LT1571EGN-2#TRPBF

LT1571 Series Constant-Current/ Constant-Voltage Battery Chargers with Preset Voltage and Termination Flag U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT ®1571 PWM battery charger is a simple, efficient solution to fast-charge rechargeable batteries including lithium-ion (Li-Ion), nickel-metal-hydride (NiMH) and nickel-cadmium (NiCd) using constant-current and/or constant-voltage control. The internal switch is capable of delivering 1.5A DC current (2A peak current). The onboard current sense resistor (0.1Ω) allows simple charge current programming to within 5% accuracy using a low cost external resistor. The constant-voltage output can be selected for 4.1V or 4.2V per cell with 0.6% accuracy. Fast Charging of Li-Ion, NiMH and NiCd Batteries Simple Charge Current Programming Requires Only One Low Cost, 1/32W Resistor High Efficiency Charger with Up to 1.5A Charge Current Precision 0.6% Internal Voltage Reference Preset Battery Voltages: 4.1V, 4.2V, 8.2V, 8.4V 500kHz or 200kHz Switching Frequency Minimizes Charger Size Low Reverse Battery Drain Current: 5µA Flag Indicates Li-Ion Charge Completion 5% Typical Charge Current Accuracy Low Shutdown Current LT1571-1: 200kHz, Adjustable Voltage LT1571-2: 200kHz, Fixed 8.2V or 8.4V LT1571-5: 500kHz, Fixed 4.1V or 4.2V LT1571 can charge batteries ranging from 1V to 20V. A saturating switch operating at 200kHz (LT1571-1, LT1571-2) or 500kHz (LT1571-5) gives high efficiency and small charger size. A logic output (flag) indicates Li-Ion near full charge when the charge current drops to 20% of the programmed value. The LT1571-1 and LT1571-2 are in a 28-pin fused lead narrow SSOP power package. The LT1571-5 is in a 16-pin fused lead narrow SSOP power package. U APPLICATIO S ■ Cellular Phones, PDAs, Notebook Computers, Portable Instruments Cradle Chargers for Li-Ion, NiCd, NiMH and Lead-Acid Rechargeable Batteries , LTC and LT are registered trademarks of Linear Technology Corporation. U ■ TYPICAL APPLICATION D3 MBRM120T3 VIN 8.2V TO 20V (ADAPTER OUTPUT) D1 MBRM120T3 VCC CIN* 10µF SW LT1571-5 PROG 100k 1µF 6.19k D2 MMBD914L VC 1k CHARGE COMPLETE 4.2V Li-Ion BATTERY + + COUT*** 22µF L1** 10µH BOOST 0.33µF 300Ω C1 0.22µF SENSE CAP FLAG BAT SELECT BAT2 GND 0.1µF *TOKIN OR MARCON CERAMIC SURFACE MOUNT **COILTRONICS TP3-100, 10µH, 2.2mm HEIGHT (0.8A CHARGING CURRENT) COILTRONICS TP1 SERIES, 10µH, 1.8mm HEIGHT ( 0.6V VC < 40mV ● Oscillator Switching Frequency LT1571-1, LT1571-2 LT1571-5 Switching Frequency Tolerance All Conditions of VCC, Temperature, LT1571-1, LT1571-2 LT1571-1, LT1571-2, TJ < 0°C LT1571-5 LT1571-5, TJ < 0°C Maximum Duty Cycle LT1571-1, LT1571-2 LT1571-1, LT1571-2, TA = 25°C (Note 7) LT1571-5 ● ● ● ● 500 87 90 77 93 81 125 210 % % % Current Amplifier (CA2) Transconductance VC = 1V, IVC = ±1µA Maximum VC for Switch OFF IVC Current (Out of Pin) ● VC ≥ 0.6V 0.2V < VC < 0.45V VC < 40mV (Shutdown) Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT1571 is guaranteed to meet 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 3: Sense resistor RS1 and package bond wires. Note 4: Current (≈ 700µA) flows into the pins during normal operation and also when an external shutdown signal on the VC pin is greater than 0.3V. Current decreases to ≈ 200µA and flows out of the pins when external shutdown holds the VC pin below 0.3V but above shutdown threshold. Current drops to near zero when input voltage collapses. See External Shutdown in Applications Information section. 4 550 µmho 0.6 V 100 3 300 µA mA µA Note 5: A linear interpolation can be used for reference voltage specification between 0°C and – 40°C. Note 6: Maximum allowable ambient temperature may be limited by power dissipation. Parts may not necessarily be operated simultaneously at maximum power dissipation and maximum ambient temperature. Temperature rise calculations must be done as shown in the Applications Information section to ensure that maximum junction temperature does not exceed the 125°C limit. With high power dissipation, maximum ambient temperature may be less than 70°C. Note 7: 91% maximum duty cycle is guaranteed by design if VBAT or VX (see Figure 8 in Application Information) is kept between 3V and 5V. Note 8: See “Lithium-Ion Charging Completion” in the Applications Information section. LT1571 Series U W TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Junction Temperature Efficiency of Figure 4 Circuit 100 2.470 VCC = 15V (EXCLUDING DISSIPATION ON INPUT DIODE D3) VBAT = 8.4V 40 2.468 94 92 90 88 86 2.466 2.464 2.462 84 2.460 2.458 0.1 0.3 0.5 0.7 0.9 IBAT (A) 1.1 1.3 1.5 0 125 50 75 100 25 JUNCTION TEMPERATURE (°C) 20 VBOOST = 21V (VX = 5V) 15 10 0 150 0 0.2 0.4 0.6 0.8 1.0 1.2 SWITCH CURRENT (A) 1.4 1.6 1571 G03 ∆VOVP vs IVA (Voltage Amplifier) VREF Line Regulation 4 0.003 510 505 0.002 LT1571-5 3 0.001 205 ALL TEMPERATURES ∆VOVP (mV) 495 ∆VREF (V) FREQUENCY (kHz) 25 1571 G02 1571 G01 Switching Frequency vs Temperature 500 VBOOST = 26V (VX = 10V) 30 5 82 80 VCC = 16V 35 BOOST CURRENT (mA) 96 REFERENCE VOLTAGE (V) 98 EFFICIENCY (%) Boost Current vs Switch Current 0 2 125°C –0.001 LT1571-1, LT1571-2 1 200 –0.002 25°C 195 190 –20 –0.003 0 20 40 60 80 100 120 140 JUNCTION TEMPERATURE (°C) 0 5 10 15 VCC (V) Maximum Duty Cycle 0 30 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 IVA (mA) 1571 G06 VC Pin Characteristic PROG Pin Characteristic –1.20 LT1571-1, LT1571-2 (VX = 5V) 97 25 1571 G05 1571 G04 98 20 6 –1.08 –0.72 94 125°C IPROG (mA) –0.84 95 IVC (mA) DUTY CYCLE (%) –0.96 96 –0.60 –0.48 93 –0.36 92 –0.24 25°C 0 –0.12 91 0 90 0 20 40 60 80 100 120 JUNCTION TEMPERATURE (°C) 140 1571 G09 0.12 –6 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VC (V) 1571 G08 0 1 2 3 VPROG (V) 4 5 1571 G09 5 LT1571 Series U U U PI FU CTIO S GND: Ground Pin. SW: NPN Power Switch Emitter. The Schottky catch diode must be placed with very short lead length in close proximity to SW pin and GND. VCC1, VCC2: Input Supply. For good bypass, a low ESR capacitor of 10µF or higher is required, with the lead length kept to a minimum. VCC should be between 8V and 26V and at least 2V higher than VBAT for VBAT less than 10V, and 2.5V higher than VBAT for VBAT greater than 10V. Undervoltage lockout starts and switching stops when VCC goes below 7V (typical). Note that there is an internal parasitic diode from SW pin to VCC pin. Do not force VCC below SW by more than 0.7V with battery present. All VCC pins should be shorted together close to the pins. BOOST: This pin is used to bootstrap and drive the NPN switch to a low on-voltage for low power dissipation. VBOOST = VCC + VBAT when switch is on. For less power dissipation use VBOOST = 3V to 6V (see Applications Information). SENSE: Current Amplifier CA1 Input. Sensing can be at either terminal of the battery. Note that current sense resistor RS1 (0.08Ω) is between SENSE and BAT pins. BAT: Current Amplifier CA1 Input. BAT2 (LT1571-2, LT1571-5): This pin is used to connect the battery to the internal preset voltage setting resistor. An internal switch disconnects the internal divider from the battery when the device is in shutdown or when input power is disconnected. This disconnect function eliminates current drain due to the resistor divider. This pin should be connected to the positive node of the battery if the internal preset divider is used. Otherwise this pin should be grounded. Maximum voltage on this pin is 20V. PROG: This pin is for programming the charge current and for system loop compensation. Charge current is regulated to 2000× the current drawn from the PROG pin. During normal operation, VPROG stays close to 2.465V. If it is shorted to GND, switching will stop. When 6 a microprocessor-controlled DAC is used to program charge current, it must be capable of sinking current at a compliance up to 2.465V. VC: This is the inner loop control signal of the current mode PWM. Switching starts at 0.9V. In normal operation, a higher VC corresponds to a higher charge current. A capacitor of at least 0.1µF to GND filters out noise and controls the rate of soft-start. To shut down switching, pull this pin below 0.6V. Typical current out of this pin is 60µA. When VC is pulled below 40mV, LT1571 supply current drops to typical 150µA. SELECT (LT1571-2, LT1571-5): This pin is used to select the preset battery voltage. For the LT1571-2, leave this pin open for 8.2V and ground it for 8.4V. For the LT1571-5, leave this pin open for 4.1V and ground it for 4.2V. For other battery voltages, use the adjustable LT1571-1. VFB (LT1571-1): This is the input to the amplifier VA (see Block Diagram) with a threshold of 2.465V. Typical input current is about 3nA. When charging batteries, VA monitors the battery voltage and reduces charging current when battery voltage reaches the preset value. If it is not used (constant-current only mode), the VFB pin should be grounded. CAP: A 0.1µF capacitor from CAP to ground is needed to filter the sampled charge current signal. This filtered signal is used to set the FLAG pin when the charge current drops to 20% of the programmed maximum charge current. This threshold level can be set as low as 7.5% of the programmed maximum charge current by adding a resistor on the CAP pin. FLAG: This pin is an open-collector output that is used to indicate end of charge. The FLAG pin is driven low when the charge current drops below a certain percentage of the programmed charge current as explained in the CAP pin function. A pull-up resistor is required if this function is used. This pin is capable of sinking at least 1mA. Maximum voltage on this pin is VCC. LT1571 Series W BLOCK DIAGRA 80mV + + – VC SHUTDOWN 0.2V BAT + + D3 VCC VIN 200kHz/500kHz OSCILLATOR – D2 BOOST S – VCC + C1 QSW R R L1 SW + GND D1 1.5V – PWM C1 – + SLOPE COMPENSATION R2 SENSE + B1 + VBAT IBAT CA1 – R1 IPROG = 500µA/A 1k IBAT RS1 BAT IBAT VBAT BATTERY R3 + IPROG A11 – BAT2 (LT1571-2, LT1571-5 ONLY) – VC CA2 + 75k IVA 4 IVA CAP – E6 VFB (LT1571-1 ONLY) + VA IPROG + FLAG R7 VREF VREF 2.465V R6 11k R5 2k + R4 SELECT (LT1571-2, LT1571-5 ONLY) – 4V PROG NOTES: LT1571-2: R4 = 7.1k, R7 = 30.24k LT1571-5: R4 = 3.33k, R7 = 8.62k LT1571-1: 200kHz, VFB PIN FOR ADJUSTABLE BATTERY VOLTAGE (VFB PIN IS NOT INTERNALLY CONNECTED TO THE RESISTORS) LT1571-2: 200kHz, PRESET 8.2V CELL (SELECT PIN OPEN) OR 8.4V (SELECT PIN GROUNDED) LT1571-5: 500KHz, PRESET 4.1V CELL (SEECT PIN OPEN) OR 4.2V (SELECT PIN GROUNDED) RPROG  2.465V  IBAT =   • 2000  RPROG  CPROG 1571 BD 7 LT1571 Series U OPERATIO The LT1571 is a current mode PWM step-down (buck) charger. The battery charge current is programmed by a resistor RPROG (or a DAC output current) at the PROG pin (see Block Diagram). Amplifier CA1 converts the charge current through RS1 to a much lower current IPROG (500µA/ A) fed into the PROG pin. Amplifier CA2 compares the output of CA1 with the programmed current and drives the PWM loop to force them to be equal. High DC accuracy is achieved with averaging capacitor CPROG. Note that IPROG has both AC and DC components. IPROG goes through R1 and generates a ramp signal that is fed to the PWM control comparator C1 through buffer B1 and level shift resistors R2 and R3, forming the current mode inner loop. The BOOST pin drives the NPN switch (QSW) into saturation and reduces power loss. For batteries like lithium-ion that require both constant-current and constant-voltage charging, the 0.5%, 2.465V reference and the amplifier VA reduce the charge current when battery voltage reaches the preset level. For NiMH and NiCd, VA can be used for overvoltage protection. When input voltage is removed, the VCC pin drops to 0.7V below the battery voltage forcing the charger into a low-battery drain (5µA typical) sleep mode. To shut down the charger, simply pull the VC pin low with a transistor. Comparator E6 monitors the charge level and signals through the FLAG pin when charging is in voltage mode and the charge current has reduced to 20% or less. This charge complete signal can be used to start a timer for charging termination. U W U U APPLICATIO S I FOR ATIO Input and Output Capacitors In the charger circuits in Figures 1 and 2, the input capacitor CIN is assumed to absorb all input switching ripple current in the converter, so it must have adequate ripple current rating. Worst-case RMS ripple current will be equal to one half of the output charge current. Actual capacitance value is not critical. Solid tantalum capacitors such as the AVX TPS and Sprague 593D series have high ripple current rating in a relatively small surface mount package, but caution must be used when tantalum capacitors are used for input bypass. High input surge currents are possible when the adapter is hot-plugged to the charger and solid tantalum capacitors have a known failure mechanism when subjected to very high turn-on surge currents. Selecting a high voltage rating on the capacitor will minimize problems. Consult with the manufacturer before use. Alternatives include new high capacity ceramic capacitors from Tokin or United Chemi-Con/ MARCON, et al. OS-CON can also be used. The output capacitor COUT is also assumed to absorb output switching ripple current. The general formula for capacitor ripple current is: 8  V  0.29(VBAT ) 1 − BAT   VCC  IRMS = (L1)( f) For example, with VCC = 16V, VBAT = 8.4V, L1 = 33µH and f = 200kHz, IRMS = 0.18A. EMI considerations usually make it desirable to minimize ripple current in the battery leads. Beads or inductors can be added to increase battery impedance at the 200kHz switching frequency. Switching ripple current splits between the battery and the output capacitor depending on the ESR of the output capacitor and the battery impedance. If the ESR of COUT is 0.2Ω and the battery impedance is raised to 4Ω with a bead of inductor, only 5% of the ripple current will flow into the battery. Soft-Start The LT1571 is soft-started by the 0.33µF capacitor on VC pin. On start-up, the VC pin voltage will rise quickly to 0.5V, then ramp at a rate set by the internal 45µA pull-up current and the external capacitor. Charge current starts ramping up when the VC pin voltage reaches 0.9V and full current LT1571 Series U W U U APPLICATIO S I FOR ATIO is achieved with VC at 1.1V. With a 0.33µF capacitor, the time to reach full charge current is about 9ms and it is assumed that input voltage to the charger will reach full value in less than 3ms. Capacitance can be increased up to 1µF if longer input start-up times are needed. The lockout voltage will be VIN = VZ + 1V. In any switching regulator, conventional time-based soft starting can be defeated if the input voltage rises much slower than the time-out period. This happens because the switching regulators in the battery charger and the computer power supply are typically supplying a fixed amount of power to the load. If the input voltage comes up slowly compared to the soft-start time, the regulators will try to deliver full power to the load when the input voltage is still well below its final value. If the adapter is current limited, it cannot deliver full power at reduced output voltages and the possibility exists for a quasi “latch” state where the adapter output stays in a current limited state at reduced output voltage. For instance, if maximum charger plus computer load power is 20W, a 24V adapter might be current limited at 1A. If adapter voltage is less than (20W/1A = 20V) when full power is drawn, the adapter voltage will be pulled down by the constant 20W load until it reaches a lower stable state where the switching regulators can no longer supply full load. This situation can be prevented by utilizing undervoltage lockout, set higher than the minimum adapter voltage where full power can be achieved. Charge Current Programming A fixed undervoltage lockout of 7V is built into the LT1571. A higher lockout voltage can be implemented with a Zener diode D2 (see Figure 2). For example, for a 24V adapter to start charging at 22VIN, choose VZ = 21V. When VIN is less than 22V, D1 keeps VC low and charger off. The basic formula for charge current is (see Block Diagram):  2.465V  IBAT = (IPROG)(2000) =   (2000)  R PROG  where RPROG is the total resistance from PROG pin to ground. For example, 1A charge current is needed. RPROG = (2.465V)(2000) = 4.93k 1A Charge current can also be programmed by pulse width modulating IPROG with a switch Q1 to RPROG at a frequency higher than a few kHz (Figure 3). Charge current will be proportional to the duty cycle of Q1 with full current at 100% duty cycle. When a microprocessor DAC output is used to control charge current, it must be capable of sinking current at a compliance up to 2.5V if connected directly to the PROG pin. LT1571 PROG D3 300Ω VIN D2 VZ D1 1N4148 VC 2k RPROG 4.64k VCC LT1571 GND 1571 F02 Figure 2. Undervoltage Lockout 5V 0V CPROG 1µF Q1 VN2222 PWM IBAT = (DC)(1A) 1571 F03 Figure 3. PWM Current Programming 9 LT1571 Series U W U U APPLICATIO S I FOR ATIO Lithium-Ion Charging The circuit in Figure 4 uses the 28-pin LT1571-2 to charge lithium-ion batteries at a constant 1A until the battery voltage reaches 8.4V preset battery voltage. The charger will then automatically go into a constant-voltage mode with current decreasing to near zero over time as the battery reaches full charge. Lithium-Ion Charge Completion Some battery manufacturers recommend termination of constant-voltage float mode after charge current has dropped below a specified level (typically around 10% to 20% of the full current) and a further time-out period of 30 minutes to 90 minutes has elapsed. Check with manufacturer for details. The LT1571 provides a signal at the FLAG pin when the charger is in voltage mode and charge current has reduced to approximately 20% of full current. Note that full current is (2.465V × 2000)/RPROG. Comparator E6 in the Block Diagram compares the charge current sample IPROG to the output current IVA voltage amplifier VA. When the charge current drops to 20% of full current, IPROG will be equal to 0.25 IVA and the open-collector output VFLAG will go low. This signal can be used to start an external timer or to terminate the charge. When this feature is used, a capacitor of at least 0.1µF is required at CAP pin to filter out the switching noise and a pull-up resistor is also needed at FLAG pin. Charge Termination Flag Threshold Setting The charge termination flag threshold can be reduced from the default 20% level to as low as 7.5% of the programmed full charge current. This is done by adding a resistor RCAP from the CAP pin to ground (see Figure 5). The formula for selecting the RCAP resistor is: Threshold = 0.20 – (1.331) or RCAP = (1.331)RPROG 0.20 – Threshold RPROG is the charge current setting resistor. LT1571 CAP 1571 F05 Figure 5. Reducing Charge Termination Threshold D3 MBRM120T3 SW L1** 33µH RCAP 0.1µF D1 MBRM120T3 C1 0.22µF VCC PROG D2 MMBD914L 1µF 0.3µF SENSE VC 4.93k 100k 300Ω 1k CAP 0.1µF VIN 11V TO 26V CIN* 10µF LT1571-2 BOOST SELECT FLAG BAT GND BAT2 NOTE: COMPLETE LITHIUM-ION CHARGER * TOKIN OR MARCON CERAMIC SURFACE MOUNT ** COILTRONICS CTX33-2 + COUT 22µF TANT + 8.4V 1571 F04 Figure 4. 200kHz Charging Lithium Batteries (Efficiency at 1A > 87%) 10 RPROG RCAP LT1571 Series U W U U APPLICATIO S I FOR ATIO For example, if 10% threshold is needed for the 1A charger (see Figure 4), then with RPROG = 4.93k: RCAP = 1.331 • 4.93k = 65.6k 0.20 – 0.10 Because of low level errors, as the threshold level is reduced, the accuracy is also reduced. It is not recommended to program a level less than 7.5%. Preset Battery Voltage Settings The LT1571-2 operates at 200kHz and is preset for 8.2V battery voltage with SELECT pin floating and 8.4V with SELECT pin grounded. The LT1571-5 operates at 500kHz and is preset for 4.1V battery voltage with SELECT pin floating and 4.2V with SELECT pin grounded. BAT2 pin is for Kelvin sensing the battery voltage and should be connected to the battery. Other Battery Voltage Settings For battery voltages other than the preset voltages, the LT1571-1 should be used. It operates at 200kHz and the battery voltage is programmed with R3 and R4 divider at VFB pin (Figure 6). VBAT VFB R3 = (R4)(VBAT − 2.465) Lithium-ion batteries typically require float voltage accuracy of 1% to 2%. Accuracy of the LT1571-1 VFB voltage is ±0.5% at 25°C and ±1% over full temperature. This leads to the possibility that very accurate (0.1%) resistors might be needed for R3 and R4. Actually, the temperature of the LT1571-1 rarely exceeds 50°C in float mode because charge currents have tapered off to a low level, so 0.25% resistors normally provide the required level of overall accuracy. External Shutdown The LT1571 can be externally shut down by pulling the VC pin low with an open-drain N-FET, such as 2N7002. The VC pin should be pulled below 0.6V to stop switching. When VC is pulled below 40mV, LT1571 supply current drops to typical 150µA. Removing input power to the charger puts the LT1571 into a sleep mode and draws only 5µA from the battery. Nickel-Cadmium and Nickel-Metal-Hydride Charging The circuit in Figure 7 uses the LT1571-1 to charge NiCd or NiMH batteries up to 20V with charge currents of 0.5A when Q1 is on and 50mA when Q1 is off. For a 2-level charger, R1 and R2 are found from: R3 LT1571-1 IBAT = R4 1571 F06 Figure 6. Programming Other Battery Voltages Current through the R3/R4 divider is set at a compromise value of 25µA to minimize battery drain when the charger is off. The VFB pin input current of 3nA contributes very little output voltage error and can be neglected. With divider current set at 25µA, R4 = 2.465/25µA = 100k and, 2.465 (2000)(2.465) R PROG (2.465)(2000) R1 = ILOW R2 = (2.465)(2000 ) IHI − ILOW All battery chargers with fast-charge rates require some means to detect full charge in the battery and terminate the high charge current. NiCd batteries are typically charged at high current until the battery temperature begins to increase or until the battery voltage reaches a peak and begins to decrease (– dV/dt). This is an indication of near full charge. The charge current is then reduced to a much 11 LT1571 Series U W U U APPLICATIO S I FOR ATIO D3 1N5819 C1 D1 0.22µF 1N5819 SW VCC CIN* 10µF BOOST PROG L1** 33µH D2 1N914 300Ω 0.1µF VC R1 100k R2 11k 1k Q1 VN2222 IBAT SENSE * TOKIN OR MARCON CERAMIC SURFACE MOUNT ** COILTRONICS CTX33-2 BAT + PBIAS = (3.5mA )(VIN) + 1.5mA(VBAT ) 2 VBAT ) ( + VIN 1µF LT1571-1 GND VIN (WALL ADAPTER) COUT 22µF TANT + 2V TO 20V ON: IBAT = 0.5A OFF: IBAT = 0.05A 1571 F07 [7.5mA + (0.012)(IBAT )] BAT  (IBAT )(VBAT )2  1+ V30   PDRIVER = 55(VIN) 2 IBAT ) (RSW )(VBAT ) ( + ( tOL )(VIN)(IBAT )( f) PSW = V IN PSENSE = (0.18Ω)(IBAT ) 2 Figure 7. Charging NiMH or NiCd Batteries with Constant Current (Efficiency at 0.5A ≈ 90%) lower value and maintained as a constant trickle charge. An intermediate “top off” current may also be used for a fixed time period to reduce total charge time. NiMH batteries are similar in chemistry to NiCd but have two differences related to charging. First, the inflection characteristic in battery voltage as full charge is approached is not nearly as pronounced. This makes it more difficult to use – dV/dt as an indicator of full charge, and an increase in temperature is more often used with a temperature sensor located in the battery pack. Secondly, constant trickle charge may not be recommended. Instead, a moderate level of current is used on a pulse basis (≈ 1% to 5% duty cycle) with the time-averaged value substituting for a constant low trickle. Thermal Calculations If the LT1571 is used for charge currents above 0.4A, a thermal calculation should be done to ensure that junction temperature will not exceed 125°C. Power dissipation in the IC is caused by bias and driver current, switch resistance, switch transition losses and the current sense resistor. The following equations show that maximum practical charge current for the 16-pin SSOP package (75° C/W thermal resistance) is about 1.2A for an 8.4V battery and 1.4A for a 4.2V battery. This assumes a 60°C maximum ambient temperature. The 28-pin SSOP, with a thermal resistance of 40°C/W, can provide a full 1.5A charge current in many situations. 12 RSW = Switch ON resistance ≈ 0.35Ω tOL = Effective switch overlap time ≈ 10ns f = 200kHz (500kHz for LT1571-5) Example: VIN = 15V, VBAT = 8.4V, IBAT = 1.2A; PBIAS = (3.5mA )(VIN) + 1.5mA(VBAT ) 2 VBAT ) ( + VIN [7.5mA + (0.012)(IBAT )] BAT  (IBAT )(VBAT )2  1+ V30   PDRIVER = 55(VIN) 2 IBAT ) (RSW )(VBAT ) ( + ( tOL )(VIN)(IBAT )( f) PSW = V IN PSENSE = (0.18Ω)(IBAT ) 2 Total power in the IC is: 0.17 + 0.13 + 0.32+ 0.26 = 0.88W Temperature rise will be (0.88W)(40°C/W) = 35°C. This assumes that the LT1571 is properly heat sunk by connecting all fused ground pins to the expanded traces and that the PC board has a backside or internal plane for heat spreading. The PDRIVER term can be reduced by connecting the boost diode D2 to a lower system voltage (lower than VBAT) LT1571 Series U W U U APPLICATIO S I FOR ATIO instead of VBAT (see Figure 8). The optimum boost voltage (VX) is from 3V to 6V. SW BOOST L1 Then, (IBAT )(VBAT )(VX) 1+ V30X  PDRIVER = 55(VIN) D2 SENSE VX 3V TO 6V IVX Total board area becomes an important factor when the area of the board drops below about 20 square inches. The graph in Figure 9 shows thermal resistance vs board area for 2-layer and 4-layer boards. Note that 4-layer boards have significantly lower thermal resistance, but both types show a rapid increase for reduced board areas. Figure 10 shows actual measured lead temperature for chargers operating at full current. Battery voltage and input voltage will affect device power dissipation, so the data sheet power calculations must be used to extrapolate these readings to other situations. Vias should be used to connect board layers together. Planes under the charger area can be cut away from the rest of the board and connected with vias to form both a low thermal resistance system and to act as a ground plane for reduced EMI. THERMAL RESISTANCE (°C/W) PDRIVER 0.045W = = 14mA 3.3V VX 55 50 2-LAYER BOARD 45 4-LAYER BOARD 40 35 GN16, MEASURED FROM AIR AMBIENT TO DIE USING COPPER LANDS AS SHOWN ON DATA SHEET 30 25 0 Maximum duty cycle for the LT1571-1/LT1571-2 is typically 90% but this may be too low for some applications. For example, if an 18V ±3% adapter is used to charge ten NiMH cells, the charger must put out approximately 15V. A total of 1.6V is lost in the input diode, switch resistance, inductor resistance and parasitics so the required duty 5 20 15 25 10 BOARD AREA (IN2) 30 35 1571 F09 Figure 9. LT1571 Thermal Resistance 90 NOTE: PEAK DIE TEMPERATURE WILL BE ABOUT 10°C HIGHER THAN LEAD TEMPERATURE AT 1.3A CHARGING CURRENT 80 70 2-LAYER BOARD 60 4-LAYER BOARD 50 ICHRG = 1.3A VIN = 16V VBAT = 8.4V VBOOST = VBAT TA = 25°C 40 30 20 0 Higher Duty Cycle 10µF 60 LEAD TEMPERATURE (°C) The average IVX required is: 1571 F08 + Figure 8. Lower VBOOST For example, VX = 3.3V, .3V  (1.2A)(8.4V)(3.3V) 1+ 330   PDRIVER = = 0.045W 55(15V ) LT1571 C1 5 20 15 25 10 BOARD AREA (IN2) 30 35 1571 F10 Figure 10. LT1571 Lead Temperature cycle is 15/16.4 = 91.4%. The duty cycle can be extended to 93% by restricting boost voltage to 5V instead of using VBAT as is normally done. This lower boost voltage VX (see Figure 8) also reduces power dissipation in the LT1571. 13 LT1571 Series U W U U APPLICATIO S I FOR ATIO Lower Dropout Voltage Layout Considerations For even lower dropout and/or reducing heat on the board, the input diode D3 can be replaced with a FET (see Figure 11). Connect a P-channel FET in place of the input diode with its gate connected to the battery (SENSE pin) causing the FET to turn off when the input voltage goes low. The problem is that the gate must be pumped low so that the FET is fully turned on even when the input is only a volt or two above the battery voltage. Also there is a turnoff speed issue. The FET should turn off instantly when the input is dead shorted to avoid large current surges from the battery back through the charger into the FET. Gate capacitance slows turn off, so a small P-FET (Q2) discharges the gate capacitance quickly in the event of an input short. The body diode of Q2 creates the necessary pumping action to keep the gate of Q1 low during normal operation. Switch rise and fall times are under 10ns for maximum efficiency. To minimize radiation, the catch diode, SW pin and input bypass capacitor leads should be kept as short as possible. A ground plane should be used under the switching circuitry to prevent interplane coupling and to act as a thermal spreading path. All ground pins should be connected to expand traces for low thermal resistance. The fast-switching high current ground path including the switch, catch diode and input capacitor should be kept very short. Catch diode and input capacitor should be close to the chip and terminated to the same point. This path contains nanosecond rise and fall times with several amps of current. The other paths contain only DC and /or 200kHz or 500kHz triwave and are less critical. Figure 12 indicates the high speed, high current switching path. Figure 13 shows critical path layout. Q1 VIN + VCC SWITCH NODE SW Q2 RX 50k D1 L1 LT1571 C3 VBAT BOOST L1 D2 SENSE VX 3V TO 6V Q1: Si4435DY Q2: TP0610L CIN VIN BAT CX 10µF VBAT HIGH FREQUENCY CIRCULATING PATH COUT + HIGH DUTY CYCLE CONNECTION 1571 F12 1571 F11 Figure 11. Replacing the Input Diode Figure 12. High Speed Switching Path GND LT1571-5 D1 L1 GND GND SW VCC2 BOOST VCC1 BAT2 CAP FLAG PROG SELECT VC SENSE BAT GND GND CIN 1571 F13 Figure 13. Critical Electrical and Thermal Path Layer for LT1571-5 14 BAT LT1571 Series U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. GN Package 16-Lead Plastic SSOP (Narrow 0.150) (LTC DWG # 05-08-1641) 0.189 – 0.196* (4.801 – 4.978) 16 15 14 13 12 11 10 9 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.007 – 0.0098 (0.178 – 0.249) 0.009 (0.229) REF 2 3 5 6 4 7 0.053 – 0.068 (1.351 – 1.727) 8 0.004 – 0.0098 (0.102 – 0.249) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.0250 (0.635) BSC 0.008 – 0.012 (0.203 – 0.305) * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE GN16 (SSOP) 1098 GN Package 28-Lead Plastic SSOP (Narrow 0.150) (LTC DWG # 05-08-1641) 0.386 – 0.393* (9.804 – 9.982) 28 27 26 25 24 23 22 21 20 19 18 17 1615 0.229 – 0.244 (5.817 – 6.198) 0.150 – 0.157** (3.810 – 3.988) 1 0.015 ± 0.004 × 45° (0.38 ± 0.10) 0.0075 – 0.0098 (0.191 – 0.249) 0.033 (0.838) REF 2 3 4 5 6 7 8 9 10 11 12 13 14 0.053 – 0.069 (1.351 – 1.748) 0.004 – 0.009 (0.102 – 0.249) 0° – 8° TYP 0.016 – 0.050 (0.406 – 1.270) 0.008 – 0.012 (0.203 – 0.305) 0.0250 (0.635) BSC * DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE ** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 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. GN28 (SSOP) 1098 15 LT1571 Series RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1505 High Current Constant-Current/Constant-Voltage Battery Charger Controller with Input Current Limit High Efficiency Synchronous Buck Topology, Uses External N-Channel FETs. Includes Preset Battery Voltages and Input Current Limiting LT1510 200kHz Constant-Current/Constant-Voltage Battery Charger Up to 1.5A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries LT1510-5 500kHz Constant-Current/Constant-Voltage Battery Charger Up to 1A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries LT1511 200kHz Constant-Current/Constant-Voltage Battery Charger with Input Current Limit Up to 3A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries LT1512 500kHz SEPIC Constant-Current/Constant-Voltage Battery Charger Up to 1.5A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries. Input Voltage Can be Higher or Lower Than Battery Voltage. 2A Internal Switch LT1513 500kHz SEPIC Constant-Current/Constant-Voltage Battery Charger Up to 2A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries. Input Voltage Can be Higher or Lower Than Battery Voltage. 3A Internal Switch LTC®1729 Li-Ion Battery Charger Termination Controller Can Be Used with Battery Chargers to Provide Charge Termination, Preset Voltages, C/10 Charge Detection and Timer Functions LTC1731 Linear Constant-Current/Constant-Voltage Charger Controller Simple Charger Uses External FET. Features Preset Voltages, C/10 Charge Detection and Programmable Timer LTC1759 SMBus Controlled Constant-Current/Constant-Voltage Smart Battery Charger Controller LT1505 Charger Functionality with SMBus LT1769 200kHz Constant-Current/Constant-Voltage Battery Charger with Input Current Limit Up to 2A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries 16 Linear Technology Corporation 1571f LT/TP 0700 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com  LINEAR TECHNOLOGY CORPORATION 2000