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LT1571 Series Constant-Current/ Constant-Voltage Battery Chargers with Preset Voltage and Termination Flag
FEATURES
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DESCRIPTIO
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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: 5A 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
The LT (R)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 (0.1) 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. 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Cellular Phones, PDAs, Notebook Computers, Portable Instruments Cradle Chargers for Li-Ion, NiCd, NiMH and Lead-Acid Rechargeable Batteries
TYPICAL APPLICATION
VIN 8.2V TO 20V (ADAPTER OUTPUT) D3 MBRM120T3 VCC CIN* 10F 100k 6.19k CHARGE COMPLETE 4.2V Li-Ion BATTERY LT1571-5 PROG 1F 300 1k FLAG BAT COUT*** 22F BAT2 0.33F VC SENSE CAP SELECT GND 0.1F BOOST D2 MMBD914L SW C1 0.22F L1** 10H D1 MBRM120T3
+
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*TOKIN OR MARCON CERAMIC SURFACE MOUNT **COILTRONICS TP3-100, 10H, 2.2mm HEIGHT (0.8A CHARGING CURRENT) COILTRONICS TP1 SERIES, 10H, 1.8mm HEIGHT (<0.5A CHARGING CURRENT) ***PANASONIC EEFCD1B220
Figure 1. Compact Li-Ion Cellular Phone Charger (0.8A)
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1571 F01
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LT1571 Series
ABSOLUTE
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Supply Voltage (VCC) .............................................. 28V BOOST Pin Voltage with Respect to VCC ................. 20V FLAG Pin Voltage ..................................................... VCC IBAT (Average)........................................................ 1.5A Switch Current (Peak) .............................................. 2A Storage Temperature Range ................. - 65C to 150C
PACKAGE/ORDER I FOR ATIO
**GND **GND
TOP VIEW **GND 1 SW 2 BOOST 3 BAT2 4 FLAG 5 SELECT 6 SENSE 7 **GND 8 16 GND** 15 VCC1* 14 VCC2* 13 CAP 12 PROG 11 VC 10 BAT 9 GND**
1 2 3 4 5 6 7 8 9
**GND SW BOOST NC FLAG NC VFB
SENSE 10 **GND 11 **GND 12 **GND 13 **GND 14
GN PACKAGE 16-LEAD NARROW PLASTIC SSOP TJMAX = 125C, JA = 75C/ W * VCC1 AND VCC2 SHOULD BE CONNECTED TOGETHER CLOSE TO THE PINS ** FOUR CORNER PINS ARE FUSED TO INTERNAL DIE ATTACH PADDLE FOR HEAT SINKING. CONNECT THESE FOUR PINS TO EXPANDED PC LANDS FOR PROPER HEAT SINKING
GN PACKAGE 28-LEAD NARROW PLASTIC SSOP
TJMAX = 125C, JA = 40C/ W * VCC1 AND VCC2 SHOULD BE CONNECTED TOGETHER CLOSE TO THE PINS ** ALL GND PINS ARE FUSED TO INTERNAL DIE ATTACH PADDLE FOR HEAT SINKING. CONNECT THESE PINS TO EXPANDED PC LANDS FOR PROPER HEAT SINKING 40C/W THERMAL RESISTANCE ASSUMES AN INTERNAL GROUND PLANE DOUBLING AS A HEAT SPREADER
ORDER PART NUMBER LT1571EGN-5
Consult factory for Industrial and Military grade parts.
ORDER PART NUMBER LT1571EGN-1
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(Note 1)
Operating Ambient Temperature Range (Note 2) .................. - 40C to 85C Operating Junction Temperature Range .............................. - 40C to 125C Lead Temperature (Soldering, 10 sec).................. 300C
TOP VIEW 28 GND** 27 GND** 26 GND** 25 GND** 24 VCC1* 23 VCC2* 22 CAP 21 PROG 20 VC 19 BAT 18 GND** 17 GND** 16 GND** 15 GND**
**GND **GND **GND SW BOOST BAT2 FLAG NC SELECT 1 2 3 4 5 6 7 8 9
TOP VIEW 28 GND** 27 GND** 26 GND** 25 GND** 24 VCC1* 23 VCC2* 22 CAP 21 PROG 20 VC 19 BAT 18 GND** 17 GND** 16 GND** 15 GND**
SENSE 10 **GND 11 **GND 12 **GND 13 **GND 14
GN PACKAGE 28-LEAD NARROW PLASTIC SSOP
TJMAX = 125C, JA = 40C/ W * VCC1 AND VCC2 SHOULD BE CONNECTED TOGETHER CLOSE TO THE PINS ** ALL GND PINS ARE FUSED TO INTERNAL DIE ATTACH PADDLE FOR HEAT SINKING. CONNECT THESE PINS TO EXPANDED PC LANDS FOR PROPER HEAT SINKING 40C/W THERMAL RESISTANCE ASSUMES AN INTERNAL GROUND PLANE DOUBLING AS A HEAT SPREADER
ORDER PART NUMBER LT1571EGN-2
LT1571 Series
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = 16V (LT1571-1, LT1571-2), VCC = 10V (LT1571-5), VBAT = 8V (LT1571-1,LT1571-2), VBAT = 4V (LT1571-5), no load on any outputs unless otherwise noted. (Note 6)
PARAMETER Overall Supply Current DC Battery Charging Current, IBAT VPROG = 2.7V 8V VCC 26V, 0V VBAT 20V (LT1571-1) RPROG = 4.93k RPROG = 4.93k, TJ < 0C RPROG = 3.28k RPROG = 49.3k RPROG = 49.3k, TJ < 0C VBAT 20V, 0C TJ 70C (LT1571-1) 40 VC 40mV RPROG = 4.93k. Measured at VFB, with VA Supplying IPROG and Switch Off 8V VCC 26V, 0C TJ 70C 8V VCC 26V, 0C TJ 125C 8V VCC 26V, TJ < 0C (Note 5) RPROG = 4.93k. Measured at BAT2 Pin TJ = 25C 8V VCC 26V, 0C TJ 125C Absolute Value, Not Matching VBAT2 = VPRESET - 1V RPROG = 4.93k RPROG = 4.93k, RCAP = 65.6k Low-to-High Threshold High-to-Low Threshold VCAP = 4.5V, IFLAG 1mA VCAP = 0.6V, VCC = 26V Output Current from 100A to 500A VPROG = VREF, VFB = VREF + 10mV At 0.75mA Output Current Undervoltage Lockout VCC - VBOOST 20V 20V < VCC - VBOOST 26V 2V VBOOST - VCC 8V (Switch ON) 8V < VBOOST - VCC 20V (Switch ON) (LT1571-1)
q q q q q q q
CONDITIONS
MIN
TYP 5.2
MAX 7 1.07 1.09 1.65 125 130 15
UNITS mA A A A mA mA A mV
0.93 0.91 1.35 75 70
1.0 1.5 100
Shutdown Auto Shutdown, Reverse Current from Battery (When Adapter in Figure 1 Circuit is Removed) Shutdown Threshold at VC Pin When VCC is Connected Shutdown Supply Current Reference Reference Voltage (LT1571-1) 5 80 0.15 0.3
mA
2.446 2.441 2.430
2.465 2.465 2.465
2.480 2.489 2.480
V V V
Preset Battery Voltage LT1571-2: 8.2V/8.4V LT1571-5: 4.1V/4.2V Voltage Setting Resistors Tolerance (R4, R5) BAT2 Pin Input Current (LT1571-2, LT1571-5) Charge Completion Flag (Comparator E6) Charge Completion Threshold (Note 8) Threshold on CAP Pin FLAG (Open Collector) Output Low FLAG Pin Leakage Current Voltage Amplifier VA Transconductance Output Source Current VFB Input Bias Current (LT1571-1) Overall Minimum Input Operating Voltage Boost Pin Current
q q q q q
0.5 -1 -40 1 40 6 0.14 0.05 0.6 0.3 3 0.3 1.3 3 6.2 7 0.10 0.25 6 8 15 7.8 20 30 11 14 0.6 2.5 0.20 0.085 4 0.28 0.13 4.5
% % % A A A V V V A mho mA nA V A A mA mA
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LT1571 Series
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = 16V (LT1571-1, LT1571-2), VCC = 10V (LT1571-5), VBAT = 8V (LT1571-1,LT1571-2), VBAT = 4V (LT1571-5), no load on any outputs unless otherwise noted. (Note 6)
PARAMETER Switch Switch ON Resistance IBOOST/ISW During Switch ON Switch OFF Leakage Current Maximum VBAT with Switch ON Minimum IPROG for Switch ON Minimum IPROG for Switch OFF Current Sense Amplifier Inputs (SENSE, BAT) Sense Resistance (RS1) Total Resistance from SENSE to BAT (Note 3) BAT Bias Current (Note 4) VC < 0.3V VC > 0.6V VC < 40mV LT1571-1, LT1571-2 LT1571-5 All Conditions of VCC, Temperature, LT1571-1, LT1571-2 LT1571-1, LT1571-2, TJ < 0C LT1571-5 LT1571-5, TJ < 0C LT1571-1, LT1571-2 LT1571-1, LT1571-2, TA = 25C (Note 7) LT1571-5 VC = 1V, IVC = 1A
q q q q q q q
CONDITIONS ISW = 1.5A, VBOOST - VSW 2V ISW = 1A, VBOOST - VSW < 2V (Unboosted) VBOOST = (VCC + 8V), ISW 1A VSW = 0V, VCC 20V VSW = 0V, 20V < VCC 26V
q q q
MIN
TYP 0.3 20 2 4
MAX 0.5 2.0 35 100 200 VCC - 2 27
UNITS mA/A A A V A mA
1 1
4 2.4 0.08 0.2 - 200 700
0.12 0.25 - 375 1300 5 220 550 230 230 575 575
A A A kHz kHz kHz kHz kHz kHz % % %
Oscillator Switching Frequency Switching Frequency Tolerance 180 440 170 160 425 400 87 90 77 125 200 500 200 500
Maximum Duty Cycle
93 81 210 550 0.6 100 3 300
Current Amplifier (CA2) Transconductance Maximum VC for Switch OFF IVC Current (Out of Pin) VC 0.6V 0.2V < VC < 0.45V VC < 40mV (Shutdown) mho V A mA A
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 0C to 70C. Specifications over the - 40C to 85C 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 ( 700A) 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 200A 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.
Note 5: A linear interpolation can be used for reference voltage specification between 0C and - 40C. 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 125C limit. With high power dissipation, maximum ambient temperature may be less than 70C. 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.
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LT1571 Series TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency of Figure 4 Circuit
100 98 96 94
EFFICIENCY (%)
REFERENCE VOLTAGE (V)
VCC = 15V (EXCLUDING DISSIPATION ON INPUT DIODE D3) VBAT = 8.4V
2.466 2.464 2.462 2.460 2.458
BOOST CURRENT (mA)
92 90 88 86 84 82 80 0.1 0.3 0.5 0.7 0.9 IBAT (A) 1.1 1.3 1.5
Switching Frequency vs Temperature
510 505 500
FREQUENCY (kHz)
LT1571-5
VOVP (mV)
VREF (V)
495
205 LT1571-1, LT1571-2 200
195 190 -20
-0.003
0
20 40 60 80 100 120 140 JUNCTION TEMPERATURE (C)
1571 G04
Maximum Duty Cycle
98 97 96 LT1571-1, LT1571-2 (VX = 5V) -1.20 -1.08 -0.96 -0.84 -0.60 -0.48 -0.36 -0.24 -0.12 91 90 0 20 120 JUNCTION TEMPERATURE (C) 40 60 80 100 140 0 0.12
DUTY CYCLE (%)
IPROG (mA)
95
IVC (mA)
94 93 92
UW
1571 G01
1571 G09
Reference Voltage vs Junction Temperature
2.470 2.468
Boost Current vs Switch Current
40 35 30 25 20 15 10 5 VBOOST = 21V (VX = 5V) VCC = 16V VBOOST = 26V (VX = 10V)
0
125 50 75 100 25 JUNCTION TEMPERATURE (C)
150
0 0 0.2
0.4 0.6 0.8 1.0 1.2 SWITCH CURRENT (A)
1.4 1.6
1571 G03
1571 G02
VREF Line Regulation
0.003 0.002
3 4
VOVP vs IVA (Voltage Amplifier)
0.001 0
ALL TEMPERATURES
2 125C 1
-0.001 -0.002
0 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
25C
0
5
10
15 VCC (V)
20
25
30
1571 G05
VC Pin Characteristic
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PROG Pin Characteristic
125C 25C
-0.72
0
-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
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LT1571 Series
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 10F 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 (0.08) 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 2000x 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 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 0.1F 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 60A. When VC is pulled below 40mV, LT1571 supply current drops to typical 150A. 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 0.1F 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.
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LT1571 Series
BLOCK DIAGRA
80mV
+
VC
0.2V BAT
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VCC
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GND VBAT PWM C1 SLOPE COMPENSATION R2 B1
-
R3 IPROG A11
VC 75k
CA2 VREF R7 VFB (LT1571-1 ONLY) R4 SELECT (LT1571-2, LT1571-5 ONLY)
IVA 4 CAP IPROG
IVA VA
FLAG E6
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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 CPROG
+
+
-
1.5V
-
+
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+ -
SHUTDOWN VCC D3 VIN
+ - -
200kHz/500kHz OSCILLATOR BOOST S C1 SW D1 SENSE IBAT RS1 IBAT L1 D2
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QSW
-
+
+
R
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CA1 R1 IPROG = 500A/A 1k IBAT
- + -
BAT
VBAT
BATTERY BAT2 (LT1571-2, LT1571-5 ONLY)
+ -
VREF 2.465V R6 11k R5 2k
2.465V IBAT = * 2000 RPROG
1571 BD
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LT1571 Series
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 (500A/ 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
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:
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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 (5A 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.
V 0.29(VBAT ) 1 - BAT VCC IRMS = (L1)( f)
For example, with VCC = 16V, VBAT = 8.4V, L1 = 33H 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 0.2 and the battery impedance is raised to 4 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 0.33F 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 45A 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
APPLICATIO S I FOR ATIO
is achieved with VC at 1.1V. With a 0.33F 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 1F if longer input start-up times are needed. 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. 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).
D3 VIN D2 VZ D1 1N4148 VC 2k VCC LT1571 GND
1571 F02
Figure 2. Undervoltage Lockout
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The lockout voltage will be VIN = VZ + 1V. 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. Charge Current Programming The basic formula for charge current is (see Block Diagram):
2.465V IBAT = (IPROG)(2000) = (2000) R PROG
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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 300 RPROG 4.64k 5V 0V Q1 VN2222 PWM
1571 F03
CPROG 1F
IBAT = (DC)(1A)
Figure 3. PWM Current Programming
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LT1571 Series
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 x 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 0.1F is required at
D1 MBRM120T3 SW C1 0.22F L1** 33H D2 MMBD914L SENSE CAP 0.1F SELECT GND FLAG BAT BAT2 VC 1k LT1571-2 BOOST PROG 0.3F 1F VCC
NOTE: COMPLETE LITHIUM-ION CHARGER * TOKIN OR MARCON CERAMIC SURFACE MOUNT ** COILTRONICS CTX33-2
Figure 4. 200kHz Charging Lithium Batteries (Efficiency at 1A > 87%)
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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 RCAP RPROG is the charge current setting resistor.
LT1571 CAP RCAP
1571 F05
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0.1F
Figure 5. Reducing Charge Termination Threshold
D3 MBRM120T3 CIN* 10F
VIN 11V TO 26V
4.93k
100k
300
+
COUT 22F TANT
+
8.4V
1571 F04
LT1571 Series
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 LT1571-1 R4
1571 F06
R3
Figure 6. Programming Other Battery Voltages
Current through the R3/R4 divider is set at a compromise value of 25A 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 25A, R4 = 2.465/25A = 100k and,
U
R3 =
W
UU
(R4)(VBAT - 2.465)
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 25C 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 50C 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 150A. Removing input power to the charger puts the LT1571 into a sleep mode and draws only 5A 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:
IBAT =
(2000)(2.465)
R2 =
R PROG (2.465)(2000) R1 = ILOW
(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
APPLICATIO S I FOR ATIO
C1 D1 0.22F 1N5819 SW VCC CIN* 10F 1F 300 VC IBAT SENSE * TOKIN OR MARCON CERAMIC SURFACE MOUNT ** COILTRONICS CTX33-2 BAT 0.1F D3 1N5819
VIN (WALL ADAPTER)
BOOST PROG L1** 33H D2 1N914 LT1571-1 GND
R1 100k R2 11k Q1 VN2222
1k
+
COUT 22F TANT
+
2V TO 20V
ON: IBAT = 0.5A OFF: IBAT = 0.05A
1571 F07
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 125C. 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 60C maximum ambient temperature. The 28-pin SSOP, with a thermal resistance of 40C/W, can provide a full 1.5A charge current in many situations.
12
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PBIAS = (3.5mA )(VIN) + 1.5mA(VBAT )
W
UU
(VBAT )2 +
VIN
[7.5mA + (0.012)(IBAT )]
BAT (IBAT )(VBAT )2 1+ V30 PDRIVER = 55(VIN) (IBAT )2 (RSW)(VBAT ) + t V I f PSW = ( OL )( IN)( BAT )( ) V IN
PSENSE = (0.18)(IBAT )
2
RSW = Switch ON resistance 0.35 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 )
(VBAT )2 +
VIN
[7.5mA + (0.012)(IBAT )]
BAT (IBAT )(VBAT )2 1+ V30 PDRIVER = 55(VIN) (IBAT )2 (RSW)(VBAT ) + t V I f PSW = ( OL )( IN)( BAT )( ) V IN
PSENSE = (0.18)(IBAT )
2
Total power in the IC is: 0.17 + 0.13 + 0.32+ 0.26 = 0.88W Temperature rise will be (0.88W)(40C/W) = 35C. 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
APPLICATIO S I FOR ATIO
Then,
instead of VBAT (see Figure 8). The optimum boost voltage (VX) is from 3V to 6V.
(IBAT )(VBAT )(VX) 1+ VX 30 PDRIVER = 55(VIN)
For example, VX = 3.3V, .3 (1.2A)(8.4V)(3.3V) 1+ 330V PDRIVER = = 0.045W 55(15V ) The average IVX required is:
PDRIVER 0.045W = = 14mA 3.3V VX
THERMAL RESISTANCE (C/W)
LEAD TEMPERATURE (C)
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. Higher Duty Cycle 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
U
SW C1 L1 D2 SENSE VX 3V TO 6V IVX LT1571 BOOST
W
UU
+
10F
1571 F08
Figure 8. Lower VBOOST
60 55 50 45 40 35 30 25 0 5 20 15 25 10 BOARD AREA (IN2) 30 35 GN16, MEASURED FROM AIR AMBIENT TO DIE USING COPPER LANDS AS SHOWN ON DATA SHEET 4-LAYER BOARD
2-LAYER BOARD
1571 F09
Figure 9. LT1571 Thermal Resistance
90 80 70 2-LAYER BOARD 60 50 40 30 20 0 ICHRG = 1.3A VIN = 16V VBAT = 8.4V VBOOST = VBAT TA = 25C 5 20 15 25 10 BOARD AREA (IN2) 30 35 4-LAYER BOARD NOTE: PEAK DIE TEMPERATURE WILL BE ABOUT 10C HIGHER THAN LEAD TEMPERATURE AT 1.3A CHARGING CURRENT
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
APPLICATIO S I FOR ATIO
Lower Dropout Voltage
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.
Q1 VIN
+
VCC Q2 RX 50k D1 L1 D2 SENSE VX 3V TO 6V Q1: Si4435DY Q2: TP0610L BAT CX 10F VBAT
VIN CIN HIGH FREQUENCY CIRCULATING PATH COUT BAT
SW C3 LT1571 BOOST
+
HIGH DUTY CYCLE CONNECTION
1571 F11
1571 F12
Figure 11. Replacing the Input Diode
L1
Figure 13. Critical Electrical and Thermal Path Layer for LT1571-5
14
U
Layout Considerations 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.
SWITCH NODE L1 VBAT
W
UU
Figure 12. High Speed Switching Path
GND LT1571-5 D1 GND SW BOOST BAT2 FLAG SELECT SENSE GND GND VCC2 VCC1 CAP PROG VC BAT GND
1571 F13
CIN
LT1571 Series
PACKAGE DESCRIPTIO
0.007 - 0.0098 (0.178 - 0.249) 0.016 - 0.050 (0.406 - 1.270)
* 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
0.0075 - 0.0098 (0.191 - 0.249) 0.016 - 0.050 (0.406 - 1.270)
0 - 8 TYP
* 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.
U
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
0.009 (0.229) REF
0.229 - 0.244 (5.817 - 6.198)
0.150 - 0.157** (3.810 - 3.988)
1 0.015 0.004 x 45 (0.38 0.10) 0 - 8 TYP 0.053 - 0.068 (1.351 - 1.727)
23
4
56
7
8 0.004 - 0.0098 (0.102 - 0.249)
0.008 - 0.012 (0.203 - 0.305)
0.0250 (0.635) BSC
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 0.033 (0.838) REF
0.229 - 0.244 (5.817 - 6.198)
0.150 - 0.157** (3.810 - 3.988)
1 0.015 0.004 x 45 (0.38 0.10) 0.053 - 0.069 (1.351 - 1.748)
23
4
56
7
8
9 10 11 12 13 14 0.004 - 0.009 (0.102 - 0.249)
0.008 - 0.012 (0.203 - 0.305)
0.0250 (0.635) BSC
GN28 (SSOP) 1098
15
LT1571 Series RELATED PARTS
PART NUMBER LT1505 LT1510 LT1510-5 LT1511 LT1512 DESCRIPTION High Current Constant-Current/Constant-Voltage Battery Charger Controller with Input Current Limit 200kHz Constant-Current/Constant-Voltage Battery Charger 500kHz Constant-Current/Constant-Voltage Battery Charger 200kHz Constant-Current/Constant-Voltage Battery Charger with Input Current Limit 500kHz SEPIC Constant-Current/Constant-Voltage Battery Charger 500kHz SEPIC Constant-Current/Constant-Voltage Battery Charger Li-Ion Battery Charger Termination Controller COMMENTS High Efficiency Synchronous Buck Topology, Uses External N-Channel FETs. Includes Preset Battery Voltages and Input Current Limiting Up to 1.5A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries Up to 1A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries Up to 3A Charge Current for Li-Ion, NiCd, NiMH or Lead Acid Batteries 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 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 Can Be Used with Battery Chargers to Provide Charge Termination, Preset Voltages, C/10 Charge Detection and Timer Functions
LT1513
LTC(R)1729 LTC1731 LTC1759 LT1769
Linear Constant-Current/Constant-Voltage Charger Controller Simple Charger Uses External FET. Features Preset Voltages, C/10 Charge Detection and Programmable Timer SMBus Controlled Constant-Current/Constant-Voltage Smart Battery Charger Controller 200kHz Constant-Current/Constant-Voltage Battery Charger with Input Current Limit LT1505 Charger Functionality with SMBus Up to 2A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
1571f LT/TP 0700 4K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2000

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