General Description
The low-cost MAX1873R/S/T provides all functions
needed to simply and efficiently charge 2-, 3-, or 4-
series 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 cur-
rent drawn from the input source so that AC adapter
size and cost can be minimized. An analog voltage out-
put 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 current-
limited source to charge NiCd or NiMH batteries in mul-
tichemistry 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.
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
Features
oLow-Cost and Simple Circuit
oCharges 2-, 3-, or 4-Series Lithium-Ion Cells
oAC Adapter Input-Current-Limit Loop
oAlso Charges Ni-Based Batteries
oAnalog Output Monitors Charge Current
o±0.75% Battery-Regulation Voltage
o5µA Shutdown Battery Current
oInput Voltage Up to 28V
o200mV Dropout Voltage/100% Duty Cycle
oAdjustable Charging Current
o300kHz PWM Oscillator Reduces Noise
oSpace-Saving 16-Pin QSOP
oMAX1873 Evaluation Kit Available to Speed
Designs
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
19-2099; Rev 0; 7/01
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT AVAILABLE
PART TEMP. RANGE PIN-PACKAGE
MAX1873REEE -40°C to +85°C 16 QSOP
MAX1873SEEE -40°C to +85°C 16 QSOP
MAX1873TEEE -40°C to +85°C 16 QSOP
Selector Guide
PART SERIES CELLS TO CHARGE
MAX1873REEE 2-Cell Li+ or 5- or 6-cell Ni Battery
MAX1873SEEE 3-Cell Li+ or 7- or 9-cell Ni Battery
MAX1873TEEE 4-Cell Li+ 10-cell Ni Battery Packs
MAX1873
GND
VADJ
REF
ICHG/EN
IOUT
DCIN
VH VL CSSP
CSSN
SYSTEM
LOAD
VIN 9V TO
28V
(9V MIN
FOR 2-
CELLS)
4V OUT PER
200mV ON RCS
EXT
BATT
CSB
CCI
CCS
CCV
2- TO 4-CELL
Li+
Typical Operating Circuit
Pin Configuration appears at end of data sheet.
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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= 0°C to +85°C. Typical values are at TA= +25°C, unless
otherwise noted.)
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.
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= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C..........+667mW
Operating Temperature Range
MAX1873_EEE................................................-40°C to +85°C
Junction Temperature..................................................... +150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................ +300°C
PARAMETER CONDITIONS MIN TYP MAX UNITS
INPUT SUPPLY AND REFERENCE
DCIN Input Voltage Range 628V
6.0V < VDCIN < 28V 4 7 mA
D C IN Qui escent S up p l y C ur r ent DCIN ≤ BATT 0.1 10 µA
DCIN to BATT Undervoltage Threshold CSSP = DCIN, input falling 0.05 0.175 V
DCIN to BATT Undervoltage Threshold CSSP = DCIN, input rising 0.22 0.38 V
VL Output Voltage 6.0V < VDCIN < 28V 5.15 5.40 5.65 V
VL Output Load Regulation IVL = 0 to 3mA 15 50 mV
REF Output Voltage IREF = 21µA (200kΩ load) 4.179 4.20 4.221 V
26mV
REF Line Regulation 6.0V < VDCIN < 28V 22 65 ppm/V
REF Load Regulation IREF = 0 to 1mA 6 13 mV
SWITCHING REGULATOR
PWM Oscillator Frequency 270 300 330 kHz
EXT Driver Source On-Resistance 47Ω
EXT Driver Sink On-Resistance 2.5 4.5 Ω
VH Output Voltage DCIN - VH, 6V < VDCIN <28V, IVH = 0 to 20mA 4.75 5.75 V
CSSN/CSSP Input Current VCSSN/VCSSP = 28V, VDCIN = 28V 70 200 µA
CSSN/CSSP Off-State Leakage VDCIN = VSSN/VCSSP = 18V, VBATT = VCSB = 18V 1.5 5 µA
ICHG/EN = 0 (charger disabled) 0.2 1
BATT, CSB Input Current ICHG/EN = REF (charger enabled) 250 500 µA
BATT, CSB Input Current DCIN ≤ BATT (input power removed) 1.5 5 µA
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
_______________________________________________________________________________________
3
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= 0°C to +85°C. Typical values are at TA= +25°C, unless
otherwise noted.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
2-cell version MAX1873R 10.45 11 11.55
3-cell version MAX1873S 15.675 16.5 17.325BATT Overvoltage Cutoff Threshold
4-cell version MAX1873T (Note 1) 17.575 18.5 19.425
V
VVADJ = 0 7.898 7.958 8.018
VVADJ = VREF/2 8.337 8.4 8.463
MAX1873R
(2 Li+ cells)
VVADJ = VREF (Note 1) 8.775 8.842 8.909
VVADJ = 0 11.847 11.937 12.027
VVADJ = VREF/2 12.505 12.6 12.695
MAX1873S
(3 Li+ cells)
VVADJ = VREF (Note 1) 13.163 13.263 13.363
VVADJ = 0 15.796 15.916 16.036
VVADJ = VREF/2 16.674 16.8 16.926
Battery Regulation Voltage
MAX1873T
(4 Li+ cells)
VVADJ = VREF (Note 1) 17.551 17.684 17.817
V
MAX1873R 4.8 5.0 5.2
MAX1873S 7.2 7.5 7.8
BATT Undervoltage Threshold For ICHG/20 trickle
charge
MAX1873T 9.6 10 10.4
V
CURRENT SENSE
VICHG/EN = VREF 190 200 210
CSB to BATT Battery Current-Sense
Voltage VICHG/EN = VREF/4 40 50 60 mV
CSB to BATT Current-Sense Voltage
when VBATT < 2.5V per Cell 51015mV
CSSP to CSSN Current-Sense Voltage 6V < VCSSP < 28V 90 100 110 mV
CONTROL INPUTS/OUTPUTS
ICHG/EN Input Threshold Includes 50mV of hysteresis 500 600 700 mV
ICHG/EN Input Voltage Range For
Charge Current Adjustment 700 VREF mV
VADJ Input Current VVADJ = VREF/2 -100 100 nA
ICHG/EN Input Current VICHG/EN = VREF -100 100 nA
VADJ Input Voltage Range 0V
REF V
Full scale VCSB - VBATT = 200mV,
0 < IOUT < 500µA 3.6 4.0 4.4
25% scale VCSB - VBATT = 50mV,
0 < IOUT < 500µA 0.9 1.0 1.1
V
Trickle charge VCSB - VBATT = 10mV 75 200 325
IOUT Voltage
No charge
current
VCSB - VBATT = 0,
IIOUT = sinking 20µA 40 70 90 mV
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
4 _______________________________________________________________________________________
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= -40°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
PARAMETER CONDITIONS MIN MAX UNITS
INPUT SUPPLY AND REFERENCE
DCIN Input Voltage Range 628V
6.0V < VDCIN < 28V 7 mA
D C IN Qui escent S up p l y C ur r ent DCIN ≤ BATT 10 µA
DCIN to BATT Undervoltage Threshold CSSP = DCIN, input falling 0.05 0.2 V
DCIN to BATT Undervoltage Threshold CSSP = DCIN, input rising 0.22 0.38 V
VL Output Voltage 6.0V < VDCIN < 28V 5.15 5.65 V
VL Output Load Regulation IVL = 0 to 3mA 50 mV
REF Output Voltage IREF = 21µA (200kΩ load) 4.179 4.221 V
6mV
REF Line Regulation 6.0V < VDCIN < 28V 65 ppm/V
REF Load Regulation IREF = 0 to 1mA 13 mV
SWITCHING REGULATOR
PWM Oscillator Frequency 270 330 kHz
EXT Driver Source On-Resistance 7Ω
EXT Driver Sink On-Resistance 4.5 Ω
VH Output Voltage DCIN - VH, 6V < VDCIN <28V, IVH = 0 to 20mA 4.75 5.75 V
CSSN/CSSP Input Current VCSSN/VCSSP = 28V, VDCIN = 28V 200 µA
CSSN/CSSP Off-State Leakage VDCIN = VSSN/VCSSP = 18V VBATT = VCSB = 18V 5 µA
ICHG/EN = 0 (charger disabled) 1
BATT, CSB Input Current ICHG/EN = REF (charger enabled) 500 µA
BATT, CSB Input Current DCIN ≤ BATT (input power removed) 5 µA
2-cell version MAX1873R 10.45 11.55
3-cell version MAX1873S 15.675 17.325
BATT Overvoltage Cutoff Threshold
4-cell version MAX1873T (Note 1) 17.575 19.425
V
VVADJ = 0 7.898 8.018
VVADJ = VREF/2 8.337 8.463
MAX1873R
(2 Li+ cells)
VVADJ = VREF (Note 1) 8.775 8.909
VVADJ = 0 11.847 12.027
VVADJ = VREF/2 12.505 12.695
MAX1873S
(3 Li+ cells)
VVADJ = VREF (Note 1) 13.163 13.363
VVADJ = 0 15.796 16.036
VVADJ = VREF/2 16.674 16.926
Battery Regulation Voltage
MAX1873T
(4 Li+ cells)
VVADJ = VREF (Note 1) 17.551 17.817
V
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
_______________________________________________________________________________________
5
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= -40°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
PARAMETER CONDITIONS MIN MAX UNITS
MAX1873R 4.8 5.2
MAX1873S 7.2 7.8
BATT Undervoltage Threshold For ICHG/20 trickle
charge
MAX1873T 9.6 10.4
V
CURRENT SENSE
VICHG/EN = VREF 190 210 mV
CSB to BATT Battery Current-Sense
Voltage VICHG/EN = VREF/4 40 60 mV
CSB to BATT Current-Sense Voltage
when VBATT < 2.5V per Cell 515mV
CSSP to CSSN Current-Sense Voltage 6V < VCSSP < 28V 90 110 mV
CONTROL INPUTS/OUTPUTS
ICHG/EN Input Threshold Includes 50mV of hysteresis 500 700 mV
ICHG/EN Input Voltage Range for
Charge Current Adjustment 700 VREF mV
VADJ Input Current VVADJ = VREF/2 -100 100 nA
ICHG/EN Input Current VICHG/EN = VREF -100 100 nA
VADJ Input Voltage Range 0V
REF V
Full scale VCSB - VBATT = 200mV,
0 < IOUT < 500µA 3.6 4.4
25% scale VCSB - VBATT = 50mV,
0 < IOUT < 500µA 0.9 1.1
V
Trickle charge VCSB - VBATT = 10mV 75 325
IOUT Voltage
No charge
current
VCSB - VBATT = 0,
IIOUT = sinking 20µA 40 90 mV
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 -40°C are guaranteed by design, not production tested.
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
6 _______________________________________________________________________________________
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= +25°C, unless otherwise noted).
0
5.0
2.5
10.0
7.5
15.0
12.5
17.5
0 1.0 1.50.5 2.0 2.5 3.0 3.5
MAX1873T (4-CELL)
BATTERY VOLTAGE vs. CHARGING CURRENT
MAX1873 toc01
CHARGING CURRENT (A)
BATTERY VOLTAGE (V)
RCSB + 0.068Ω
0
1.0
0.5
2.5
2.0
1.5
4.0
3.5
3.0
4.5
0 10050 150 200 250
IOUT VOLTAGE
vs. CSB-BATT VOLTAGE
MAX1873 toc02
CSB-BATT VOLTAGE (mV)
IOUT VOLTAGE (V)
4.180
4.190
4.185
4.200
4.195
4.205
4.210
RECENT VOLTAGE VS. TEMPERATURE
MAX1873 toc04
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
-50 0 25-25 50 75 100
MAX1873T
15.5
16.0
17.0
16.5
17.5
18.0
021345
MAX1873T (4-CELL)
BATTERY REGULATION VOLTAGE
vs. VADJ VOLTAGE
MAX1873 toc03
VADJ VOLTAGE (V)
BATTERY REGULATION VOLTAGE (V)
4.180
4.195
4.190
4.185
4.200
4.205
4.210
00.40.30.1 0.2 0.5 0.6 0.7 0.8 0.9 1.0
RECENT VOLTAGE
VS. REFERENCE CURRENT
MAX1873 toc05
REFERENCE VOLTAGE (mA)
REFERENCE VOLTAGE (V)
MAX1873T
50
60
80
70
90
100
81612 20 24 28
MAX1873R (2-CELL)
EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc06
INPUT VOLTAGE (V)
EFFICIENCY (%)
VBATT = 7V
ICHG = 3A
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
_______________________________________________________________________________________
7
50
60
80
70
90
100
MAX1873S (3-CELL)
EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc07
INPUT VOLTAGE (V)
EFFICIENCY (%)
12 2016 24 28
VBATT = 10.5V
ICHG = 3A
50
60
80
70
90
100
16 2018 22 24 26 28
MAX1873T (4-CELL)
EFFICIENCY vs. INPUT VOLTAGE
MAX1873 toc08
INPUT VOLTAGE (V)
EFFICIENCY (%)
VBATT = 14V
ICHG + 3A
0
1.0
0.5
2.0
1.5
2.5
3.0
0 1.0 1.50.5 2.0 2.5 3.0
CHARGING CURRENT
vs. SYSTEM LOAD CURRENT
MAX1873 toc10
SYSTEM LOAD CURRENT (A)
CHARGING CURRENT (A)
4
8
6
12
10
16
14
18
0507525 100 125 150
4-CELL BATTERY VOLTAGE AND
CHARGING CURRENT vs. TIME
MAX1873 toc09
TIME (MINUTES)
BATTERY VOLTAGE (V)
BATTERY VOLTAGE
CHARGING CURRENT
CHARGING CURRENT (A)
0
1.0
0.5
2.0
1.5
3.0
2.5
3.5
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= +25°C, unless otherwise noted).
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 CSSN 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.
2 CSSP Source Current-Sense Positive Input. Also used for input source undervoltage sensing.
3 CCS Input-Source-Current Regulation Loop Compensation Point
4 CCV Battery Regulation Voltage Control-Loop Compensation Point. Pulling CCV high (to VL) through a 1.5kΩ
resistor disables the voltage control loop for charging NiCd or NiMH batteries.
5 CCI Battery Charge Current Control-Loop Compensation Point
6 ICHG/EN
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 5µA.
7 IOUT 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).
8 VADJ
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 100kΩ each).
9 REF 4.2V Reference Voltage Output. Bypass to GND with a 1µF ceramic capacitor.
10 BATT Battery Voltage-Sense Input and Battery Current-Sense Negative Input. Bypass to GND with a 68µF for
MAX1873R, 47µF for MAX1873S, and 33µF for MAX1873T. Use capacitors with ESR < 1Ω.
11 CSB Battery Current-Sense Positive Input
12 GND Ground
13 VH Internal VH Regulator. VH internally supplies power to the EXT driver. Connect a 0.22µF ceramic
capacitor between VH and DCIN.
14 EXT Drive Output for External PFET. EXT swings from VDCIN to VDCIN - 5V.
15 DCIN Power-Supply Input. DCIN is the input supply for charger IC. Bypass to GND with a 0.22µF ceramic
capacitor.
16 VL Internal VL Regulator. VL powers the MAX1873’s control logic at 5.4V. Bypass to GND with a 2.2µF or
larger ceramic capacitor.
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 MOS-
FET switch, inductor, and diode to convert the input volt-
age 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 differ-
ent 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 main-
tained 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 opti-
mal compensation of each. A typical CCV compensa-
tion network is shown in Figure 1 and will suffice for
most designs.
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
_______________________________________________________________________________________ 9
MAX1873
GND
VADJ
REF
ICHG/EN
IOUT
DCIN
VH VL CSSP
CSSN
VIN 17V TO 28V
(9V MIN FOR 2- CELLS)
4V OUT PER
200mV ON RCSB
EXT
BATT
CSB
CCI
CCS
CCV
D1
MBRS340
CVL
2.2μF
RP
4.7Ω
CP
0.01μF
RN
4.7Ω
CN
0.01μF
P
RCSS
0.033Ω
D2
MBRS340
SYSTEM
LOAD
CL
47μF
CBATT
68μF
LI+
BATTERY
(2- TO 4-CELLS)
RCSB
0.068Ω
L1
10μH
R1
R2
CREF
1μF
CCCVP
1nF
RCCV
10kΩ
CCCVS
0.1μF
CCCS
47nF
CCC
47nF
N
DISABLE
CDCIN
0.22μF
CVH
0.22μF
100kΩ
Figure 1. Typical Application Circuit
MAX1873
Charging-Current Regulator
The charging-current regulator limits the battery charg-
ing 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 pro-
vides suitable performance for most applications.
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
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 cur-
rent. 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 input-
current 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 suit-
able performance for most applications.
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 con-
trolling signal to prevent delay when switching between
the battery-voltage control, charging-current control,
and input-current regulation loops.
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 1.5µA from the battery.
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.
Simple Current-Limited Switch-Mode
Li+ Charger Controller
10 ______________________________________________________________________________________
VL
CSSN
CSSP
CCS
CCV
CCI
IOUT
VADJ
ICHG
/EN
DCIN
EXT
VH
GND
CSB
BATT
REF
5.4
REGULATOR
UNDERVOLTAGE
COMPARATOR
BATT
SHUTDOWN
FOR ALL
BLOCKS
VH
DRIVER
CONTROL
LOGIC
4.2V
REFERENCE
GND
VOLTAGE
ERROR AMP
CURRENT
ERROR AMP
R
R
9R
A = 1
Figure 2. Functional Block Diagram
Charge-Current Monitor Output
IOUT is an analog voltage output that is proportional to
the actual charge current. With the aid of a microcon-
troller, the IOUT signal can facilitate gas-gauging, indi-
cate percent of charge, or charge-time remaining. The
equation governing this output is:
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.
Design Procedure
Setting the Battery-Regulation Voltage
For Li+ batteries, VADJ sets the per-cell battery-regula-
tion voltage limit. To set the VADJ voltage, use a resis-
tive-divider from REF to GND (Figure 1). For a battery
voltage of 4.2V per cell, use resistors of equal value
(100kΩeach) 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 manu-
facturer to determine this voltage before charging any
Li+ battery. Once the per-cell voltage is determined,
the VADJ voltage is calculated by the equation:
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 200kΩ.
R2 can then be calculated as follows:
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 Charging-Current Limit
The charging current ICHG is sensed by the current-
sense 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:
In some cases, common values for RCSB may not allow
the desired charge-current value. It may also be desir-
able 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-cur-
rent-sense threshold. In those cases the equation for
charge current becomes:
Setting the Input-Current Limit
The input-source current limit, IIN, is set by the input-
current sense resistor, RCSS, (Figure 1) connected
between CSSP and CSSN. The equation for the source
current is:
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 cur-
rent (LIR) can be used to calculate the inductor value:
where fSW is the switching frequency (nominally
300kHz) and ICHG is the charging current. The peak
inductor current is given by:
LV V V
V f I LIR
BATT DCIN MAX BATT
DCIN MAX SW CHG
=−
[]
{}
×× ×
[]
()
()
/
IVR
IN CSS
=01./
IVVVR
CHG ICH EN REF CSB
=
()
02.//
/
IVR
CHG CSB
=02./
RV V V R
VADJ REF VADJ
21=−
()
[]
×/
VVNV
VADJ BATTR REF
=
()
[]
−
()
95 9./
VVVor
VRI
IOUT CSB BATT
OUT CSB CHG
=−
()
=×
()
20
20
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
______________________________________________________________________________________ 11
MAX1873
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 11.2µH with a peak current of 3.75A. Therefore a
10µH inductor would be satisfactory.
MOSFET Selection
The MAX1873 uses a P-channel power MOSFET
switch. The MOSFET must be selected to meet the effi-
ciency or power dissipation requirements of the charg-
ing 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 propor-
tional.
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:
And the catch-diode duty cycle (D') will be 1 - D or:
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 source-
voltage and maximum battery voltage. PRcan be
approximated by the equation:
Transition losses (PT) can be approximated by the
equation:
where tTR is the MOSFET transition time and fSW is the
switching frequency. The total power dissipation of the
MOSFET is then:
Diode Selection
A Schottky rectifier with a current rating of at least the
charge current limit must be connected from the MOS-
FET drain to GND. The voltage rating of the diode must
exceed the maximum expected input voltage.
Capacitor Selection
The input capacitor shunts the switching current from
the charger input and prevents that current from circu-
lating 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 con-
verter will typically operate in continuous conduction. In
this case, the RMS current of the input capacitor can
be approximated with the equation:
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 pre-
venting AC currents from flowing back into the wall
cube. This requirement varies depending on the wall
cube’s impedance and the requirements of any con-
ducted or radiated EMI specifications that must be met.
Low ESR aluminum electrolytic capacitors may be
used, however, tantalum or high-value ceramic capaci-
tors generally provide better performance.
The output filter capacitor absorbs the inductor-ripple
current. The output-capacitor impedance must be sig-
nificantly 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 sta-
bility is:
C
VV
V
VfR
OUT
REF BATT
DCIN MIN
BATT SW CSB
>
+
⎛
⎝
⎜⎞
⎠
⎟
××
1
()
II DD
CIN CHG
≈−
2
PPP
TOT R T
=+
PVIft
TDCIN CHG SW TR
=×××
3
PV
VRI
RBATT MAX
DCIN MIN DS ON CHG
=××
()
() () 2
DVV
V
DCIN BATT
DCIN
'≈−
DV
V
BATT
DCIN
≈
I I LIR
PEAK CHG
=+
()
12/
Simple Current-Limited Switch-Mode
Li+ Charger Controller
12 ______________________________________________________________________________________
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 (68mΩfor 3A charging current)
from CSB to BATT.
The maximum output capacitor ESR allowed for stability is:
where RESR is the output capacitor ESR.
Compensation Components
The three regulation loops: input current limit, charging
current limit, and charging voltage limit are compensat-
ed 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 error-
amplifier 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 connect-
ing a capacitor in parallel with a series resistor-capaci-
tor from CCV to GND. Recommended values are shown
in Figure 1.
Applications Information
VL, VH, and REF Bypassing
The MAX1873 uses two internal linear regulators to
power internal circuitry. The outputs of the linear regu-
lators 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 2.2µF bypass capacitor is required from VL to GND
to ensure stability. A 0.22µF capacitor is required from
VH to DCIN. A 1µF 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.
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 kΩresistor. This disables the voltage
control loop so the Li+ battery-regulation voltage set-
tings do not interfere with charging. However, the bat-
tery 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, 7-
to 9-cells with the S version, and 10-cells with the T ver-
sion.
The MAX1873 contains no charge-termination algo-
rithms 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.
Chip Information
PROCESS: BiCMOS
TRANSISTOR COUNT: 1397
RRV
V
ESR CSB BATT
REF
<×
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
______________________________________________________________________________________ 13
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
CSSN VL
DCIN
EXT
VH
GND
CSB
BATT
REF
TOP VIEW
MAX1873R/S/T
16 QSOP
CSSP
CCS
ICHG/EN
CCV
CCI
IOUT
VADJ
Pin Configuration
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
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
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Package Information
