Rainbow Electronics MAX1873 User Manual

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.

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.

EVALU

ATIO

IT A

ILA

BLE

Selector Guide

Typical Operating Circuit

Pin Configuration appears at end of data sheet.

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

VIN 9V TO

28V

(9V MIN

FOR 2-

CELLS)

VH VL CSSP

SYSTEM LOAD

MAX1873

PART SERIES CELLS TO CHARGE

MAX1873REEE

MAX1873SEEE

MAX1873TEEE 4-Cell Li+ 10-cell Ni Battery Packs

2-Cell Li+ or 5- or 6-cell Ni Battery

3-Cell Li+ or 7- or 9-cell Ni Battery

4V OUT PER

200mV ON R

DCIN

IOUT

ICHG/EN

REF

VADJ

GND

CSSN

EXT

CSB

BATT

CCI

CCS

CCV

2- TO 4-CELL

Li+

MAX1873

Simple Current-Limited Switch-Mode Li+ Charger Controller

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 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 ......................................(V

DCIN

+ 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 (T

= +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

INPUT SUPPLY AND REFERENCE

DCIN Input Voltage Range 628V

D C IN Qui escent S up p l y C ur r ent

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 < V

VL Output Load Regulation IVL = 0 to 3mA 15 50 mV

REF Output Voltage I

REF Line Regulation 6.0V < V

REF Load Regulation I

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 < V

CSSN/CSSP Input Current V

CSSN/CSSP Off-State Leakage V

BATT, CSB Input Current

BATT, CSB Input Current DCIN ≤ BATT (input power removed) 1.5 5 µA

PARAMETER CONDITIONS MIN TYP MAX UNITS

6.0V < V DCIN ≤ BATT 0.1 10 µA

= 21µA (200kΩ load) 4.179 4.20 4.221 V

REF

= 0 to 1mA 6 13 mV

REF

CSSN/VCSSP

DCIN

ICHG/EN = 0 (charger disabled) 0.2 1

ICHG/EN = REF (charger enabled) 250 500

< 28V 4 7 mA

DCIN

< 28V 5.15 5.40 5.65 V

DCIN

< 28V

DCIN

= 28V, V

= V

SSN/VCSSP

<28V, IVH = 0 to 20mA 4.75 5.75 V

DCIN

= 28V 70 200 µA

DCIN

= 18V, V

BATT

= V

= 18V 1.5 5 µA

CSB

26mV

22 65 ppm/V

µA

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 16.8V; TA= 0°C to +85°C. Typical values are at TA= +25°C, unless

otherwise noted.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Battery Regulation Voltage

BATT Undervoltage Threshold

CURRENT SENSE

CSB to BATT Battery Current-Sense Voltage

CSB to BATT Current-Sense Voltage when V

< 2.5V per Cell

BATT

CSSP to CSSN Current-Sense Voltage 6V < V

CONTROL INPUTS/OUTPUTS

ICHG/EN Input Threshold Includes 50mV of hysteresis 500 600 700 mV

ICHG/EN Input Voltage Range For Charge Current Adjustment

VADJ Input Current V

ICHG/EN Input Current V

VADJ Input Voltage Range 0V

IOUT Voltage

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

= 0 7.898 7.958 8.018

MAX1873R (2 Li+ cells)

MAX1873S (3 Li+ cells)

MAX1873T (4 Li+ cells)

VADJ

= V

/2 8.337 8.4 8.463

REF

= V

(Note 1) 8.775 8.842 8.909

REF

= 0 11.847 11.937 12.027

= V

/2 12.505 12.6 12.695

REF

= V

(Note 1) 13.163 13.263 13.363

REF

= 0 15.796 15.916 16.036

= V

/2 16.674 16.8 16.926

REF

= V

(Note 1) 17.551 17.684 17.817

REF

VADJ

MAX1873R 4.8 5.0 5.2

For I charge

CHG

ICHG/EN

/20 trickle

MAX1873S 7.2 7.5 7.8

MAX1873T 9.6 10 10.4

= V

REF

= V

/4 40 50 60

REF

190 200 210

51015mV

< 28V 90 100 110 mV

CSSP

700 V

= V

VADJ

ICHG/EN

Full scale

25% scale

Trickle charge V

No charge current

/2 -100 100 nA

REF

= V

REF

- V

CSB

0 < I

OUT

- V

CSB

0 < I

OUT

- V

CSB

- V

CSB

= sinking 20µA

IOUT

= 200mV,

BATT

< 500µA

= 50mV,

BATT

< 500µA

= 10mV 75 200 325

BATT

= 0,

BATT

-100 100 nA

3.6 4.0 4.4

0.9 1.0 1.1

40 70 90

REF

MAX1873

Simple Current-Limited Switch-Mode Li+ Charger Controller

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 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

D C IN Qui escent S up p l y C ur r ent

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 < V

VL Output Load Regulation IVL = 0 to 3mA 50 mV

REF Output Voltage I

REF Line Regulation 6.0V < V

REF Load Regulation I

SWITCHING REGULATOR

PWM Oscillator Frequency 270 330 kHz

EXT Driver Source On-Resistance 7 Ω

EXT Driver Sink On-Resistance 4.5 Ω

VH Output Voltage DCIN - VH, 6V < V

CSSN/CSSP Input Current V

CSSN/CSSP Off-State Leakage V

BATT, CSB Input Current

BATT, CSB Input Current DCIN ≤ BATT (input power removed) 5 µA

BATT Overvoltage Cutoff Threshold

Battery Regulation Voltage

6.0V < V

< 28V 7 mA

DCIN

DCIN ≤ BATT 10 µA

< 28V 5.15 5.65 V

DCIN

= 21µA (200kΩ load) 4.179 4.221 V

REF

< 28V

DCIN

= 0 to 1mA 13 mV

REF

<28V, IVH = 0 to 20mA 4.75 5.75 V

DCIN

CSSN/VCSSP

= V

DCIN

SSN/VCSSP

= 28V, V

= 28V 200 µA

DCIN

= 18V V

BATT

= V

= 18V 5 µA

CSB

6mV

65 ppm/V

ICHG/EN = 0 (charger disabled) 1

ICHG/EN = REF (charger enabled) 500

2-cell version MAX1873R 10.45 11.55

3-cell version MAX1873S 15.675 17.325

4-cell version MAX1873T (Note 1) 17.575 19.425

= 0 7.898 8.018

MAX1873R (2 Li+ cells)

MAX1873S (3 Li+ cells)

MAX1873T (4 Li+ cells)

VADJ

= V

/2 8.337 8.463

REF

= V

(Note 1) 8.775 8.909

REF

= 0 11.847 12.027

= V

/2 12.505 12.695

REF

= V

(Note 1) 13.163 13.363

REF

= 0 15.796 16.036

= V

/2 16.674 16.926

REF

= V

(Note 1) 17.551 17.817

REF

VADJ

µA

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

_______________________________________________________________________________________ 5

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 16.8V; TA= -40°C to +85°C. Typical values are at TA= +25°C,

unless otherwise noted.)

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.

PARAMETER CONDITIONS MIN MAX UNITS

BATT Undervoltage Threshold

CURRENT SENSE

CSB to BATT Battery Current-Sense Voltage

CSB to BATT Current-Sense Voltage when V

< 2.5V per Cell

BATT

CSSP to CSSN Current-Sense Voltage 6V < V

CONTROL INPUTS/OUTPUTS

ICHG/EN Input Threshold Includes 50mV of hysteresis 500 700 mV

ICHG/EN Input Voltage Range for Charge Current Adjustment

VADJ Input Current V

ICHG/EN Input Current V

VADJ Input Voltage Range 0V

IOUT Voltage

For I

CHG

/20 trickle

charge

ICHG/EN

CSSP

= V

VADJ

ICHG/EN

= V

< 28V 90 110 mV

REF

= V

Full scale

25% scale

Trickle charge V

No charge current

MAX1873R 4.8 5.2

MAX1873S 7.2 7.8

MAX1873T 9.6 10.4

REF

/4 40 60 mV

REF

190 210 mV

515mV

700 V

REF

/2 -100 100 nA

REF

- V

CSB

0 < I

OUT

- V

CSB

0 < I

OUT

- V

CSB

- V

CSB

= sinking 20µA

IOUT

= 200mV,

BATT

< 500µA

= 50mV,

BATT

< 500µA

= 10mV 75 325

BATT

= 0,

BATT

-100 100 nA

REF

3.6 4.4

0.9 1.1

40 90

MAX1873

Simple Current-Limited Switch-Mode Li+ Charger Controller

6 _______________________________________________________________________________________

Typical Operating Characteristics

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 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)

CSB

+ 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 (%)

BATT

= 7V

CHG

= 3A

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

_______________________________________________________________________________________ 7

Typical Operating Characteristics (continued)

(Circuit of Figure 1, V

DCIN

= V

CSSP

= V

CSSN

= 18V, V

ICHG/EN

= V

REF

, V

VADJ

= V

REF

/2. MAX1873R: V

BATT

= V

CSB

= 8.4V;

MAX1873S: V

BATT

= V

CSB

= 12.6V; MAX1873T: V

BATT

= V

CSB

= 16.8V; TA= +25°C, unless otherwise noted).

100

EFFICIENCY (%)

BATTERY VOLTAGE (V)

EFFICIENCY vs. INPUT VOLTAGE

= 10.5V

BATT

= 3A

CHG

12 2016 24 28

4-CELL BATTERY VOLTAGE AND

CHARGING CURRENT vs. TIME

MAX1873S (3-CELL)

INPUT VOLTAGE (V)

BATTERY VOLTAGE

CHARGING CURRENT

MAX1873 toc07

3.5

3.0

MAX1873 toc09

2.5

2.0

1.5

1.0

0.5

MAX1873T (4-CELL)

EFFICIENCY vs. INPUT VOLTAGE

100

EFFICIENCY (%)

= 14V

BATT

+ 3A

CHG

16 2018 22 24 26 28

INPUT VOLTAGE (V)

CHARGING CURRENT

vs. SYSTEM LOAD CURRENT

3.0

2.5

2.0

1.5

1.0

CHARGING CURRENT (A)

0.5

MAX1873 toc08

MAX1873 toc10

0507525 100 125 150

TIME (MINUTES)

0 1.0 1.50.5 2.0 2.5 3.0

SYSTEM LOAD CURRENT (A)

MAX1873

Simple Current-Limited Switch-Mode Li+ Charger Controller

8 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

1 CSSN

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

5 CCI Battery Charge Current Control-Loop Compensation Point

6 ICHG/EN

7 IOUT

8 VADJ

9 REF 4.2V Reference Voltage Output. Bypass to GND with a 1µF ceramic capacitor.

10 BATT

11 CSB Battery Current-Sense Positive Input

12 GND Ground

13 VH

14 EXT Drive Output for External PFET. EXT swings from V

15 DCIN

16 VL

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.

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.

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.

Charge Current Monitor Output. Analog Voltage Output that is proportional to charging current. V = 20 (V

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).

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.

Internal VH Regulator. VH internally supplies power to the EXT driver. Connect a 0.22µF ceramic capacitor between VH and DCIN.

Power-Supply Input. DCIN is the input supply for charger IC. Bypass to GND with a 0.22µF ceramic capacitor.

Internal VL Regulator. VL powers the MAX1873’s control logic at 5.4V. Bypass to GND with a 2.2µF or larger ceramic capacitor.

CSB

- V

) or 4V for a 200mV current-sense voltage (maximum load capacitance = 5nF).

BATT

DCIN

to V

DCIN

- 5V.

IOUT

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 R

CSB

, 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.

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

_______________________________________________________________________________________ 9

DISABLE

Figure 1. Typical Application Circuit

VIN 17V TO 28V

(9V MIN FOR 2- CELLS)

100kΩ

MBR5340

2.2µF

0.01µF

REF

1µF

4.7Ω

CSS

0.033Ω

MBR5340

10µH

CSB

0.068Ω

47µF

BATT

68µF

BATTERY

(2- TO 4-CELLS)

SYSTEM LOAD

LI+

DCIN

0.22µF

4V OUT PER

200mV ON R

CCVS

0.1µF

47nF

CCS

47nF

10kΩ

CSB

CCV

0.22µF

DCIN

MAX1873

IOUT

ICHG/EN

CCI

CCS

CCV

CSSP

CSSN

EXT

CSB

BATT

REF

VADJ

GND

CCVP

1nF

MAX1873

Charging-Current Regulator

The charging-current regulator limits the battery charging current. Current is sensed by the current-sense resistor (R

CSB

in Figure 1) connected between BATT and CSB. The voltage on ICHG/EN can also adjust the charging current. Full-scale charging current (I

CHG

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.

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 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, R

CSS

, 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.

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.

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 ______________________________________________________________________________________

Figure 2. Functional Block Diagram

CSSN

CSSP

CCS

CCV

CCI

ICHG

/EN

IOUT

VADJ

A = 1

GND

SHUTDOWN

FOR ALL BLOCKS

CONTROL

LOGIC

CURRENT

ERROR AMP

5.4

REGULATOR

UNDERVOLTAGE

COMPARATOR

BATT

DRIVER

VOLTAGE

ERROR AMP

4.2V

REFERENCE

DCIN

EXT

GND

CSB

BATT

REF

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:

where V

CSB

and V

BATT

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-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 (100kΩ 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:

where V

BATTR

is the desired battery-regulation voltage (for the total series-cell stack), N is the number of Li+ battery cells, and V

REF

is the reference voltage (4.2V).

Set V

VADJ

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 currentsense resistor R

CSB

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 R

CSB

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:

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:

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:

where f

is the switching frequency (nominally

300kHz) and I

CHG

is the charging current. The peak

inductor current is given by:

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

______________________________________________________________________________________ 11

VVVor

=−

IOUT CSB BATT

VRI

OUT CSB CHG

()

=×

()

VVNV

95 9./

()

VADJ BATTR REF

RV V V R

[]

21=−

[]

()

VADJ REF VADJ

−

()

×/

IVVVR

CHG ICH EN REF CSB

IVR

= 02./

CHG CSB

02.//

()

IVR

= 01./

IN CSS

LV V V

=−

BATT DCIN MAX BATT

[]

{}

VfILIR

DCIN MAX SW CHG

()

[]

()

×× ×

MAX1873

For example, for a 4-cell charging current of 3A, a V

DCIN(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 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:

And the catch-diode duty cycle (D') will be 1 - D or:

where V

BATT

is the battery-regulation voltage (typically

4.2V per cell) and V

DCIN

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. PRcan be approximated by the equation:

Transition losses (PT) can be approximated by the equation:

where tTRis the MOSFET transition time and fSWis 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 MOSFET 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 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:

where I

CIN

is the input capacitor RMS current, D is the

PWM converter duty cycle (typically V

BATT/VDCIN

), and

CHG

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 I

CHG

. 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:

Simple Current-Limited Switch-Mode Li+ Charger Controller

12 ______________________________________________________________________________________

PPP

II LIR

PEAK CHG

12/

()

TOT R T

BATT

≈

DCIN

−

DCIN BATT

'≈

DCIN

BATT MAX

=××

()

DCIN MIN

()

DS ON CHG

()

VIft

×××

DCIN CHG SW TR

II DD

≈−

CIN CHG



REF

 

OUT

VfR

BATT SW CSB

DCIN MIN

××

BATT

()

 



where C

OUT

is the total output capacitance, V

REF

is the

reference voltage (4.2V), V

BATT

is the maximum battery

regulation voltage (typically 4.2V per cell), V

DCIN (MIN)

is the minimum source-input voltage, and R

CSB

is the

current-sense resistor (68mΩ for 3A charging current) from CSB to BATT.

The maximum output capacitor ESR allowed for stability is:

where R

ESR

is the output capacitor ESR.

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.

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 2.2µF 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 kΩ resistor. This disables the voltage control loop so the Li+ battery-regulation voltage set-

tings do not interfere with charging. However, the battery undervoltage-protection features remain active so charging current is reduced when V

BATT

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.

Chip Information

PROCESS: BiCMOS

TRANSISTOR COUNT: 1397

MAX1873

Simple Current-Limited Switch-Mode

Li+ Charger Controller

______________________________________________________________________________________ 13

Pin Configuration

CSB BATT

REF

ESR

TOP VIEW

CSSN VL

CSSP

CCS

MAX1873R/S/T

CCV

CCI

ICHG/EN

IOUT

VADJ

16 QSOP

DCIN

EXT

GND

CSB

BATT

REF

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

Rainbow Electronics MAX1873 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual