LTC4123 Low Power Wireless Charger for Hearing Aids Features Description Complete Low Power Wireless NiMH Charger nn Low Minimum Input Voltage: 2.2V nn Small Total Solution Volume nn 1.5V, 25mA Linear Single-Cell NiMH Charger nn Temperature Compensated Charge Voltage nn Integrated Rectifier with Overvoltage Limit nn Zinc-Air Battery Detection nn Reverse Polarity Protection nn Thermally Enhanced 6-Lead (2mm × 2mm) DFN package The LTC®4123 is a low power wireless receiver and a constant-current/constant-voltage linear charger for NiMH batteries. An external programming resistor sets the charge current up to 25mA. The temperature compensated charge voltage feature protects the NiMH battery and prevents overcharging. Applications The LTC4123 prevents charging of Zinc-Air batteries as well as batteries inserted with reverse polarity. The LTC4123 pauses charging if its temperature is too hot or too cold. An internal timer provides time-based charging termination. nn Wireless charging with the LTC4123 allows products to be charged while sealed within enclosures and eliminates bulky connectors in space constrained environments. The LTC4123 also makes it possible to charge NiMH batteries used in moving or rotating equipment. Hearing Aids Smart Cards nn Fitness Devices nn Moving and/or Rotating Equipment nn nn The 2mm × 2mm DFN package and low external component count make the LTC4123 well-suited for hearing aid applications or other low power portable devices where small solution size is mandatory. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application 25mA NiMH Wireless Battery Charger Complete Wireless Charging Solution for a Hearing Aid AIR GAP ACIN Tx COIL VIN + – TRANSMITTER CIRCUIT LRX 13µH BAT ICHARGE = 25mA MAX VCC LED LTC4123 CRX 33nF + CHRG CIN 4.7µF 1.5V NiMH BATTERY GND PROG RPROG 953Ω 4123 TA01 4123f For more information www.linear.com/LTC4123 1 LTC4123 Absolute Maximum Ratings Pin Configuration (Notes 1, 3) Input Supply Voltages VCC........................................................ –0.3V to 5.5V ACIN................................................... –10V to VCC+1V Input Supply Currents I(ACIN)............................................................. 200mA BAT.................................................................. –2V to 2V PROG, CHRG..................................... –0.3V to VCC+0.3V Operating Junction Temperature Range (Note 2)......................................................... –20 to 85°C Storage Temperature Range.......................–65 to 150°C TOP VIEW 6 GND ACIN 1 VCC 2 7 GND 5 BAT 4 PROG CHRG 3 DC PACKAGE 6-LEAD (2mm × 2mm) PLASTIC DFN TJMAX = 85°C, θJA = 80.6°C/W EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC4123EDC#PBF LTC4123EDC#TRPBF LGSY 6-Lead (2mm × 2mm) Plastic DFN –20°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. 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/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VACIN = 0V, VCC = 5V unless otherwise noted (Notes 2, 3, 4). SYMBOL PARAMETER VCC Input Supply Operating Range IVCC Input Quiescent Operating Current VUVLO Input Supply Undervoltage Lockout Threshold VCC Rising Hysteresis VBAT Battery Charge Voltage IBAT(LEAK) Battery Pin Discharge Current VPROG PROG Pin Servo Voltage hPROG Ratio of BAT Current to PROG Current ICHG Constant-Current Mode Charge Current VUVCL Undervoltage Current Limit TCHG Charge Termination Period 2 CONDITIONS MIN l Charging Terminated. IBAT and IPROG = 0A TYP 2.2 l 1.88 MAX UNITS 5 V 125 200 µA 1.95 2.02 V 40 mV TA = 25°C 1.4955 1.5075 1.5195 V TA = –10°C (Note 4) 1.580 1.595 1.610 V TA = 75°C (Note 4) 1.3675 1.3825 1.3975 V 100 nA Charger Terminated or VCC < VUVLO, VBAT = 2V 0.25 V 96 mA/mA RPROG = 23.7kΩ l 0.73 1 1.27 mA RPROG = 953Ω l 22 25 28 mA 7.2 Hours RPROG = 4.99kΩ 2.2 4.8 6 V 4123f For more information www.linear.com/LTC4123 LTC4123 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VACIN = 0V, VCC = 5V unless otherwise noted (Notes 2, 3, 4). SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Thermal Sensing Cold Temperature Fault Threshold Die Temperature Falling Hysteresis Hot Temperature Fault Threshold Die Temperature Rising Hysteresis –5 °C 5 °C 70 °C 5 °C Zinc-Air Battery Detection VZn-AIR Zinc-Air Fault Threshold Voltage VBAT Rising 1.60 Hysteresis TZn-AIR 1.65 V 40 Zinc-Air Detection Period Charge Voltage Limit During Zinc-Air Battery Detection Zinc-Air Detection Charge Current RPROG = 23.7kΩ mV 80 s 1.8 V 1 mA –50 mV 40 mV 5 V Reverse Polarity Detection VREVPOL Reverse Polarity Threshold Voltage VBAT Falling Hysteresis AC Rectification VCC(HIGH) VCC(LOW) VCC High Voltage Limit VCC Rising VCC Low Voltage Limit VCC Falling ACIN to VCC Voltage Drop IVCC = –20mA, Charger Terminated 3 V 0.65 V Status Pin (CHRG) ICHRG CHRG Pin Pull-Down Current VCHRG = 450mV CHRG Leakage Current CHRG = 5V 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 LTC4123 is tested under conditions such that TJ ≈ TA. The LTC4123E is guaranteed to meet specifications from 0°C to 85°C junction temperature. Specifications over the –20°C to 85°C operating junction temperature are assured by design, characterization and correlation with statistical process controls. Note that the maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the rated package thermal 250 340 430 µA 1 µA impedance and other environmental factors. The junction temperature (TJ, in °C) is calculated from the ambient temperature (TA, in °C) and power dissipation (PD, in Watts) according to the following formula: TJ = TA + (PD • θJA), where θJA (in °C/W) is the package thermal impedance. Note 3: All currents into pins are positive; all voltages are referenced to GND unless otherwise noted. Note 4: These parameters are guaranteed by design and are not 100% tested. The battery charge voltage variation over temperature is guaranteed in a ±15mV band as shown in the Typical Performance Characteristics curve. 4123f For more information www.linear.com/LTC4123 3 LTC4123 Typical Performance Characteristics Battery Charge Current vs Battery Charge Voltage Battery Charge Voltage vs Temperature 12.0 Battery Charge Voltage vs Supply Voltage 1.600 1.520 CHARGE VOLTAGE CHARGE VOLTAGE MAX CHARGE VOLTAGE MIN 1.580 10.0 1.560 1.515 1.540 8.0 6.0 VBAT (V) 1.520 VBAT (V) ICHG (mA) TA = 25°C, unless otherwise noted. 1.500 1.480 1.460 4.0 1.510 1.505 1.440 1.500 1.420 2.0 1.400 RPROG = 2.49kΩ 0 1.30 1.35 1.40 1.45 1.50 VBAT (V) 1.55 1.380 1.60 RPROG = 23.7kΩ –5 10 4123 G01 25 40 TEMPERATURE (°C) 55 RPROG = 23.7kΩ 1.495 2.5 3 3.5 4 4.5 SUPPLY VOLTAGE (V) 70 5 4123 G02 PROG Pin Voltage vs Temperature (Constant Current Mode) 4123 G03 Undervoltage Current Limit: Charge Current vs Supply Voltage 260 Charge Current vs PROG Pin Voltage 1.00 12.0 RPROG = 2.49kΩ 10.0 0.80 255 250 ICHG (mA) ICHG (mA) VPROG (mV) 8.0 6.0 0.60 0.40 4.0 245 0.20 2.0 240 RPROG = 23.7kΩ RPROG = 23.7k –5 10 25 40 TEMPERATURE (°C) 55 0 70 2 2.2 2.4 2.6 2.8 SUPPLY VOLTAGE (V) 4123 G04 3.0 Battery Leakage Current vs Temperature 150 140 SUPPLY VOLTAGE (V) IBAT(LEAK) (nA) IVCC (µA) 40 20 0 –20 –40 –60 –80 2 2.5 3 3.5 4 SUPPLY VOLTAGE (V) 4.5 5 4123 G07 4 200 250 4123 G06 1.98 60 110 100 150 VPROG (mV) 2.00 80 120 50 UVLO Threshold vs Temperature (Rising and Falling) 100 VBAT = –100mV 130 0 4123 G05 Input Quiescent Current vs Supply Voltage 100 0 –5 1.94 1.92 1.90 1.88 VCC = 0V VBAT = 2V –100 –20 1.96 10 25 40 55 TEMPERATURE (°C) 70 85 4123 G08 1.86 –20 UVLO FALLING UVLO RISING –5 10 25 40 55 TEMPERATURE (°C) 70 85 4123 G09 4123f For more information www.linear.com/LTC4123 LTC4123 Typical Performance Characteristics TA = 25°C, unless otherwise noted. CHRG Pull-Down Current vs Temperature 5.00 380 4.50 360 4.00 340 ICHRG (µA) SUPPLY VOLTAGE (V) VCC High and Low Thresholds vs Temperature 3.50 320 300 3.00 2.50 –20 VCC(HIGH) VCC(LOW) –5 10 25 40 55 TEMPERATURE (°C) 70 280 –20 85 –5 10 25 40 55 TEMPERATURE (°C) 70 4123 G11 4123 G10 Charge Timer Accuracy vs Supply Voltage Charge Termination Period vs Temperature 7.20 CHARGE TIMER ACCURACY (%) 20.0 TCHG (Hours) 6.60 6.00 5.40 4.80 –5 10 25 40 TEMPERATURE (°C) 55 15.0 10.0 5.0 0 –5.0 –10.0 –15.0 –20.0 2.2 70 2.9 3.6 4.3 SUPPLY VOLTAGE (V) 5.0 4123 G12 4123 G13 Maximum Available Wireless Power vs Coil Spacing Typical Wireless Charging Cycle 24 75 18 50 RPROG(MIN) = 953Ω 12 LRX = 760308101208 25 LTX = 760308103206 fDRIVE = 244kHz See Figure 4 0 1.5 3.5 6 5.5 7.5 9.5 COIL SPACING (mm) 0 11.5 1.6 VBAT 195 1.2 130 0.8 See Figure 4 P675 NiMH RPROG=976Ω fDRIVE=244kHz LTX=760308103206 LRX=760308101208 65 0 0 4123 G14 1 2 VPROG 3 4 VBAT (V) 100 260 VPROG (mV) MAX POWER MAX CHARGE CURRENT 30 MAXIMUM CHARGE CURRENT AVAILABLE (mA) 125 MAXIMUM AVAILABLE POWER (mW) 85 0.4 5 6 0.0 TIME (HOURS) 4123 G15 4123f For more information www.linear.com/LTC4123 5 LTC4123 Pin Functions ACIN (Pin 1): AC Input Voltage. Connect the external LC tank, which includes the receive inductor, to this pin. Short this pin to ground when not used. VCC (Pin 2): The DC input voltage range is 2.2V to 5V. An internal diode is connected from the ACIN pin (anode) to this pin (cathode). When an AC voltage is present at the ACIN pin, the voltage on this pin is the rectified AC voltage. Connect a 4.7µF capacitor to ground on this pin. When the ACIN pin is not used (shorted to ground), connect this pin to a DC voltage source to provide power to the part and to charge the battery. CHRG (Pin 3): Open-Drain charge status output. CHRG requires a pull-up resistor and/or LED to indicate the status of the battery charger. This pin has four possible states: powered on/charging (blink slow), no power /not charging (high impedance), charging complete (pull-down), and Zinc-Air battery/reverse polarity detection/ battery temperature out of range/UVCL at the beginning of the charge cycle (blink fast). To conserve power, this pin implements a 300µA pull-down current source. 6 PROG (Pin 4): The charge current program pin. A 1% resistor, RPROG, connected from PROG to ground programs the charge current. In constant-current charging mode, the voltage at this pin is regulated to 0.25V. The voltage on this pin sets the constant current charge current to: ICHG = 96 • VPROG 24V = RPROG RPROG BAT (Pin 5): Battery connection pin. Connect the NiMH battery to this pin. At 25°C, the battery voltage is regulated to 1.5075V. This charge voltage is temperature compensated with a temperature coefficient of –2.5mV/ºC. GND (Pin 6, Exposed Pad Pin 7): Ground. Connect the ground pins to a suitable PCB copper ground plane for proper electrical operation. The exposed pad must be soldered to PCB ground for the rated thermal performance. 4123f For more information www.linear.com/LTC4123 LTC4123 Block Diagram VCC ACIN RECTIFICATION AND INPUT POWER CONTROL IBAT VUVLO + – 96 CONSTANT CURRENT (CC) + CONSTANT VOLTAGE (CV) + UNDERVOLTAGE CURRENT LIMIT (UVCL) VUVCL VCC + – + UVCL – CHARGING (SLOW BLINK) CHARGING COMPLETE (ON) 300µA TEMP FAULT – LOGIC + CHRG BAT FAULT (BLINK FAST) ZINC-AIR BAT FAULT – + + REVERSE POLARITY FAULT TREF TDIE PROG PROG CC VPROG + CV – BAT VCC NEGATIVE TC VOLTAGE REFERENCE VZn-AIR BAT + BAT BAT VREVPOL – GND 4123 BD Figure 1. Block Diagram operation The LTC4123 is a low power battery charger designed to wirelessly charge single-cell NiMH batteries. The charger uses a constant-current/constant-voltage charge algorithm with a charge current programmable up to 25mA. The final charge voltage is temperature compensated to reach an optimum state-of-charge and prevent overcharging of the battery. The LTC4123 also guarantees the accuracy of the charge voltage to ±15mV from –5°C to 70˚C (see typical performance characteristics). An external resonant LC tank connected to the ACIN pin allows the part to receive power wirelessly from an alternating magnetic field generated by a transmit coil. A complete wireless power transfer system consists of transmit circuitry, with a transmit coil, and receive circuitry, with a receive coil. The Rectification and Input Power control circuitry (Figure 1) rectifies the AC voltage at the ACIN pin and regulates the rectified voltage at VCC to less than VCC(HIGH) (typically 5V). 4123f For more information www.linear.com/LTC4123 7 LTC4123 operation UNIT POWERED *BAT < –50mV? *REVERSE BATTERY CONDITION IS CHECKED THROUGHOUT THE ALGORITHM YES BATTERY IN REVERSE STOP CHARGING PULSE LED FAST YES ZINC-AIR BATTERY PRESENT STOP CHARGING PULSE LED FAST YES STOP CHARGING PAUSE CHARGE TIMER PULSE LED FAST YES CHARGING COMPLETE STOP CHARGING LED ON NO **IF THE DIE TEMPERATURE IS TOO HIGH OR TOO LOW DURING ZINC-AIR BATTERY DETECTION (80 SECONDS), THIS 80 SECOND TIMER WILL BE RESET START CHARGE TIMER START CHARGING PULSE LED SLOWLY BAT > 1.65V? NO NO TIME = 80sec? YES NiMH PRESENT CONTINUE CHARGING PULSE LED SLOWLY **DIE TEMPERATURE TOO HIGH OR TOO LOW? NO NO CHARGE TIMER EXPIRED? 4123 F02 ALL THE VALUES LISTED ABOVE ARE TYPICAL. SEE ELECTRICAL CHARACTERISTICS TABLE FOR MORE INFORMATION Figure 2. Charge Algorithm 8 4123f For more information www.linear.com/LTC4123 LTC4123 operation An LED can be connected to the CHRG pin to indicate the status of the charge cycle and any fault conditions. An internal thermal limit will stop charging and pause the 6-hour charge timer if the die temperature rises above 70˚C or falls below –5˚C. UVLO threshold and switch on the charger again. This oscillatory behavior will result in intermittent charging. The UVCL circuitry prevents this undesirable behavior. In a typical charge cycle (see Figure 2), the 6-hour charge timer will begin when the part is powered. At the beginning of the charge cycle, the LTC4123 will determine if the battery is connected in reverse or if a Zinc-Air battery is connected to the BAT pin. If any of the above fault conditions is true, the BAT pin goes to a high impedance state and charging is stopped immediately. An LED connected to CHRG will blink fast (typically at 6Hz). If the battery is a NiMH battery inserted with correct polarity, it will continue to charge at the programmed current level in constant-current mode and CHRG will blink slowly (typically at 0.8Hz). The LTC4123 detects the presence of Zinc-Air batteries at the beginning of the charge cycle. Initially, the LTC4123 will charge the battery at full charge current and if the BAT pin rises above VZn-AIR (typically 1.65V) in TZn-AIR (typically 80 seconds) or less from the start of the charge timer, the LTC4123 determines the battery connected is a Zinc-Air battery and charging is disabled immediately. The charging cycle continues normally otherwise. The charge resistance of a Zinc-Air battery is higher than a NiMH battery and therefore the battery voltage of Zinc-Air rises significantly. An LED connected to CHRG will blink fast indicating a battery fault condition. When the BAT pin approaches the final charge voltage, the LTC4123 enters constant-voltage mode and the charge current begins to drop. The charge current will continue to drop and the BAT pin voltage will be maintained at the proper charge voltage. After the charge termination timer expires, charge current ceases and the BAT pin assumes a high impedance state. Once the charge cycle terminates, the CHRG pin stops blinking and assumes a pull-down state. To start a new charge cycle, remove the input voltage at ACIN or VCC and reapply it. Input Voltage Qualification An internal undervoltage lockout (UVLO) circuit monitors the input voltage at VCC and disables the LTC4123 until VCC rises above VUVLO (typically 1.95V). The UVLO circuit has a built-in hysteresis of approximately 40mV. During undervoltage conditions, maximum battery drain current is IBAT(LEAK) (typically 100nA). The LTC4123 also includes undervoltage current limiting (UVCL) that prevents charging at the programmed current until the input supply voltage is above VUVCL (typically 2.2V). UVCL is particularly useful in situations when the wireless power available is limited. Without UVCL if the magnetic coupling between the receive coil and transmit coil is low, UVLO could be easily tripped if the charger tries to provide the full charge current. UVLO forces the charge current to zero, which allows the supply voltage to rise above the Battery Fault Conditions If the LTC4123 is in UVCL mode at the beginning of the charge cycle (typically 3 seconds after power is first applied), it is unable to provide full charge current to perform Zinc-Air battery detection. In this case, a battery fault will be indicated at CHRG (blink fast). Adjust the magnetic coupling between the receive and transmit coils to restart the charging cycle. When a battery is inserted in reverse or the die temperature is above 70˚C or below –5˚C, an LED connected to CHRG will blink fast. Table 1 summarizes the four different possible states of the CHRG pin when the charger is active. Table 1. CHRG Pin Status Summary CHRG Blink Frequency Charge Status On (Pull-Down) Charging complete Blink Slow (0.8Hz) Charging Blink Fast (6Hz) Fault-No Charging; Temperature Fault/ Battery in Reverse/Zinc-Air Battery Present/UVCL at the beginning of charge cycle Off (High Impedance) No power/No Charging Operation without Wireless Power LTC4123 can be powered by connecting a DC voltage source to the VCC pin instead of receiving power wirelessly through the ACIN pin. Ground the ACIN pin if an input supply voltage is connected to VCC. 4123f For more information www.linear.com/LTC4123 9 LTC4123 Applications Information Wireless Power Transfer In a wireless power transfer system, power is transmitted using an alternating magnetic field. An AC current in the transmit coil generates a magnetic field. When the receive coil is placed in this field, an AC current is induced in the receive coil. The AC current induced at the receive coil is a function of the applied AC current at the transmitter, and the magnetic coupling between the transmit and receive coils. The LTC4123 internal diode rectifies the AC voltage at the ACIN pin. IAC-TX AIR GAP IAC-RX LTX LRX 1:n 4123 F03 Figure 3. Wireless Power Transfer System The power transmission range across the air gap can be improved using resonance by connecting an LC tank to the ACIN pin tuned to the same frequency as the transmit coil AC current frequency. Receiver and Single Transistor Transmitter The Single Transistor Transmitter shown in Figure 4 is an example of a DC/AC converter that can be used to drive AC current into a transmit coil, LTX. The NMOS, M1, is driven by a 50% duty cycle square wave generated by the LTC6990 oscillator. During the first half of the cycle, M1 is switched on and the current through LTX rises linearly. During the second half of the cycle, M1 is switched off and the current through LTX circulates through the LC tank formed by CTX and LTX. The current through LTX is shown in Figure 5. TRANSMITTER RECEIVER VIN 5V C2 100µF C1 4.7µF OE V+ U1 DIV OUT LTC6990 SET GND GND fLC_TANK = 315kHz CTX1 33nF CTX2 1nF LTX 7.5µH AIR GAP (3mm-5mm) ACIN LRX 13µH CRX 33nF CIN 4.7µF BAT LED VCC ICHG = 25mA MAX LTC4123 + 1.5V NiMH CHRG fDRIVE = 244kHz M1 Si2312CDS GND PROG RPROG 953Ω R1 205k 4123 F04 Figure 4. DC/AC Converter, Transmit/Receive Coils, Tuned Resonant LTC4123 Receiver 10 4123f For more information www.linear.com/LTC4123 LTC4123 Applications Information If the transmit LC tank frequency is set to 1.29 times the driving frequency, switching losses in M1 are significantly reduced due to zero voltage switching (ZVS). Figure 6 and Figure 7 illustrate the ZVS condition at different fTX-TANK frequencies. 500mA/DIV 0A fTX−TANK =1.29 • f DRIVE 2µs/DIV 4123 F05 Figure 5. Current Through Transmit Coil, LTX, in Transmitter DRAIN VOLTAGE 5V/DIV fDRIVE is set by resistor RSET in LTC6990. fTX-TANK is set by: 1 fTX−TANK = 2 • π L TX •CTX The peak voltage of the transmit coil, LTX, that appears at the drain of M1 is: VTX−PEAK =1.038 • π • VIN And the peak current through LTX is: 0V GATE VOLTAGE 2V/DIV I TX−PEAK = 0.36 • VIN fTX−TANK •L TX And the RMS current through LTX is: 0V 2µs/DIV 4123 F06 Figure 6. Voltage on the Drain and Gate of NMOS, M1, when fTX_TANK = fDRIVE ITX-RMS = 0.66 • ITX-PEAK The LC tank at the receiver, LRX and CRX, is tuned to the same frequency as the driving frequency of the transmit LC tank: fRX−TANK = fDRIVE DRAIN VOLTAGE 5V/DIV where fRX-TANK is given by, 0V GATE VOLTAGE 2V/DIV 0V 2µs/DIV 4123 F07 Figure 7. Voltage on the Drain and Gate of NMOS, M1, when fTX_TANK = 1.29 • fDRIVE fRX−TANK = 1 2 • π LRX •CRX Note: fDRIVE can be easily adjusted therefore it is best practice to choose fRX-TANK using minimum component count (i.e. CRX) then adjusting fDRIVE to match. The amount of AC current in the transmit coil can be increased by increasing the supply voltage (VIN), decreasing the driving frequency (fDRIVE), or decreasing the inductance (LTX) of the transmit coil. Since the amount of power transmitted is proportional to the AC current in the transmit coil, VIN, fDRIVE and LTX can be varied to adjust the power delivery to the receive coil. 4123f For more information www.linear.com/LTC4123 11 LTC4123 Applications Information The overall power transfer efficiency is also dependent on the quality factor (Q) of the components used in the transmitter and receiver circuitry. Select components with low resistance for transmit/receive coils and capacitors. Choosing Transmit Power Level As discussed in the previous section, several parameters can be used to adjust the transmit power of the transmitter shown in Figure 4. These include the supply voltage, (VIN), the driving frequency (fDRIVE) and the inductance of the transmit coil (LTX). Transmit power should be set as low as possible to receive the desired output power at worst-case coupling conditions (e.g. maximum transmit distance with the worst-case misalignment). Increased transmit power can deliver more power to the LTC4123based receiver, but care must be taken not to exceed the rated current of the transmit coil. Furthermore, the LTC4123 has the ability to shunt excess received power, but this will start to increase the temperature of the LTC4123. Since the LTC4123 die temperature is assumed to be approximately equal to the battery temperature, it is important to minimize the die temperature rise to maintain an accurate battery charge voltage. Using the rated current of the transmit inductor to set an upper limit, transmit power should be adjusted downward until charge current is negatively impacted at worst-case coupling conditions. Charge current can easily be monitored using the PROG pin voltage. Once the transmit power level is determined, the transmit and receive coils should be arranged under best-case coupling conditions with a fully-charged battery or a battery simulator. In this scenario, the LTC4123 will shunt excess power. Measure the LTC4123 temperature using an infrared sensor or use the negative temperature coefficient of the battery charge voltage as an indication of temperature. Charge voltage measured under the best-case coupling condition should be within ten to fifteen millivolts of the charge voltage measured under worst-case coupling conditions (given the same battery current). Single Transistor Transmitter and LTC4123 Receiver – Design Example The example in Figure 4 illustrates the design of the resonant coupled single transistor transmitter and LTC4123 charger. The steps needed to complete the design are reviewed below. 1.Set the charge current for the LTC4123: In this example, the charge current required is 25mA: RPROG = 24V = 960Ω 25mA Since 960Ω is not a standard 1% value, a 953Ω resistor with a 1% tolerance is selected to obtain a charge current within 1% of the desired value. 2.Determine the receiver resonant frequency and set component values for the receiver LC tank: It is best practice to select a resonant frequency that yields a low component count. In this example, 244kHz is selected as the receiver resonant frequency. At 244kHz, the tank capacitance (CRX) required with the selected receive coil (13µH) is 33nF. 33nF is a standard value for capacitors, therefore the tank capacitance requires only one component. The tank capacitance calculation is shown below. CRX = 1 4• π 2 • f 2RX−TANK •LRX = 32.7nF = 33nF Select a 33nF capacitor with a minimum voltage rating of 25V and 5% (or 1%) tolerance for CRX. A higher voltage rating usually corresponds to a higher quality factor which is preferable. However, the higher the voltage rating, the larger the package size usually is. 3.Set the driving frequency (fDRIVE) for the Single Transistor Transmitter: fDRIVE is set to the same value as the receiver resonant frequency: RSET = 1MHz 50kΩ • = 205kΩ NDIV 244kHz where NDIV = 1 as the DIV pin in LTC6990 is grounded. Select a 205kΩ (standard value) resistor with 1% tol- 12 4123f For more information www.linear.com/LTC4123 LTC4123 Applications Information erance. For more information regarding the LTC6990 oscillator see the data sheet. 5.Verify if the AC current through the transmit coil is well within the rated current. 4.Set the LC tank component values for the single transistor transmitter: If fdrive is 244kHz, the transmit LC tank frequency (fTX-TANK) is: In this example, the supply voltage to the basic transistor transmitter is 5V. The peak AC current through the transmit (LTX) coil can be calculated: fTX−TANK =1.29 • 244kHz = 315kHz The transmit coil (LTX) used in the example is 7.5µH. The value of transmit tank capacitance (CTX) can be calculated: 1 C = = 34nF TX 2 2 4 • π • f TX−TANK •L TX Since 34nF is not a standard capacitor value, use a 33nF capacitor and a 1nF capacitor in parrallel to obtain a value 1% of the calculated CTX. The recommended rating for CTX capacitors is 50V with 5% (or 1%) tolerance. ITX–PEAK = 0.36 • VIN 0.36 • 5V = = 0.76A fTX–TANK •L TX 315kHz • 7.5µH And ITX-RMS = 0.66 • 0.76 = 0.5A The rated current for the transmit coil is 1.55A (please see the Würth 760308103206 data sheet for more information). The ITX–RMS calculated is well below the rated current. Verify the transmit power level chosen does not result in excessive heating of the LTC4123. Please refer to the Choosing Transmit Power Level section for more information. Table 2. Recommended Components for LTC4123 Receiver Item Part Description Manufacturer/Part Number CIN CAP, CHIP, X5R, 4.7µF, ±10%, 10V, 0402 Samsung Electro-Mechanics America Inc. CL05A475KP5NRNC LRX 13µH, 10mm, Receive Coil Würth 760308101208 CRX1 CAP, CHIP, C0G, 33nF, ±5%, 50V, 0805 or TDK C2012C0G1H333J125AA CAP, CHIP, C0G, 33nF, ±1%, 50V, 1206 MURATA GCM3195C1H333FA16D D1 LED, 630nm, Red, 0603, SMD Rohm Semiconductor SML-311UTT86 RPROG RES, CHIP, 953Ω, ±1%, 1/16W, 0402 VISHAY CRCW0402953RFKED Table 3. Recommended Components for Single Transistor Transmitter Item Part Description Manufacturer/Part Number C1 CAP, CHIP, X5R, 4.7μF, ±20%, 6.3V, 0402 TDK C1005X5R0J475M C2 CAP, CHIP, X5R, 100μF, ±20%, 6.3V, 1206 MURATA GRM31CR60J107ME39L LTX 7.5µH, 28mm × 15mm, Transmit Coil Würth 760308103206 CTX1 CAP, CHIP, C0G, 33nF, ±5%, 50V, 0805 TDK C2012C0G1H333J125AA CTX2 CAP, CHIP, C0G, 1nF, ±5%, 50V, 0603 TDK C1608C0G1H102J080AA D1 LED, RED, SMT, 0603 LITEON LTST-C193KRKT-5A M1 MOSFET, N-CH 20V, 6A, SOT-23-3 Vishay Si2312CDS-T1-GE3 RSET RES, CHIP, 205kΩ, ±1%, 1/16W, 0402 Vishay CRCW0402205KFKED U1 IC, TimerBlox: Voltage Controlled Silicon Oscillator, 2mm × 3mm DFN Linear Tech. LTC6990IDCB 4123f For more information www.linear.com/LTC4123 13 LTC4123 Applications Information Component Selection for Transmitter and Receiver To ensure optimum performance from the LTC4123 in the design example discussed in the previous section, it is recommended to use the components listed in Table 2 and Table 3 for the receiver and transmitter respectively. Select receive and transmit coil with good quality factors to improve the overall power transmission efficiency. Use ferrite to improve the magnetic coupling between transmit and receive coils and to shield the rest of the transmit and receive circuitry from the AC magnetic field. Capacitors with low ESR and low thermal coefficients such as C0G ceramics should be used in receive and transmit LC tanks. Component Selection for CHRG Status Indicator The LED connected at CHRG is powered by a 300uA pulldown current source. Select a high efficiency LED with low forward voltage drop. Some recommended components are shown in Table 4. Table 4. Recommended LED Manufacturer/Part Number Part Description SML-311UTT86 Rohm Semiconductor, LED, 630nm, RED, 0603, SMD LTST-C193KRKT-5A Lite-On Inc. LED, RED, SMT, 0603 Stability Considerations The LTC4123 has three control loops: constant-current (CC), constant-voltage (CV) and undervoltage current limit (UVCL). In constant-current mode, the PROG pin is in the feedback loop. An additional pole is created by the PROG pin capacitance. Therefore, capacitance on this pin must be kept to a minimum. With no additional capacitance on the PROG pin, the LTC4123 charger is stable with program resistor values as high as 23.7kΩ. However, any additional capacitance on the PROG pin limits the minimum allowed charge current. In UVCL mode, the VCC pin is in the feedback loop. Any series resistance from the supply to the VCC pin and the decoupling capacitor at VCC pin will create an additional 14 pole. The series resistance at the VCC pin is highly variable and is dependent on the LC tank connected at the ACIN pin. The LTC4123 is internally compensated to operate with 1µF to 10µF decoupling capacitor and/or up to 100Ω to 10kΩ equivalent series resistance from the supply to the VCC pin. Zinc-Air Battery Detection During Zinc-Air battery detection, the full programmed charge current is applied to the battery for up to 80 (TZn-AIR) seconds after the charger is powered on. The full programmed charge current is necessary to perform successful Zinc-Air battery detection. Upon initial application of input power, if the charger is unable to provide the programmed charge current, it signals a fault mode and the LED at CHRG will blink fast. For instance, the programmed charge current could drop at the beginning of the charge cycle due to misalignment between transmit and receive coils. To restart a charge cycle, it is necessary to remove the receiver from the transmitter’s magnetic field and try again. At colder temperatures, if multiple charge cycles are initiated with a fully-charged NiMH battery, it is possible for the LTC4123 to detect that battery as a Zinc-Air battery and signal a fault (blink fast). This is because the internal impedance of a fully-charged NiMH battery is significantly higher at colder temperatures. Board Layout Considerations The VCC bypass capacitor should be connected as close as possible to the VCC pin. The trace connection from the ground return of the bypass capacitor to the ground return of the LC tank should be as short as possible to minimize and localize AC noise. To minimize the parasitic capacitance on the PROG pin, the trace connection from the PROG pin to the programming resistor should be as short as possible. The ground return for the resistor should be connected to GND via the exposed pad with the shortest possible trace length. 4123f For more information www.linear.com/LTC4123 LTC4123 Package Description Please refer to http://www.linear.com/product/LTC4123#packaging for the most recent package drawings. DC6 Package 6-Lead Plastic DFN (2mm × 2mm) (Reference LTC DWG # 05-08-1703 Rev C) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 0.60 ±0.10 (2 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 1.37 ±0.10 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.125 TYP 0.60 ±0.10 (2 SIDES) 0.40 ±0.10 4 6 2.00 ±0.10 (4 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER R = 0.05 TYP 0.200 REF 0.75 ±0.05 3 (DC6) DFN REV C 0915 1 0.25 ±0.05 0.50 BSC 1.37 ±0.10 (2 SIDES) 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WCCD-2) 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 4123f 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. For more information www.linear.com/LTC4123 15 LTC4123 Typical Application Wireless 25mA p675 NiMH Linear Charger Tuned at 244kHz VIN 5V C2 100µF AIR GAP (3mm-5mm) fLC_TANK = 315kHz C1 4.7µF OE V+ U1 CTX1 33nF LTX 7.5µH CTX2 1nF LRX 13µH BAT CIN 4.7µF LED VCC ICHG = 25mA MAX LTC4123 + CHRG fDRIVE = 244kHz GND M1 Si2312CDS OUT DIV LTC6990 SET GND 1.5V POWER ONE NiMH (P675) PROG RPROG 953Ω CTX1, CRX: C2012C0G1H333J125AA CTX2: C1608C0G1H102J080AA LTX: 760308103206 LRX: 760308101208 R1 205k GND ACIN CRX 33nF 4123 TA03 Wireless 25mA p675 NiMH Linear Charger Tuned at 255kHz VIN 5V C2 100µF fLC_TANK = 329kHz C1 4.7µF OE V+ U1 CTX1 33nF LTX 5.9µH CTX2 6.8nF ACIN LRX 5.8µH CRX 68nF BAT CIN 4.7µF LED VCC ICHG = 25mA MAX LTC4123 + CHRG fDRIVE = 255kHz GND M1 Si2312CDS OUT DIV LTC6990 SET GND CTX1: C2012C0G1H333J125AA CTX2: C1608C0G1H682J080AA LTX: L41200T23 CRX: GRM31C5C1H683JA01L LRX: L4120R19 R1 196k GND AIR GAP (4mm – 6.5mm) 1.5V POWER ONE NiMH (P675) PROG RPROG 953Ω 4123 TA04 Related Parts PART NUMBER DESCRIPTION COMMENTS LTC4120 400mA Wireless Power Receiver Buck Battery Charger Wireless 1 to 2 Cell Li-Ion Charger, 400mA Charge Current, Dynamic Harmonization Control, Wide Input Range: 12.5V to 40V, 16-Lead 3mm × 3mm QFN Package. LTC4125 5W AutoResonant Wireless Power Transmitter Monolithic AutoResonant Full Bridge Driver. Transmit power automatically adjusts to receiver load, Foreign Object Detection, Wide Operating Switching Frequency Range: 50kHz-250kHz, Input Voltage Range 3V to 5.5V, 20-Lead 4mm × 5mm QFN Package LTC4071 Li-Ion/Polymer Shunt Battery Charger System with Low Battery Disconnect Charger Plus Pack Protection in One IC, Low Operating Current (550nA), 50mA Internal Shunt Current, Pin Selectable Float Voltages (4.0V, 4.1V, 4.2V), 8-Lead 2mm × 3mm DFN and MSOP Packages. 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LTC4123 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4123 4123f LT 1115 • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2015