MAX1873REEE+ - Maxim Integrated Products, Inc.

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

♦Low-Cost and Simple Circuit

♦Charges 2-, 3-, or 4-Series Lithium-Ion Cells

♦AC Adapter Input-Current-Limit Loop

♦Also Charges Ni-Based Batteries

♦Analog Output Monitors Charge Current

♦±0.75% Battery-Regulation Voltage

♦5µA Shutdown Battery Current

♦Input Voltage Up to 28V

♦200mV Dropout Voltage/100% Duty Cycle

♦Adjustable Charging Current

♦300kHz PWM Oscillator Reduces Noise

♦Space-Saving 16-Pin QSOP

♦MAX1873 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

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

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

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

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

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

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

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

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).

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

1.0

0.5

2.5

2.0

1.5

4.0

3.5

3.0

4.5

010050 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

0 0.40.30.1 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

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

100

MAX1873S (3-CELL)

EFFICIENCY vs. INPUT VOLTAGE

MAX1873 toc07

INPUT VOLTAGE (V)

EFFICIENCY (%)

12 2016 24 28

BATT

= 10.5V

CHG

= 3A

100

16 2018 22 24 26 28

MAX1873T (4-CELL)

EFFICIENCY vs. INPUT VOLTAGE

MAX1873 toc08

INPUT VOLTAGE (V)

EFFICIENCY (%)

BATT

= 14V

CHG

+ 3A

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)

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)

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

MBR5340

CVL

2.2µF

4.7Ω

0.01µF

4.7Ω

0.01µF

RCSS

0.033Ω

MBR5340

SYSTEM

LOAD

47µF

CBATT

68µF

LI+

BATTERY

(2- TO 4-CELLS)

RCSB

0.068Ω

10µH

CREF

1µF

CCCVP

1nF

RCCV

10kΩ

CCCVS

0.1µF

CCCS

47nF

CCC

47nF

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 ______________________________________________________________________________________

CSSN

CSSP

CCS

CCV

CCI

IOUT

VADJ

ICHG

/EN

DCIN

EXT

GND

CSB

BATT

REF

5.4

REGULATOR

UNDERVOLTAGE

COMPARATOR

BATT

SHUTDOWN

FOR ALL

BLOCKS

DRIVER

CONTROL

LOGIC

4.2V

REFERENCE

GND

VOLTAGE

ERROR AMP

CURRENT

ERROR AMP

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

VfILIR

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

=−

()

=×

()

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:

VfR

OUT

REF BATT

DCIN MIN

BATT SW CSB











××

()

II DD

CIN CHG

≈−

PPP

TOT R T

PVIft

TDCIN CHG SW TR

=×××

VRI

RBATT MAX

DCIN MIN DS ON CHG

=××

()

() () 2

DVV

DCIN BATT

DCIN

'≈−

BATT

DCIN

≈

II 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

ESR CSB BATT

REF

<×

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

______________________________________________________________________________________ 13

CSSN VL

DCIN

EXT

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 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

Package Information

QSOP.EPS