to reduce the charge current is: TA = 105°C – 1.495W • 40°C/W TA = 105°C – 59.8°C TA = 45.2°C The LTC3550-1 can be used above 45.2°C ambient, but the charge current will be reduced from 650mA. Assuming no power dissipation from the buck converter, the approximate current at a given ambient temperature can be approximated by: IBAT = 105 °C – TA ( VIN – VBAT ) • θ JA (6) Using the previous example with an ambient temperature of 60°C, the charge current will be reduced to approximately: IBAT = 105 °C – 60 °C 45 °C = (5V – 2 . 7 V) • 40 °C/W 92 °C/A IBAT = 489mA TA = 105°C – (PD • θJA) TA = 105°C – (PD(CHARGER) + PD(BUCK)) • θJA Most of the charger’s power dissipation is generated from the internal charger MOSFET. Thus, the power dissipation is calculated to be: (4) Because the regulator typically dissipates significantly less heat than the charger (even in worst-case situations), the calculations here should work well as an approximation. 35501f 18 LTC3550-1 U W U U APPLICATIO S I FOR ATIO However, the user may wish to repeat the previous analysis to take the buck regulator’s power dissipation into account. Equation (6) can be modified to take into account the temperature rise due to the buck regulator: IBAT = 105 °C – TA − (PD(BUCK ) • θ JA ) ( VIN – VBAT ) • θ JA (7) For optimum performance, it is critical that the exposed metal pad on the backside of the LTC3550-1 package is properly soldered to the PC board ground. When correctly soldered to a 2500mm2 double sided 1oz copper board, the LTC3550-1 has a thermal resistance of approximately 40°C/W. Failure to make thermal contact between the exposed pad on the backside of the package and the copper board will result in thermal resistances far greater than 40°C/W. As an example, a correctly soldered LTC3550-1 can deliver over 800mA to a battery from a 5V supply at room temperature. Without a good backside thermal connection, this number would drop to much less than 500mA. Battery Charger Stability Considerations The constant-voltage mode feedback loop is stable without any compensation provided a battery is connected to the charger output. When the charger is in constant-current mode, the charge current program pin (IDC or IUSB) is in the feedback loop, not the battery. The constant-current mode stability is affected by the impedance at the charge current program pin. With no additional capacitance on this pin, the charger is stable with program resistor values as high as 20k (ICHG = 50mA); however, additional capacitance on these nodes reduces the maximum allowed program resistor value. Checking Regulator Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ΔILOAD • ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady state value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76. A second, more severe transient is caused by switching in loads with large (>1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD). Thus, a 10µF capacitor charging to 3.3V would require a 250µs rise time, limiting the charging current to about 130mA. Protecting the USB Pin and Wall Adapter Input from Overvoltage Transients Caution must be exercised when using ceramic capacitors to bypass the USBIN pin or the wall adapter inputs. High voltage transients can be generated when the USB or wall adapter is hot-plugged. When power is supplied via the USB bus or wall adapter, the cable inductance along with the self resonant and high Q characteristics of ceramic capacitors can cause substantial ringing which could exceed the maximum voltage ratings and damage the LTC3550-1. Refer to Linear Technology Application Note 88, entitled “Ceramic Input Capacitors Can Cause Overvoltage Transients” for a detailed discussion of this problem. The long cable lengths of most wall adapters and USB cables makes them especially susceptible to this problem. To bypass the USB and the wall adapter inputs, add a 1Ω resistor in series with a ceramic capacitor to lower the effective Q of the network and greatly reduce the ringing. A tantalum, OS-CON, or electrolytic capacitor can be used in place of the ceramic and resistor, as their higher ESR reduces the Q, thus reducing the voltage ringing. The oscilloscope photograph in Figure 4 shows how serious the overvoltage transient can be for the USB and wall adapter inputs. For both traces, a 5V supply is hot-plugged using a three foot long cable. For the top trace, only a 4.7µF ceramic X5R capacitor (without the recommended 1Ω series resistor) is used to locally bypass the input. This trace shows excessive ringing when the 5V cable is inserted, with the overvoltage spike reaching 10V. For 35501f 19 LTC3550-1 U W U U APPLICATIO S I FOR ATIO the bottom trace, a 1Ω resistor is added in series with the 4.7µF capacitor to locally bypass the 5V input. This trace shows the clean response resulting from the addition of the 1Ω resistor. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3550-1. These items are also illustrated graphically in Figures 5 and 6. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VCC trace should be kept short, direct and wide. 4.7μF ONLY 2V/DIV 2. Does the VOUT pin connect directly to the output? 4.7μF + 1Ω 2V/DIV 20μs/DIV 3550-1 F04 Figure 4. Waveforms Resulting from Hot-Plugging a 5V Input Supply When Using Ceramic Bypass Capacitors Even with the additional 1Ω resistor, bad design techniques and poor board layout can often make the overvoltage problem even worse. System designers often add extra inductance in series with input lines in an attempt to minimize the noise fed back to those inputs by the application. In reality, adding these extra inductances only makes the overvoltage transients worse. Since cable inductance is one of the fundamental causes of the excessive ringing, adding a series ferrite bead or inductor increases the effective cable inductance, making the problem even worse. For this reason, do not add additional inductance (ferrite beads or inductors) in series with the USB or wall adapter inputs. For the most robust solution, 6V transorbs or zener diodes may also be added to further protect the USB and wall adapter inputs. Two possible protection devices are the SM2T from STMicroelectronics and the EDZ series devices from ROHM. Always use an oscilloscope to check the voltage waveforms at the USBIN and DCIN pins during USB and wall adapter hot-plug events to ensure that overvoltage transients have been adequately removed. 3. Does the (+) plate of CIN connect to VCC as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the (–) plates of CIN and COUT as close as possible. 5. Solder the exposed pad on the backside of the package to PC board ground for optimum thermal performance. The thermal resistance of the package can be further enhanced by increasing the area of the copper used for PC board ground. Design Example As a design example, assume the LTC3550-1 is used in a single lithium-ion battery-powered cellular phone application. The battery is charged by either plugging a wall adapter into the phone or putting the phone in a USB cradle. The optimum charge current for this particular lithium-ion battery is determined to be 800mA. Starting with the charger, choosing RIDC to be 1.24k programs the charger for 806mA. Choosing RIUSB to be 2.1k programs the charger for 475mA when charging from the USB cradle, ensuring that the charger never exceeds the 500mA maximum current supplied by the USB port. A good rule of thumb for ITERMINATE is onetenth the full charge current, so RITERM is picked to be 1.24k (ITERMINATE = 80mA). Moving on to the step-down converter, VCC will be powered from the battery which can range from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 600mA but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low 35501f 20 LTC3550-1 U U W U APPLICATIO S I FOR ATIO and high load currents is important. With this information we can calculate L using Equation (1), ∆ IL = VOUT fO • L A 2.2µH inductor works well for this application. For best efficiency choose a 720mA or greater inductor with less than 0.2Ω series resistance. CIN will require an RMS current rating of at least 0.3A = ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.25Ω. In most cases, a ceramic capacitor will satisfy this requirement. ⎞ ⎛ V • ⎜ 1 − OUT ⎟ VCC ⎠ ⎝ Substituting VOUT = 1.875V, VCC = 4.2V, ΔIL = 240mA and fO = 1.5MHz in Equation (3) gives: L= Figure 7 shows the complete circuit along with its efficiency curve. 1 . 875V ⎛ 1 . 875V ⎞ • ⎜1− = 2 . 88µH 1 . 5MHz • (240mA) ⎝ 4 . 2V ⎟⎠ LTC3550-1 6 7 VCC + – BOLD LINES INDICATE HIGH CURRENT PATHS VOUT SW VCC 10 CIN 8 GND GND L1 9 – 17 COUT + VOUT 3550-1 F05 Figure 5. DC-DC Converter Layout Diagram VIA TO VOUT VIA TO VCC SW CIN L1 VCC GND COUT VOUT 3550-1 F06 Figure 6. DC-DC Converter Suggested Layout 35501f 21 LTC3550-1 U U W U APPLICATIO S I FOR ATIO LTC3550-1 SW RUN WALL ADAPTER USB POWER 1µF 2.1k 1% DCIN VOUT USBIN VCC IUSB BAT IDC 1µF VOUT 1.875V 600mA 10µF** CER 2.2µH* 800mA (WALL) 475mA (USB) 4.7µF† ITERM GND + 1.24k 1% 1.24k 1% 4.2V SINGLECELL Li-Ion BATTERY 3550-1 F07a * MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN LMK212BJ475MG Figure 7a. Design Example Circuit 100 VCC = 2.7V EFFICIENCY (%) 90 VCC = 4.2V 80 70 VCC = 3.6V 60 50 0.1 1 10 ILOAD (mA) 100 1000 35501 F07b Figure 7b. Buck Regulator Efficiency vs Output Current U TYPICAL APPLICATIO S Full Featured Dual Input Charger Plus Step-Down Converter WALL ADAPTER USB POWER 800mA (WALL) 475mA (USB) LTC3550-1 BAT DCIN USBIN 1µF 4.7µF + 1k PWR 1µF 1k CHRG VCC IUSB 2.2µH SW IDC 2.1k 1% 1.24k 1% VOUT ITERM 1k 1% 4.2V SINGLE-CELL Li-Ion BATTERY GND 10µF CER VOUT 1.875V 600mA 3550-1 TA03 35501f 22 LTC3550-1 U PACKAGE DESCRIPTIO DHC Package 16-Lead Plastic DFN (5mm × 3mm) (Reference LTC DWG # 05-08-1706) 0.65 ±0.05 3.50 ±0.05 1.65 ±0.05 2.20 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 4.40 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 5.00 ±0.10 (2 SIDES) R = 0.20 TYP 3.00 ±0.10 (2 SIDES) 9 0.40 ± 0.10 16 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) PIN 1 NOTCH (DHC16) DFN 1103 8 0.200 REF 1 0.25 ± 0.05 0.50 BSC 0.75 ±0.05 4.40 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WJED-1) IN JEDEC PACKAGE OUTLINE MO-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 35501f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23 LTC3550-1 U TYPICAL APPLICATIO S Dual Input Charger Plus Step-Down Converter with Wall Adapter PowerPath™ LTC3550-1 DCIN WALL ADAPTER 1µF USBIN USB POWER VCC 1k 4.7µF 1µF 800mA (WALL) 475mA (USB) IUSB BAT IDC SW 2.2µH 2.1k 1% 1.24k 1% VOUT ITERM 1k 1% GND 10µF CER VOUT 1.875V 600mA + 4.2V SINGLE-CELL Li-Ion BATTERY 3550-1 TA04 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20µA, ThinSOT Package LTC3406/LTC3406B 1.5MHz, 600mA Synchronous Step-Down DC/DC Converter in ThinSOTTM LTC3455 Dual DC/DC Converter with USB Power Management and Li-Ion Battery Charger Efficiency >96%, Accurate USB Current Limiting (500mA/100mA), 4mm × 4mm QFN-24 Package LTC3456 2-Cell Multi-Output DC/DC Converter with USB Power Manager Seamless Transition Between 2-Cell Battery, USB and AC Wall Adapter Input Power Sources, QFN Package LTC4054 Standalone Linear Li-Ion Battery Charger with Integrated Pass Transistor in ThinSOT Thermal Regulation Prevents Overheating, C/10 Termination, Up to 800mA Charge Current LTC4055 USB Power Controller and Battery Charger Charges Single-Cell Li-Ion Batteries Directly from USB Port, Thermal Regulation, 4mm × 4mm QFN-16 Package LTC4058 Standalone 950mA Lithium-Ion Charger in DFN C/10 Charge Termination, Battery Kelvin Sensing, ±7% Charge Accuracy LTC4063 Standalone Li-Ion Charger Plus LDO 4.2V, ±0.35% Float Voltage, Up to 1A Charge Current, 100mA LDO LTC4068 Standalone Linear Li-Ion Battery Charger with Programmable Termination Charge Current up to 950mA, Thermal Regulation, 3mm × 3mm DFN-8 Package LTC4075 Dual Input Standalone Li-Ion Battery Charger Charges Single-Cell Li-Ion Batteries from Wall Adapter and USB Inputs with Automatic Input Power Detection and Selection, 950mA Charger Current, Thermal Regulation, C/X Charge Termination, 3mm × 3mm DFN Package LTC4076 Dual Input Standalone Li-Ion Battery Charger Charges Single-Cell Li-Ion Batteries from Wall Adapter and USB Inputs with Automatic Input Power Detection and Selection, 950mA Charger Current, Thermal Regulation, USB Low Power Mode Select, C/X Charge Termination, 3mm × 3mm DFN Package LTC4077 Dual Input Standalone Li-Ion Battery Charger Charges Single-Cell Li-Ion Batteries from Wall Adapter and USB Inputs with Automatic Input Power Detection and Selection, 950mA Charger Current, Thermal Regulation, Programmable USB Low Power Mode, C/10 Charge Termination, 3mm × 3mm DFN Package PowerPath and ThinSOT are trademarks of Linear Technology Corporation. 35501f 24 Linear Technology Corporation LT 1205 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005