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19-2099; Rev 0; 7/01
ILABLE N KIT AVA EVALUATIO
Simple Current-Limited Switch-Mode Li+ Charger Controller
General Description
The low-cost MAX1873R/S/T provides all functions needed to simply and efficiently charge 2-, 3-, or 4series lithium-ion cells at up to 4A or more. It provides a regulated charging current and voltage with less than 0.75% total voltage error at the battery terminals. An external P-channel MOSFET operates in a step-down DC-DC configuration to efficiently charge batteries in low-cost designs. The MAX1873R/S/T regulates the battery voltage and charging current using two control loops that work together to transition smoothly between voltage and current regulation. An additional control loop limits current drawn from the input source so that AC adapter size and cost can be minimized. An analog voltage output proportional to charging current is also supplied so that an ADC or microcontroller can monitor charging current. The MAX1873 may also be used as an efficient currentlimited source to charge NiCd or NiMH batteries in multichemistry charger designs. The MAX1873R/S/T is available in a space-saving 16-pin QSOP package. Use the evaluation kit (MAX1873EVKIT) to help reduce design time. o Low-Cost and Simple Circuit o Charges 2-, 3-, or 4-Series Lithium-Ion Cells o AC Adapter Input-Current-Limit Loop o Also Charges Ni-Based Batteries o Analog Output Monitors Charge Current o 0.75% Battery-Regulation Voltage o 5A Shutdown Battery Current o Input Voltage Up to 28V o 200mV Dropout Voltage/100% Duty Cycle o Adjustable Charging Current o 300kHz PWM Oscillator Reduces Noise o Space-Saving 16-Pin QSOP o MAX1873 Evaluation Kit Available to Speed Designs
Features
MAX1873
Ordering Information
PART MAX1873REEE MAX1873SEEE MAX1873TEEE TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP 16 QSOP
Applications
Notebook Computers Portable Internet Tablets 2-, 3-, or 4-cell Li+ Battery Pack Chargers 6-, 9-, or 10-cell Ni Battery Pack Chargers Hand-Held Instruments Portable Desktop Assistants (PDAs) Desktop Cradle Chargers
Typical Operating Circuit
VIN 9V TO 28V (9V MIN FOR 2CELLS) VH VL CSSP SYSTEM LOAD
Selector Guide
PART MAX1873REEE MAX1873SEEE MAX1873TEEE SERIES CELLS TO CHARGE 2-Cell Li+ or 5- or 6-cell Ni Battery 3-Cell Li+ or 7- or 9-cell Ni Battery 4-Cell Li+ 10-cell Ni Battery Packs
4V OUT PER 200mV ON RCS
DCIN
MAX1873
CSSN
IOUT ICHG/EN REF CSB BATT CCI VADJ EXT
Pin Configuration appears at end of data sheet.
CCS GND CCV 2- TO 4-CELL Li+
________________________________________________________________ Maxim Integrated Products
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Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
ABSOLUTE MAXIMUM RATINGS
CSSP, CSSN, DCIN to GND ...................................-0.3V to +30V VL, ICHG/EN to GND................................................-0.3V to +6V VH, EXT to DCIN.......................................................-6V to +0.3V VH, EXT to GND ......................................(VDCIN + 0.3V) to -0.3V EXT to VH .................................................................+6V to -0.3V DCIN to VL..............................................................+30V to -0.3V VADJ, REF, CCI, CCV, CCS, IOUT to GND.............................................-0.3V to (VL + 0.3V) BATT, CSB to GND.................................................-0.3V to +20V CSSP to CSSN.......................................................-0.3V to +0.6V CSB to BATT..........................................................-0.3V to +0.6V VL Source Current ............................................................+50mA VH Sink Current ................................................................+40mA Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C..........+667mW Operating Temperature Range MAX1873_EEE ................................................-40C to +85C Junction Temperature ..................................................... +150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) ................................ +300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER INPUT SUPPLY AND REFERENCE DCIN Input Voltage Range DCIN Quiescent Supply Current DCIN to BATT Undervoltage Threshold DCIN to BATT Undervoltage Threshold VL Output Voltage VL Output Load Regulation REF Output Voltage REF Line Regulation REF Load Regulation SWITCHING REGULATOR PWM Oscillator Frequency EXT Driver Source On-Resistance EXT Driver Sink On-Resistance VH Output Voltage CSSN/CSSP Input Current CSSN/CSSP Off-State Leakage BATT, CSB Input Current BATT, CSB Input Current DCIN - VH, 6V < VDCIN <28V, IVH = 0 to 20mA VCSSN/VCSSP = 28V, VDCIN = 28V VDCIN = VSSN/VCSSP = 18V, VBATT = VCSB = 18V ICHG/EN = 0 (charger disabled) ICHG/EN = REF (charger enabled) DCIN BATT (input power removed) 4.75 70 1.5 0.2 250 1.5 270 300 4 2.5 330 7 4.5 5.75 200 5 1 500 5 kHz V A A A A 6.0V < VDCIN < 28V DCIN BATT CSSP = DCIN, input falling CSSP = DCIN, input rising 6.0V < VDCIN < 28V IVL = 0 to 3mA IREF = 21A (200k load) 6.0V < VDCIN < 28V IREF = 0 to 1mA 4.179 0.05 0.22 5.15 5.40 15 4.20 2 22 6 6 4 0.1 28 7 10 0.175 0.38 5.65 50 4.221 6 65 13 V mA A V V V mV V mV ppm/V mV CONDITIONS MIN TYP MAX UNITS
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Simple Current-Limited Switch-Mode Li+ Charger Controller
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER BATT Overvoltage Cutoff Threshold CONDITIONS 2-cell version MAX1873R 3-cell version MAX1873S 4-cell version MAX1873T (Note 1) MAX1873R (2 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) Battery Regulation Voltage MAX1873S (3 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) MAX1873T (4 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) MAX1873R BATT Undervoltage Threshold CURRENT SENSE CSB to BATT Battery Current-Sense Voltage CSB to BATT Current-Sense Voltage when VBATT < 2.5V per Cell CSSP to CSSN Current-Sense Voltage CONTROL INPUTS/OUTPUTS ICHG/EN Input Threshold ICHG/EN Input Voltage Range For Charge Current Adjustment VADJ Input Current ICHG/EN Input Current VADJ Input Voltage Range Full scale 25% scale Trickle charge No charge current VCSB - VBATT = 200mV, 0 < IOUT < 500A VCSB - VBATT = 50mV, 0 < IOUT < 500A VCSB - VBATT = 10mV VCSB - VBATT = 0, IIOUT = sinking 20A Includes 50mV of hysteresis 500 700 VVADJ = VREF/2 VICHG/EN = VREF -100 -100 0 3.6 0.9 75 40 4.0 1.0 200 70 600 700 VREF 100 100 VREF 4.4 V 1.1 325 90 mV mV mV nA nA V 6V < VCSSP < 28V VICHG/EN = VREF VICHG/EN = VREF/4 190 40 5 90 200 50 10 100 210 60 15 110 mV mV mV For ICHG/20 trickle charge MAX1873S MAX1873T MIN 10.45 15.675 17.575 7.898 8.337 8.775 11.847 12.505 13.163 15.796 16.674 17.551 4.8 7.2 9.6 TYP 11 16.5 18.5 7.958 8.4 8.842 11.937 12.6 13.263 15.916 16.8 17.684 5.0 7.5 10 MAX 11.55 17.325 19.425 8.018 8.463 8.909 12.027 12.695 13.363 16.036 16.926 17.817 5.2 7.8 10.4 V V V UNITS
MAX1873
IOUT Voltage
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Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER INPUT SUPPLY AND REFERENCE DCIN Input Voltage Range DCIN Quiescent Supply Current DCIN to BATT Undervoltage Threshold DCIN to BATT Undervoltage Threshold VL Output Voltage VL Output Load Regulation REF Output Voltage REF Line Regulation REF Load Regulation SWITCHING REGULATOR PWM Oscillator Frequency EXT Driver Source On-Resistance EXT Driver Sink On-Resistance VH Output Voltage CSSN/CSSP Input Current CSSN/CSSP Off-State Leakage BATT, CSB Input Current BATT, CSB Input Current BATT Overvoltage Cutoff Threshold DCIN - VH, 6V < VDCIN <28V, IVH = 0 to 20mA VCSSN/VCSSP = 28V, VDCIN = 28V VDCIN = VSSN/VCSSP = 18V VBATT = VCSB = 18V ICHG/EN = 0 (charger disabled) ICHG/EN = REF (charger enabled) DCIN BATT (input power removed) 2-cell version MAX1873R 3-cell version MAX1873S 4-cell version MAX1873T (Note 1) MAX1873R (2 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) Battery Regulation Voltage MAX1873S (3 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) MAX1873T (4 Li+ cells) VVADJ = 0 VVADJ = VREF/2 VVADJ = VREF (Note 1) 10.45 15.675 17.575 7.898 8.337 8.775 11.847 12.505 13.163 15.796 16.674 17.551 4.75 270 330 7 4.5 5.75 200 5 1 500 5 11.55 17.325 19.425 8.018 8.463 8.909 12.027 12.695 13.363 16.036 16.926 17.817 V V kHz V A A A A 6.0V < VDCIN < 28V DCIN BATT CSSP = DCIN, input falling CSSP = DCIN, input rising 6.0V < VDCIN < 28V IVL = 0 to 3mA IREF = 21A (200k load) 6.0V < VDCIN < 28V IREF = 0 to 1mA 4.179 0.05 0.22 5.15 6 28 7 10 0.2 0.38 5.65 50 4.221 6 65 13 V mA A V V V mV V mV ppm/V mV CONDITIONS MIN MAX UNITS
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Simple Current-Limited Switch-Mode Li+ Charger Controller
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = -40C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER BATT Undervoltage Threshold CURRENT SENSE CSB to BATT Battery Current-Sense Voltage CSB to BATT Current-Sense Voltage when VBATT < 2.5V per Cell CSSP to CSSN Current-Sense Voltage CONTROL INPUTS/OUTPUTS ICHG/EN Input Threshold ICHG/EN Input Voltage Range for Charge Current Adjustment VADJ Input Current ICHG/EN Input Current VADJ Input Voltage Range Full scale 25% scale Trickle charge No charge current VCSB - VBATT = 200mV, 0 < IOUT < 500A VCSB - VBATT = 50mV, 0 < IOUT < 500A VCSB - VBATT = 10mV VCSB - VBATT = 0, IIOUT = sinking 20A VVADJ = VREF/2 VICHG/EN = VREF Includes 50mV of hysteresis 500 700 -100 -100 0 3.6 0.9 75 40 700 VREF 100 100 VREF 4.4 V 1.1 325 90 mV mV mV nA nA V 6V < VCSSP < 28V VICHG/EN = VREF VICHG/EN = VREF/4 190 40 5 90 210 60 15 110 mV mV mV mV CONDITIONS MAX1873R For ICHG/20 trickle charge MAX1873S MAX1873T MIN 4.8 7.2 9.6 MAX 5.2 7.8 10.4 V UNITS
MAX1873
IOUT Voltage
Note 1: While it may appear possible to set the Battery Regulation Voltage higher than the Battery Overvoltage Cutoff Threshold, this cannot happen because both parameters are derived from the same reference and track each other. Note 2: Specifications to -40C are guaranteed by design, not production tested.
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Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
Typical Operating Characteristics
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = +25C, unless otherwise noted).
MAX1873T (4-CELL) BATTERY VOLTAGE vs. CHARGING CURRENT
MAX1873 toc01
IOUT VOLTAGE vs. CSB-BATT VOLTAGE
4.0 3.5 IOUT VOLTAGE (V)
MAX1873 toc02
17.5 15.0 BATTERY VOLTAGE (V) 12.5 10.0 7.5 5.0 2.5 RCSB + 0.068 0 0 0.5 1.0 1.5 2.0 2.5 3.0
4.5
3.0 2.5 2.0 1.5 1.0 0.5 0
3.5
0
50
100
150
200
250
CHARGING CURRENT (A)
CSB-BATT VOLTAGE (mV)
MAX1873T (4-CELL) BATTERY REGULATION VOLTAGE vs. VADJ VOLTAGE
MAX1873 toc03
RECENT VOLTAGE VS. TEMPERATURE
MAX1873 toc04
18.0 BATTERY REGULATION VOLTAGE (V)
4.210 4.205
REFERENCE VOLTAGE (V)
17.5
4.200 4.195 4.190 4.185 MAX1873T 4.180
17.0
16.5
16.0
15.5 0 1 2 3 4 5 VADJ VOLTAGE (V)
-50
-25
0
25
50
75
100
TEMPERATURE (C)
RECENT VOLTAGE
VS. REFERENCE CURRENT
MAX1873 toc05
MAX1873R (2-CELL) EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc06
4.210 4.205 REFERENCE VOLTAGE (V) 4.200 4.195 4.190 4.185 MAX1873T 4.180 0
100
90 EFFICIENCY (%)
80
70
60 VBATT = 7V ICHG = 3A 50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 REFERENCE VOLTAGE (mA) 8 12 16 20 24 28 INPUT VOLTAGE (V)
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Simple Current-Limited Switch-Mode Li+ Charger Controller
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VDCIN = VCSSP = VCSSN = 18V, VICHG/EN = VREF, VVADJ = VREF/2. MAX1873R: VBATT = VCSB = 8.4V; MAX1873S: VBATT = VCSB = 12.6V; MAX1873T: VBATT = VCSB = 16.8V; TA = +25C, unless otherwise noted).
MAX1873S (3-CELL) EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc07
MAX1873
MAX1873T (4-CELL) EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc08
100
100
90 EFFICIENCY (%)
90 EFFICIENCY (%)
80
80
70
70
60
VBATT = 10.5V ICHG = 3A 12 16 20 INPUT VOLTAGE (V) 24 28
60
VBATT = 14V ICHG + 3A 16 18 20 22 24 26 28
50
50 INPUT VOLTAGE (V)
4-CELL BATTERY VOLTAGE AND CHARGING CURRENT vs. TIME
MAX1873 toc09
CHARGING CURRENT vs. SYSTEM LOAD CURRENT
MAX1873 toc10
18 16 BATTERY VOLTAGE (V) 14 12 10 CHARGING CURRENT 8 6 4 0 25 50 75 100 125 BATTERY VOLTAGE
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 CHARGING CURRENT (A) CHARGING CURRENT (A)
3.0 2.5 2.0 1.5 1.0 0.5 0 0 0.5 1.0 1.5 2.0 2.5
150
3.0
TIME (MINUTES)
SYSTEM LOAD CURRENT (A)
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Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
Pin Description
PIN 1 2 3 4 5 NAME CSSN CSSP CCS CCV CCI FUNCTION Source Current-Sense Negative Input. Connect a current-sense resistor between CSSP and CSSN to limit total current drawn from the input source. To disable input current sensing, connect CSSN to CSSP. Source Current-Sense Positive Input. Also used for input source undervoltage sensing. Input-Source-Current Regulation Loop Compensation Point Battery Regulation Voltage Control-Loop Compensation Point. Pulling CCV high (to VL) through a 1.5k resistor disables the voltage control loop for charging NiCd or NiMH batteries. Battery Charge Current Control-Loop Compensation Point Battery Charging Current Adjust/Shutdown Input. This pin can be connected to a resistive-divider between REF and GND to adjust the charge current sense threshold between CSB and BATT. When ICHG/EN is connected to REF, the CSB-BATT threshold is 200mV. Pull ICHG/EN low (below 500mV) to disable charging and reduce the supply current to 5A. Charge Current Monitor Output. Analog Voltage Output that is proportional to charging current. VIOUT = 20 (VCSB - VBATT) or 4V for a 200mV current-sense voltage (maximum load capacitance = 5nF). Battery Regulation Voltage Adjust. Set the battery regulation voltage from 3.979V per cell to 4.421V per cell with 1% resistors. Output accuracy remains better than 0.75% even with 1% adjusting resistors due to reduced adjustment range. For 4.2V, the voltage-divider resistors must be equal value (nominally 100k each). 4.2V Reference Voltage Output. Bypass to GND with a 1F ceramic capacitor. Battery Voltage-Sense Input and Battery Current-Sense Negative Input. Bypass to GND with a 68F for MAX1873R, 47F for MAX1873S, and 33F for MAX1873T. Use capacitors with ESR < 1. Battery Current-Sense Positive Input Ground Internal VH Regulator. VH internally supplies power to the EXT driver. Connect a 0.22F ceramic capacitor between VH and DCIN. Drive Output for External PFET. EXT swings from VDCIN to VDCIN - 5V. Power-Supply Input. DCIN is the input supply for charger IC. Bypass to GND with a 0.22F ceramic capacitor. Internal VL Regulator. VL powers the MAX1873's control logic at 5.4V. Bypass to GND with a 2.2F or larger ceramic capacitor.
6
ICHG/EN
7
IOUT
8
VADJ
9 10 11 12 13 14 15 16
REF BATT CSB GND VH EXT DCIN VL
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Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
D1 MBR5340 VIN 17V TO 28V (9V MIN FOR 2- CELLS) CVL 2.2F CVH 0.22F VH DCIN CSSN VL RP 4.7 CSSP CP 0.01F RN 4.7 CN 0.01F EXT
P
CDCIN 0.22F
RCSS 0.033 SYSTEM LOAD D2 MBR5340 CL 47F
MAX1873
4V OUT PER 200mV ON RCSB CCC 47nF IOUT ICHG/EN CSB CCI CCCS 47nF CCCVS 0.1F RCCV 10k CCS VADJ CCV GND BATT REF
100k DISABLE
N
L1 10H
RCSB 0.068
CBATT 68F
CREF 1F
R1
LI+ BATTERY (2- TO 4-CELLS)
R2
CCCVP 1nF
Figure 1. Typical Application Circuit
Detailed Description
The MAX1873 includes all of the functions necessary to charge 2-, 3-, or 4-series cell lithium-ion (Li+) battery packs. It includes a high-efficiency step-down DC-DC converter that controls charging voltage and current. It also features input source current limiting so that an AC adapter that supplies less than the total system current in addition to charging current can be used without fear of overload. The DC-DC converter uses an external P-channel MOSFET switch, inductor, and diode to convert the input voltage to charging current or charging voltage. The typical application circuit is shown in Figure 1. Charging current is set by RCSB, while the battery voltage is measured at BATT. The battery regulation voltage limit is nominally set to 8.4V for the R version (2-cells), 12.6V for the S version (3-cells), and 16.8V for the T version (4-cells),
but it can also be adjusted to other voltages for different Li+ chemistries.
Voltage Regulator
Li+ batteries require a high-accuracy voltage limit while charging. The battery regulation voltage is nominally set to 4.2V per cell and can be adjusted 5.25% by setting the voltage at VADJ between REF and ground. By limiting the adjust range of the regulation voltage, an overall voltage accuracy of better than 0.75% is maintained while using 1% resistors. An internal error amplifier maintains voltage regulation to within 0.75%. The amplifier is compensated at CCV (see Figure 1). Individual compensation of the voltage regulation and current regulation loops allows for optimal compensation of each. A typical CCV compensation network is shown in Figure 1 and will suffice for most designs.
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Simple Current-Limited Switch-Mode Li+ Charger Controller
CSSN 5.4 REGULATOR UNDERVOLTAGE COMPARATOR BATT DCIN VH CCS DRIVER CCV CONTROL LOGIC VH GND CSB ICHG /EN CURRENT ERROR AMP IOUT BATT EXT VL
CSSP
SHUTDOWN FOR ALL BLOCKS
CCI
VOLTAGE ERROR AMP
portions of the system are powered up or put to sleep. Without the benefit of input-current regulation, the input source would have to be able to supply the maximum system current plus the maximum charger-input current. The MAX1873 input-current loop ensures that the system always gets adequate power by reducing charging current as needed. By using the input-current limiter, the size and cost of the AC adapter can be reduced. See Setting the Input-Current Limit section for design details. Input current is measured through an external sense resistor, RCSS, between CSSP and CSSN. The inputcurrent limit feature may be bypassed by connecting CSSP to CSSN. The input-current error amplifier is compensated at CCS. A 47nF capacitor from CCS to GND provides suitable performance for most applications.
MAX1873
PWM Controller
The pulse-width modulation (PWM) controller drives the external MOSFET at a constant 300kHz to regulate the charging current and voltage while maintaining low noise. The controller accepts inputs from the CCI, CCV, and CCS error amplifiers. The lowest signal of these three drives the PWM controller. An internal clamp limits the noncontrolling signals to within 200mV of the controlling signal to prevent delay when switching between the battery-voltage control, charging-current control, and input-current regulation loops.
9R VADJ A=1 R GND
R REF 4.2V REFERENCE
Shutdown
The MAX1873 stops charging when ICHG/EN is pulled low (below 0.5V) and shuts down when the voltage at DCIN falls below the voltage at BATT. In shutdown, the internal resistive voltage-divider is disconnected from BATT to reduce the battery drain. When AC-adapter power is removed, or when the part is shut down, the MAX1873 typically draws 1.5A from the battery.
Figure 2. Functional Block Diagram
Charging-Current Regulator
The charging-current regulator limits the battery charging current. Current is sensed by the current-sense resistor (RCSB in Figure 1) connected between BATT and CSB. The voltage on ICHG/EN can also adjust the charging current. Full-scale charging current (ICHG = 0.2V / RCSB) is achieved by connecting ICHG/EN to REF. See Setting the Charging-Current Limit section for more details. The charging-current error amplifier is compensated at CCI (Figure 1). A 47nF capacitor from CCI to GND provides suitable performance for most applications.
Source Undervoltage Shutdown (Dropout)
The DCIN voltage is compared to the voltage at BATT. When the voltage at DCIN drops below BATT + 50mV, the charger turns off, preventing drain on the battery when the input source is not present or is below the battery voltage. A diode is typically connected between the input source and the charger input. This diode prevents the battery from discharging through the body diode of the high-side MOSFET should the input be shorted to GND. It also protects the charger, battery, and systems from reversed polarity adapters and negative input voltages.
Input-Current Regulator
The input-current regulator limits the source current by reducing charging current when the input current reaches the set input-current limit. In a typical portable design, system load current will normally fluctuate as
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Simple Current-Limited Switch-Mode Li+ Charger Controller
Charge-Current Monitor Output
IOUT is an analog voltage output that is proportional to the actual charge current. With the aid of a microcontroller, the IOUT signal can facilitate gas-gauging, indicate percent of charge, or charge-time remaining. The equation governing this output is: VIOUT = 20( VCSB - VBATT ) or VOUT = 20(RCSB x ICHG ) where VCSB and VBATT are the voltages at the CSB and BATT pins, and ICHG is the charging current. IOUT can drive a load capacitance of 5nF.
Setting the Charging-Current Limit
The charging current ICHG is sensed by the currentsense resistor RCSB between CSB and BATT, and is also adjusted by the voltage at ICHG/EN. If ICHG/EN is connected to REF (the standard connection), the charge current is given by: ICHG = 0.2V / RCSB In some cases, common values for RCSB may not allow the desired charge-current value. It may also be desirable to reduce the 0.2V CSB-to-BATT sense threshold to reduce power dissipation. In such cases, the ICHG/EN input may be used to reduce the charge-current-sense threshold. In those cases the equation for charge current becomes: ICHG = 0.2V VICH / EN / VREF / RCSB
MAX1873
Design Procedure
Setting the Battery-Regulation Voltage
For Li+ batteries, VADJ sets the per-cell battery-regulation voltage limit. To set the VADJ voltage, use a resistive-divider from REF to GND (Figure 1). For a battery voltage of 4.2V per cell, use resistors of equal value (100k each) in the VADJ voltage-divider. To set other battery-regulation voltages, see the remainder of this section. The per-cell battery regulation voltage is a function of Li+ battery chemistry and construction and is usually clearly specified by the manufacturer. If this is not clearly specified, be sure to consult the battery manufacturer to determine this voltage before charging any Li+ battery. Once the per-cell voltage is determined, the VADJ voltage is calculated by the equation: VVADJ = 9.5( VBATTR ) / N - (9VREF ) where VBATTR is the desired battery-regulation voltage (for the total series-cell stack), N is the number of Li+ battery cells, and VREF is the reference voltage (4.2V). Set VVADJ by choosing R1. R1 should be selected so that the total divider resistance (R1+ R2) is near 200k. R2 can then be calculated as follows: R2 = VVADJ / ( VREF - VVADJ ) x R1 Since the full range of VADJ (from 0 to VREF) results in a 5.263% adjustment of the battery-regulation limit (3.979V to 4.421V), the resistive-divider's accuracy need not be as tight as the output-voltage accuracy. Using 1% resistors for the voltage-divider still provides 0.75% battery-voltage-regulation accuracy.
(
)
Setting the Input-Current Limit
The input-source current limit, IIN, is set by the inputcurrent sense resistor, R CSS , (Figure 1) connected between CSSP and CSSN. The equation for the source current is: IIN = 0.1V / RCSS This limit is typically set to the current rating of the input power source or AC adapter to protect the input source from overload. Short CSSP and CSSN to DCIN if the input-source current-limit feature is not used.
[
]
Inductor Selection
The inductor value may be selected for more or less ripple current. The greater the inductance, the lower the ripple current. However, as the physical size is kept the same, larger inductance value typically results in higher inductor series resistance and lower inductor saturation current. Typically, a good tradeoff is to choose the inductor such that the ripple current is approximately 30% to 50% of the DC average charging current. The ratio of ripple current to DC charging current (LIR) can be used to calculate the inductor value: L = VBATT VDCIN(MAX) - VBATT
[
]
{
[VDCIN(MAX) x fSW x ICHG x LIR]
[
]} /
where f SW is the switching frequency (nominally 300kHz) and ICHG is the charging current. The peak inductor current is given by:
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11
Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
IPEAK = ICHG (1+ LIR / 2) For example, for a 4-cell charging current of 3A, a VDCIN(MAX) of 24V, and an LIR of 0.5, L is calculated to be 11.2H with a peak current of 3.75A. Therefore a 10H inductor would be satisfactory. PTOT = PR + PT
Diode Selection
A Schottky rectifier with a current rating of at least the charge current limit must be connected from the MOSFET drain to GND. The voltage rating of the diode must exceed the maximum expected input voltage.
MOSFET Selection
The MAX1873 uses a P-channel power MOSFET switch. The MOSFET must be selected to meet the efficiency or power dissipation requirements of the charging circuit as well as the maximum temperature of the MOSFET. Characteristics that affect MOSFET power dissipation are drain-source on-resistance (RDS(ON)) and gate charge. Generally these are inversely proportional. To determine MOSFET power dissipation, the operating duty cycle must first be calculated. When the charger is operating at higher currents, the inductor current will be continuous (the inductor current will not drop to 0). In this case, the high-side MOSFET duty cycle (D) can be approximated by the equation: V D BATT VDCIN And the catch-diode duty cycle (D') will be 1 - D or: V - VBATT D' DCIN VDCIN where VBATT is the battery-regulation voltage (typically 4.2V per cell) and VDCIN is the source-input voltage. For MOSFETs, the worst-case power dissipation due to on-resistance (PR) occurs at the maximum duty cycle, where the operating conditions are minimum sourcevoltage and maximum battery voltage. P R can be approximated by the equation: PR = VBATT(MAX) VDCIN(MIN) x RDS(ON) x ICHG2
Capacitor Selection
The input capacitor shunts the switching current from the charger input and prevents that current from circulating through the source, typically an AC wall cube. Thus the input capacitor must be able to handle the input RMS current. At high charging currents, the converter will typically operate in continuous conduction. In this case, the RMS current of the input capacitor can be approximated with the equation: ICIN ICHG D - D2 where ICIN is the input capacitor RMS current, D is the PWM converter duty cycle (typically VBATT/VDCIN), and ICHG is the battery-charging current. The maximum RMS input current occurs at 50% duty cycle, so the worst-case input-ripple current is 0.5 x ICHG. If the input-to-output voltage ratio is such that the PWM controller will never work at 50% duty cycle, then the worst-case capacitor current will occur where the duty cycle is nearest 50%. The impedance of the input capacitor is critical to preventing AC currents from flowing back into the wall cube. This requirement varies depending on the wall cube's impedance and the requirements of any conducted or radiated EMI specifications that must be met. Low ESR aluminum electrolytic capacitors may be used, however, tantalum or high-value ceramic capacitors generally provide better performance. The output filter capacitor absorbs the inductor-ripple current. The output-capacitor impedance must be significantly less than that of the battery to ensure that it will absorb the ripple current. Both the capacitance and the ESR rating of the capacitor are important for its effectiveness as a filter and to ensure stability of the PWM circuit. The minimum output capacitance for stability is: VBATT VREF 1+ VDCIN(MIN) COUT > VBATT x fSW x RCSB
Transition losses (P T ) can be approximated by the equation: V xI xf x t TR PT = DCIN CHG SW 3 where tTR is the MOSFET transition time and fSW is the switching frequency. The total power dissipation of the MOSFET is then:
12
______________________________________________________________________________________
Simple Current-Limited Switch-Mode Li+ Charger Controller
where COUT is the total output capacitance, VREF is the reference voltage (4.2V), VBATT is the maximum battery regulation voltage (typically 4.2V per cell), VDCIN (MIN) is the minimum source-input voltage, and RCSB is the current-sense resistor (68m for 3A charging current) from CSB to BATT. The maximum output capacitor ESR allowed for stability is: R xV RESR < CSB BATT VREF where RESR is the output capacitor ESR. tings do not interfere with charging. However, the battery undervoltage-protection features remain active so charging current is reduced when VBATT is less than the levels stated in the BATT Undervoltage Threshold line in the Electrical Characteristics Table. 5- or 6-series Ni cells may be charged with the R version device, 7to 9-cells with the S version, and 10-cells with the T version. The MAX1873 contains no charge-termination algorithms for Ni cells; it acts only as a current source. A separate microcontroller or Ni-cell charge controller must instruct the MAX1873 to terminate charging.
MAX1873
Compensation Components
The three regulation loops: input current limit, charging current limit, and charging voltage limit are compensated separately using the CCS, CCI, and CCV pins, respectively. The charge-current loop error-amplifier output is brought out at CCI. Likewise, the source-current erroramplifier output is brought out at CCS. 47nF capacitors to ground at CCI and CCS compensate the current loops in most charger designs. Raising the value of these capacitors reduces the bandwidth of these loops. The voltage-regulating loop error-amplifier output is brought out at CCV. Compensate this loop by connecting a capacitor in parallel with a series resistor-capacitor from CCV to GND. Recommended values are shown in Figure 1. PROCESS: BiCMOS TRANSISTOR COUNT: 1397
Chip Information
Applications Information
VL, VH, and REF Bypassing
The MAX1873 uses two internal linear regulators to power internal circuitry. The outputs of the linear regulators are at VL and VH. VL powers the internal control circuitry while VH powers the MOSFET gate driver. VL may also power a limited amount of external circuitry, as long as its maximum current (3mA) is not exceeded. A 2.2F bypass capacitor is required from VL to GND to ensure stability. A 0.22F capacitor is required from VH to DCIN. A 1F bypass capacitor is required between REF and GND to ensure that the internal 4.2V reference is stable. In all cases, use low-ESR ceramic capacitors.
TOP VIEW
CSSN 1 CSSP 2 CCS 3 CCV 4 CCI 5 ICHG/EN 6 IOUT 7 VADJ 8 16 VL 15 DCIN 14 EXT
Pin Configuration
MAX1873R/S/T
13 VH 12 GND 11 CSB 10 BATT 9 REF
Charging NiMH and NiCd Cells
The MAX1873 may be used in multichemistry chargers. When charging NiMH or NiCd cells, pull CCV high (to VL) with a 1.5 k resistor. This disables the voltage control loop so the Li+ battery-regulation voltage set-
16 QSOP
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13
Simple Current-Limited Switch-Mode Li+ Charger Controller MAX1873
Package Information
QSOP.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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