19-0756; Rev 0; 1/07
KIT
ATION
EVALU
E
L
B
A
IL
AVA
SMBus Level 2 Battery Charger
with Remote Sense
The MAX8731A is an SMBus™ programmable multichemistry battery charger. The MAX8731A uses a minimal command set to easily program the charge voltage,
charge current, and adapter current limit.
The MAX8731A charges one to four Li+ series cells and
delivers up to 8A charge current. The MAX8731A drives
n-channel MOSFETs for improved efficiency and
reduced cost. Low-offset current-sense amplifiers provide high accuracy with 10mΩ sense resistors.
The MAX8731A current-sense amplifiers provide high
accuracy (3% at 3.5A) and also provide fast cycle-bycycle current-mode control to protect against battery
short circuit and system load transients.
The charger employs dual remote sense, which reduces
charge time by measuring the feedback voltage directly
at the battery, improving accuracy of initial transition into
constant-voltage mode. The MAX8731A provides 0.5%
battery voltage accuracy directly at the battery terminal.
The MAX8731A provides a digital output that indicates
the presence of the AC adapter, as well as an analog output that indicates the adapter current within 4% accuracy.
The MAX8731A is available in a small 5mm x 5mm,
28-pin, thin (0.8mm) QFN package. An evaluation kit is
available to reduce design time. The MAX8731A is available in a lead-free package.
Features
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
0.5% Battery Voltage Accuracy
3% Input Current-Limit Accuracy
3% Charge-Current Accuracy
SMBus 2-Wire Serial Interface
Cycle-by-Cycle Current Limit
Battery Short-Circuit Protection
Fast Response for Pulse Charging
Fast System-Load-Transient Response
Dual-Remote-Sense Inputs
Monitor Outputs for
Adapter Current (4% Accuracy)
AC Adapter Detection
11-Bit Battery Voltage Setting
6-Bit Charge-Current/Input-Current Setting
8A (max) Battery Charger Current
11A (max) Input Current
+8V to +26V Input Voltage Range
Charges Li+, NiMH, and NiCd Battery Chemistries
Ordering Information
PART
TEMP RANGE
PINPACKAGE
PKG
CODE
MAX8731AETI+
-40°C to +85°C
28 Thin QFN
(5mm x 5mm)
T2855-6
SMBus is a trademark of Intel Corp.
Applications
Notebook Computers
Tablet PCs
Medical Devices
Portable Equipment with Rechargeable Batteries
+Indicates a lead-free package.
Typical Operating Circuit
EXTERNAL LOAD
OPTIONAL
DLO
CSIP
CSIN
FBSB
FBSA
21
PGND
TOP VIEW
LDO
Pin Configuration
20
19
18
17
16
15
CSSP
ACIN
DCIN
ACOK
14
BATSEL
LX 23
13
ACOK
DHI 24
12
GND
BST 25
11
VDD
GND
VCC 26
10
SCL
CSSN 27
9
SDA
8
IINP
*EXPOSED PADDLE
4
5
6
7
CCI
CCV
DAC
GND
3
CCS
2
REF
1
ACIN
CSSP 28
N
LX
MAX8731A DLO
REF
DCIN 22
MAX8731A
CSSN
DHI
N
PGND
BST
CSIP
CSIN
SELECTOR
HOST
SCL
SDA
VDD
BATSEL
FBSA
BATTERY
A
FBSB
BATTERY
B
SCL
SDA
VDD
GND
IINP
DAC CCS
BATSEL
LDO
VCC
CCV
CCI
THIN QFN
5mm x 5mm
________________________________________________________________ Maxim Integrated Products
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.
1
MAX8731A
General Description
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
ABSOLUTE MAXIMUM RATINGS
DCIN, CSSN, CSIN, FBSA, FBSB to GND..............-0.3V to +28V
CSSP to CSSN, CSIP to CSIN, PGND to GND ......-0.3V to +0.3V
BST to GND ............................................................-0.3V to +32V
BST to LX..................................................................-0.3V to +6V
DHI to LX.................................................-0.3V to +(VBST + 0.3)V
DLO to PGND..........................................-0.3V to +(LDO + 0.3)V
LX to GND .................................................................-6V to +28V
CCI, CCS, CCV, DAC, REF,
IINP to GND...........................................-0.3V to (VVCC + 0.3)V
VDD, SCL, SDA, BATSEL, ACIN, ACOK, VCC to GND,
LDO to PGND ......................................................-0.3V to +6V
Continuous Power Dissipation (TA = +70°C)
28-Pin Thin QFN
(derate 20.8mW/°C above +70°C) ........................1666.7 mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
16.716
16.8
16.884
V
+0.5
%
CHARGE-VOLTAGE REGULATION
ChargingVoltage() = 0x41A0
ChargingVoltage() = 0x3130
Battery Full-Charge Voltage and
Accuracy
ChargingVoltage() = 0x20D0
ChargingVoltage() = 0x1060
-0.5
12.491
12.592
12.693
V
+0.8
%
8.4
8.467
V
+0.8
%
4.192
4.234
V
+1.0
%
-0.8
8.333
-0.8
4.150
-1.0
Battery Undervoltage-Lockout
Trip Point for Trickle Charge
2.5
V
CHARGE-CURRENT REGULATION
CSIP-to-CSIN Full-Scale CurrentSense Voltage
Charge Current and Accuracy
Charge-Current Gain Error
FBSA/FBSB/CSIP/CSIN
Input Voltage Range
2
78.22
80.64
83.06
mV
RS2, Figure 1 = 10mΩ;
ChargingCurrent() = 0x1f80
7.822
8.064
8.306
A
+3
%
RS2, Figure 1 = 10mΩ;
ChargingCurrent() = 0x0f80
3.809
3.968
4.126
A
-4
+4
%
RS2, Figure 1 = 10mΩ;
ChargingCurrent() = 0x0080 (128mA)
64
400
mA
Based on ChargeCurrent() = 128mA and 8.064A
-2
+2
%
0
19
V
-3
_______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
Battery Quiescent Current
CONDITIONS
MIN
Adapter present, not charging, ICSIP + ICSIN + ILX + IFBS,
VFBS_ = VLX = VCSIN = VCSIP = 19V
TYP
MAX
2
5
µA
Adapter absent, ICSIP + ICSIN + ILX + IFBSA + IFBSB + ICSSP
+ ICSSN, VFBS_ = VLX = VCSIN = VCSIP = 19V, VDCIN = 0V
+1
VAdapter = 26V, VBattery = 16.8V, not
charging
Adapter Quiescent Current
IDCIN +
ICSSP +
ICSSN
UNITS
200
500
µA
mA
VAdapter = 19V,
VBattery = 16.8V
Charging
0.4
1
Not charging
200
500
µA
VAdapter = 8V,
VBattery = 4V
Charging
0.4
1
mA
Not charging
200
500
µA
110
113.3
mV
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage
Input Current Accuracy
POR Input Current
VFBS_ = 19V
106.7
RS1, Figure 1 = 10mΩ, InputCurrent() = 11004mA or
3584mA
-3
+3
RS1, Figure 1 = 10mΩ, InputCurrent() = 2048mA
-5
+5
RS1, Figure 1 = 10mΩ
Input Current-Limit Gain Error
Input Current-Limit Offset
Based on InputCurrent() = 1024mA and 11004mA
CSSP/CSSN Input Voltage Range
256
mA
-2
+2
%
-1
+1
mV
8
26
V
3.15
mA/V
-1.5
+1.5
mV
VCSSP - CSSN = 110mV
-5
+5
VCSSP - CSSN = 55mV or 35mV
-4
+4
VCSSP - CSSN = 20mV
-10
+10
0
3.5
V
8.0
26.0
V
IINP Transconductance
VCSSP - CSSN = 110mV
2.85
IINP Offset
Based on VCSSP - CSSN = 110mV and 20mV
IINP Accuracy
%
IINP Output Voltage Range
3.0
%
SUPPLY AND LINEAR REGULATOR
DCIN, Input Voltage Range
DCIN Undervoltage-Lockout
Trip Point
Power-Fail Threshold
DCIN falling
7
DCIN rising
7.4
7.5
7.85
VCSSP - VCSIN falling
9
15
21
VCSSP - VCSIN rising
160
210
271
V
mV
_______________________________________________________________________________________
3
MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
LDO Output Voltage
8.0V < VDCIN < 28V, no load
LDO Load Regulation
0 < ILDO < 30mA
LDO Undervoltage-Lockout Threshold
VDCIN = 8.0V, VLDO falling
VDD Range
MIN
TYP
MAX
5.25
5.4
5.55
V
34
100
mV
4.00
5.15
V
5.5
V
2.7
V
3.20
2.7
VDD UVLO Rising
2.5
VDD UVLO Hysteresis
VDD Quiescent Current
100
DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V
UNITS
mV
16
27
µA
4.096
4.120
V
3.1
3.9
V
REFERENCE
REF Output Voltage
0 < IREF < 500µA
REF Undervoltage-Lockout Trip Point
REF falling
4.071
ACOK
ACOK Sink Current
VACOK = 0.4V, ACIN = 1.5V
ACOK Leakage Current
VACOK = 5.5V, ACIN = 2.5V
1
mA
1
µA
ACIN
ACIN Threshold
2.007
2.048
2.089
V
ACIN Threshold Hysteresis
10
20
30
mV
ACIN Input Bias Current
-1
+1
µA
REMOTE-SENSE INPUTS
FBS_ Range
VCSIN - VFBS
FBS_ Gain
ΔVCSIN / Δ(VCSIN - VFBS_)
CSIN-FBS_ Clamp Voltage
FBS_ Bias Current
Charger switching, FBS_ selected
FBS_ Bias Current
Charger not switching or FBS_ not selected
200
mV
0.95
0
1.00
1.05
V/V
225
250
275
mV
14
µA
+2
µA
-2
SWITCHING REGULATOR
Off-Time
VCSIN = 16.0V, VCSSP = 19V
360
400
440
VCSIN = 16.0V, VCSSP = 17V
260
300
360
500
800
µA
2
µA
BST Supply Current
DHI high
LX Input Bias Current
VDCIN = 28V, VCSIN = VLX = 20V, DHI low
Maximum Discontinuous-Mode Peak
Current (IMIN)
0.5
ns
A
DHI On-Resistance Low
IDHI = -10mA
1
3
Ω
DHI On-Resistance High
IDHI = 10mA
3
5
Ω
DLO On-Resistance High
IDLO = 10mA
3
5
Ω
DLO On-Resistance Low
IDLO = -10mA
1
3
Ω
4
_______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF, VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.0625
0.125
0.2500
mA/V
ERROR AMPLIFIERS
GMV Amplifier Transconductance ChargingVoltage() = 16.8V, VFBS_ = 16.8V
GMI Amplifier Transconductance
0.5
1.0
2.0
mA/V
GMS Amplifier Transconductance InputCurrent() = 3968mA, VCSSP - VCSSN = 79.36mV
ChargingCurrent() = 3968mA, VCSIP - VCSIN = 39.68mV
0.5
1.0
2.0
mA/V
CCI/CCS/CCV Clamp Voltage
120
250
600
mV
0.8
V
0.25V < VCCI/S/V < 2.0V
LOGIC LEVELS
SDA/SCL Input-Low_Voltage
VDD = 2.7V to 5.5V
SDA/SCL Input-High_Voltage
VDD = 2.7V to 5.5V
2.1
SDA/SCL Input Bias Current
VDD = 2.7V to 5.5V
-1
V
BATSEL Input-Low_Voltage
BATSEL Input-High_Voltage
µA
V
2.1
BATSEL Input Bias Current
SDA, Output Sink Current
+1
0.8
V
-1
V(SDA) = 0.4V
+1
6
µA
mA
SMBus TIMING SPECIFICATIONS (VDD = 2.7V to 5.5V) (see Figures 4 and 5)
PARAMETERS
SYMBOL
CONDITIONS
MIN
SMBus Frequency
fSMB
10
Bus Free Time
TYP
MAX
UNITS
100
kHz
tBUF
4.7
µs
Start Condition Hold Time from
SCL
tHD:STA
4
µs
Start Condition Setup Time from
SCL
tSU:STA
4.7
µs
Stop Condition Setup Time from
SCL
tSU:STO
4
µs
SDA Hold Time from SCL
tHD:DAT
300
ns
SDA Setup Time from SCL
tSU:DAT
250
ns
SCL Low Timeout
tTIMEOUT
(Note 1)
25
35
ms
SCL Low Period
TLOW
4.7
µs
SCL High Period
THIGH
4
µs
Maximum Charging Period
Without a ChargeVoltage() or
ChargeCurrent() Command
140
175
210
s
_______________________________________________________________________________________
5
MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
ELECTRICAL CHARACTERISTICS
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
16.968
V
%
CHARGE-VOLTAGE REGULATION
16.632
ChargingVoltage() = 0x41A0
-1
+1
12.466
12.717
V
-1
+1
%
8.316
8.484
V
-1
+1
%
4.129
4.255
V
-1.5
+1.5
%
78.22
83.05
mV
RS2, Figure 1 = 10mΩ;
ChargingCurrent()= 0x1f80
7.822
8.305
A
-3
+3
%
RS2, Figure 1 = 10mΩ;
ChargingCurrent() = 0x0f80
3.809
4.126
A
-4
+4
%
RS2, Figure 1 =10mΩ;
ChargingCurrent() = 0x0080
30
400
mA
Based on ChargeCurrent() = 128mA and 8.064A
-2
+2
%
0
19
V
ChargingVoltage() = 0x3130
Battery Full-Charge Voltage and
Accuracy
ChargingVoltage() = 0x20D0
ChargingVoltage() = 0x1060
CHARGE-CURRENT REGULATION
CSIP-to-CSIN Full-Scale CurrentSense Voltage
Charge Current and Accuracy
Charge-Current Gain Error
FBSA/FBSB/CSIP/CSIN InputVoltage Range
Battery Quiescent Current
Adapter present, not charging, ICSIP + ICSIN + ILX + IFBS,
VFBS_ = VLX = VCSIN = VCSIP = 19V
5
Adapter absent, ICSIP + ICSIN + ILX + IFBSA + IFBSB +
ICSSP + ICSSN, VFBS_= VLX = VCSIN = VCSIP = 19V,
VDCIN = 0V
1
VAdapter = 26V, VBattery = 16.8V, not charging
Adapter Quiescent Current
6
IDCIN +
ICSSP +
ICSSN
VAdapter = 19V,
VBattery = 16.8V
Charging
VAdapter = 8V,
VBattery = 4V
Charging
Not charging
Not charging
_______________________________________________________________________________________
µA
500
µA
1
mA
500
µA
1
mA
500
µA
SMBus Level 2 Battery Charger
with Remote Sense
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
103.3
116.6
mV
RS1, Figure 1 = 10mΩ;
InputCurrent() = 11004mA or 3584mA
-6
+6
RS1, Figure 1 = 10mΩ;
InputCurrent() = 2048mA
-5
+5
Input Current-Limit Gain Error
Based on InputCurrent() = 1024mA and 11004mA
-5
+5
%
Input Current-Limit Offset
Based on InputCurrent() = 1024mA and 11004mA
-1
+1
mV
8
26
V
INPUT-CURRENT REGULATION
CSSP-to-CSSN Full-Scale
Current-Sense Voltage
VFBS_ = 19V
Input Current Accuracy
%
CSSP/CSSN Input Voltage Range
IINP Transconductance
VCSSP - CSSN = 110mV
2.7
3.3
mA/V
IINP Offset
Based on VCSSP - CSSN = 110mV and 20mV
mV
IINP Accuracy
-1.5
+1.5
VCSSP - CSSN = 110mV
-5
+5
VCSSP - CSSN = 55mV or 35mV
-4
+4
VCSSP - CSSN = 20mV
-10
+10
0
3.5
V
8.0
26.0
V
IINP Output Voltage Range
%
SUPPLY AND LINEAR REGULATOR
DCIN, Input Voltage Range
DCIN Undervoltage-Lockout
Trip Point
DCIN falling
7
DCIN rising
7.85
—
VCSSP - VCSIN falling
9
21
VCSSP - VCSIN rising
160
271
LDO Output Voltage
8.0V < VDCIN < 28V, no load
5.25
5.55
V
LDO Load Regulation
0 < ILDO < 30mA
100
mV
LDO Undervoltage-Lockout
Threshold
VDCIN = 8.0V, VLDO falling
3.20
5.15
V
2.7
5.5
V
POWER_FAIL Threshold
VDD Range
VDD UVLO Rising
VDD Quiescent Current
DCIN < 6V, VDD = 5.5V, SCL = SDA = 5.5V
mV
2.7
V
27
µA
4.139
V
3.9
V
REFERENCE
REF Output Voltage
0 < IREF < 500µA
REF Undervoltage-Lockout
Trip Point
REF falling
4.053
ACOK
ACOK Sink Current
VACOK = 0.4V, ACIN = 1.5V
1
mA
_______________________________________________________________________________________
7
MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
ELECTRICAL CHARACTERISTICS (continued)
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated per
Figure 1; TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
2.007
2.089
V
10
30
mV
0
200
mV
0.9
1.1
V/V
220
280
mV
14
µA
ACIN
ACIN Threshold
ACIN Threshold Hysteresis
REMOTE-SENSE INPUTS
FBS_ Range
VCSIN - VFBS
FBS_ Gain
ΔVCSIN / Δ(VCSIN - VFBS_)
CSIN-FBS_ Clamp Voltage
FBS_ Bias Current
Charger switching, FBS_ selected
SWITCHING REGULATOR
Off-Time
VCSIN = 16.0V, VCSSP = 19V
360
440
VCSIN = 16.0V, VCSSP = 17V
260
350
ns
BST Supply Current
DHI high
800
µA
DHI On-Resistance Low
IDHI = -10mA
3
Ω
DHI On-Resistance High
IDHI = 10mA
5
Ω
DLO On-Resistance High
IDLO = 10mA
5
Ω
DLO On-Resistance Low
IDLO = -10mA
3
Ω
0.0625
0.2500
mA/V
0.5
2.0
mA/V
ERROR AMPLIFIERS
GMV Amplifier Transconductance ChargingVoltage() = 16.8V, VFBS_ = 16.8V
GMI Amplifier Transconductance
ChargingCurrent() = 3968mA, VCSIP - VCSIN = 39.68mV
GMS Amplifier Transconductance InputCurrent() = 3968mA, VCSSP - VCSSN = 79.36mV
0.5
2.0
mA/V
CCI/CCS/CCV Clamp Voltage
150
600
mV
0.8
V
0.25V < VCCI/S/V < 2.0V
LOGIC LEVELS
SDA/SCL Input Low Voltage
VDD = 2.7V to 5.5V
SDA/SCL Input High Voltage
VDD = 2.7V to 5.5V
2.3
BATSEL Input Low Voltage
BATSEL Input High Voltage
SDA, Output Sink Current
8
V
0.8
V(SDA) = 0.4V
V
2.3
V
6
mA
_______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
(VDCIN = VLX = VCSSP = VCSSN = 19V, VBST - VLX = 4.5V, VFBSA = VFBSB = VCSIP = VCSIN = 16.8V, BATSEL = GND = PGND = 0,
CLDO = 1µF, VCC = LDO, CREF = 1µF, CDAC = 0.1µF , VDD = 3.3V, ACIN = 2.5V; pins CCI, CCV, and CCS are compensated
per Figure 1; TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
SMB TIMING SPECIFICATION (VDD = 2.7V to 5.5V) (see Figures 4 and 5)
PARAMETERS
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
100
kHz
SMBus Frequency
fSMB
10
Bus Free Time
tBUF
4.7
µs
START Condition Hold Time from
SCL
tHD:STA
4
µs
START Condition Setup Time
from SCL
tSU:STA
4.7
µs
STOP Condition Setup Time from
SCL
tSU:STO
4
µs
SDA Hold Time from SCL
tHD:DAT
300
ns
SDA Setup Time from SCL
SCL Low Timeout
tSU:DAT
tTIMEOUT
250
(Note 1)
25
ns
35
ms
SCL Low Period
TLOW
4.7
µs
SCL High Period
THIGH
4
µs
Maximum Charging Period
Without a ChargeVoltage() or
ChargeCurrent() Command
140
210
s
Note 1: Devices participating in a transfer time out when any clock low exceeds the 25ms minimum timeout period. Devices that
have detected a timeout condition must reset the communication no later than the 35ms maximum timeout period. Both a
master and a slave must adhere to the maximum value specified as it incorporates the cumulative stretch limit for both a
master (10ms) and a slave (25ms).
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
_______________________________________________________________________________________
9
MAX8731A
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)
TYPICAL
0
-2
MINIMUM
INPUT CURRENT LIMIT = 3.584A
0.2
0
INPUT CURRENT
LIMIT = 4.096A
-0.2
1.0
2
4
6
8
10
INPUT CURRENT-LIMIT SETTING (A)
0
1
2
3
SYSTEM CURRENT (A)
-0.6
VBATT = 8.4V
-0.2
8
6
NOT SWITCHING
4
1.5
VBATT = 16.8V
VBATT = 8.4V
1.0
-0.4
0
-2
TYPICAL
-8
INPUT CURRENT LIMIT = 3.584A
OPERATING AT INPUT CURRENT LIMIT
0
-1.0
0.5
1.0 1.5 2.0 2.5 3.0
SYSTEM CURRENT (A)
3.5
-10
0
4.0
MAXIMUM
4
2
0
-2
TYPICAL
MINIMUM
-6
4
2
3.072A
3.968A
0
8.064A
-2
-8
-4
-10
0
2
4
6
CHARGE-CURRENT SETTING (A)
0
8
1
2
3
4
INPUT CURRENT (A)
5
6
TRICKLE-CHARGE CURRENT ERROR
vs. BATTERY VOLTAGE
MAX8731A toc08
8
4
CHARGE-CURRENT ERROR (%)
MAX8731A toc07
10
-4
2
3
SYSTEM CURRENT (A)
CHARGE-CURRENT ERROR
vs. BATTERY VOLTAGE
CHARGE-CURRENT ERROR
vs. CHARGE CURRENT-LIMIT SETTING
6
1
5
TRICKLE-CHARGE CURRENT ERROR (%)
0
MINIMUM
-6
MAX8731A toc09
-0.8
0.5
MAXIMUM
2
-4
INPUT CURRENT LIMIT = 4.096A
-0.6
4
MAX8731A toc06
VBATT = 12.6V
IINP ERROR (%)
0
2
3
SYSTEM CURRENT (A)
IINP ERROR vs. INPUT CURRENT
2.0
IINP ERROR (%)
INPUT CURRENT LIMIT = 3.584A
1
10
0.4
IINP ERROR (%)
-0.4
0
MAX8731A toc05
0.6
10
0
-0.2
4
2.5
MAX8731A toc04
INPUT CURRENT LIMIT = 2.048A
0.2
VBATT = 16.8V
VBATT = 12.6V
0.2
IINP ERROR vs. SYSTEM CURRENT
IINP ERROR vs. SYSTEM CURRENT
1.0
0.8
0.4
-1.0
-0.4
0
0.6
-0.8
INPUT CURRENT LIMIT = 2.048A
-6
INPUT CURRENT LIMIT = 3.584A
0.8
INPUT CURRENT-LIMIT ERROR (%)
MAXIMUM
2
MAX8731A toc02
4
0.4
INPUT CURRENT-LIMIT ERROR (%)
MAX8731A toc01
INPUT CURRENT-LIMIT ERROR (%)
6
-4
INPUT CURRENT-LIMIT ERROR
vs. SYSTEM CURRENT
INPUT CURRENT-LIMIT ERROR
vs. SYSTEM CURRENT
MAX8731A toc03
INPUT CURRENT-LIMIT ERROR
vs. INPUT CURRENT-LIMIT SETTING
CHARGE-CURRENT LIMIT ERROR (%)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
0
-5
-10
-15
-20
-25
ChargeCurrent( ) = 128mA
-30
3
6
9
12
15
BATTERY VOLTAGE (V)
18
0
3
6
9
12
BATTERY VOLTAGE (V)
______________________________________________________________________________________
15
18
SMBus Level 2 Battery Charger
with Remote Sense
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)
CHARGE-VOLTAGE ERROR
vs. CHARGE-VOLTAGE SETTING
BATTERY-VOLTAGE ERROR
vs. CHARGE CURRENT
0.2
0.0
-0.2
-0.4
MAX8731A toc11
0.4
0.3
BATTERY-VOLTAGE ERROR (%)
MAX8731A toc10
CHARGE-VOLTAGE ERROR (%)
0.6
0.2
0.1
2 CELLS
0
4 CELLS
-0.1
-0.2
3 CELLS
-0.6
-0.3
8
12
16
CHARGE-VOLTAGE SETTING (V)
20
0
BATTERY REMOVAL
2
3
4
CHARGE CURRENT (A)
13.5V
12.5V
VOUT OUTPUT CAPACITOR = 22μF
5A
LOAD
CURRENT
0A
ADAPTER
CURRENT
5A
0A
5A
INDUCTOR
CURRENT
ChargeVoltage( ) = 12.6V
CCS
CCS VOLTAGE
500mV/div
CCI VOLTAGE
500mV/div
500mV/div
LDO LOAD REGULATION
EFFICIENCY vs. CHARGE CURRENT
0
MAX8731A toc14
95
4 CELLS
-5
CHARGER OFF
-10
LDO ERROR (mV)
EFFICIENCY (%)
CCI
CCS
200μs/div
100
2 CELLS
85
0A
500mV/div
CCI
20μs/div
3 CELLS
6
MAX8731A toc13
13.0V
90
5
SYSTEM LOAD TRANSIENT
MAX8731A toc12
VOUT OUTPUT CAPACITOR = 10μF
1
MAX8731A toc15
4
80
75
-15
-20
-25
70
-30
65
-35
60
-40
0
2
4
6
CHARGE CURRENT (A)
8
0
20
40
60
ILDO (mA)
80
100
______________________________________________________________________________________
11
MAX8731A
Typical Operating Characteristics (continued)
Typical Operating Characteristics (continued)
(Circuit of Figure 1, adapter = 19.5V, ChargeVoltage() = 16.8V, ChargeCurrent() = 3.854A, InputCurrent() = 3.584A, TA = +25°C,
unless otherwise noted.)
REF LOAD REGULATION
NOT SWITCHING
0.15
0.3
MAX8731A toc18
MAX8731A toc16
-1
REF ERROR vs. TEMPERATURE
0.20
MAX8731A toc17
LDO LINE REGULATION
0
0.2
REF ERROR (%)
-2
-3
-4
REF ERROR (%)
0.10
LDO ERROR (mV)
0.05
0
-0.05
0.1
0
-0.1
-0.10
-0.2
-0.15
-6
-0.20
13
18
VDCIN (V)
23
-0.3
0
0.2
0.4
0.6
IREF (mA)
0.8
0
20
40
TEMPERATURE (°C)
MAX8731A toc19
350
300
250
MAX8731A toc20
13.0
2.8Ah x 3S3P BATTERY
4
12.5
BATTERY VOLTAGE
12.0
3
11.5
2
11.0
1
200
10.5
CHARGE CURRENT
0
150
0
5
10
15
VADAPTER - VBATTERY (V)
10.0
0
20
ADAPTER CURRENT
vs. ADAPTER VOLTAGE
2
3
TIME (h)
4
2.5
MAX8731A toc21
2.5
ADAPTER PRESENT OR ABSENT
2.0
BATTERY CURRENT (μA)
SWITCHING, NO LOAD
2.0
ChargeVoltage( ) = 4.192V
1.5
NOT SWITCHING
1.0
1
5
6
BATTERY-LEAKAGE CURRENT
vs. BATTERY VOLTAGE
3.0
1.5
1.0
0.5
0.5
0
0
0
12
-20
5
CHARGE CURRENT (A)
FREQUENCY (kHz)
400
-40
BATTERY-CHARGE CURVE
SWITCHING FREQUENCY
450
1.0
MAX8731A toc22
8
5
10
15
20
ADAPTER VOLTAGE (V)
25
30
0
5
10
15
BATTERY VOLTAGE (V)
20
______________________________________________________________________________________
BATTERY VOLTAGE (V)
NOT SWITCHING
-5
ADAPTER CURRENT (mA)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
60
80
SMBus Level 2 Battery Charger
with Remote Sense
PIN
NAME
FUNCTION
1, 12
GND
Analog Ground. Connect directly to the paddle.
2
ACIN
AC Adapter Detect Input. ACIN is the input to an uncommitted comparator.
3
REF
4.096V Voltage Reference. Bypass REF with a 1µF capacitor to GND.
4
CCS
Input Current Regulation Loop-Compensation Point. Connect 0.01µF from CCS to GND.
5
CCI
Output Current Regulation Loop-Compensation Point. Connect 0.01µF from CCI to GND.
6
CCV
Voltage Regulation Loop-Compensation Point. Connect 10kΩ in series with 0.01µF to GND.
7
DAC
DAC Voltage Output. Bypass with 0.1µF from DAC to GND.
8
IINP
Input Current Monitor Output. IINP sources the current proportional to the current sensed across
CSSP and CSSN. The transconductance from (CSSP - CSSN) to IINP is 3mA/V.
9
SDA
SMBus Data I/O. Open-drain output. Connect an external pullup resistor according to SMBus specifications.
10
SCL
SMBus Clock Input. Connect an external pullup resistor according to SMBus specifications.
11
VDD
Logic Circuitry Supply-Voltage Input. Bypass with a 0.1µF capacitor to GND.
13
ACOK
AC Detect Output. This open-drain output is high impedance when ACIN is greater than REF/2. The
ACOK output remains low when the MAX8731A is powered down. Connect a 10kΩ pullup resistor from
VCC to ACOK.
14
BATSEL
Battery Voltage Select Input. Drive BATSEL high to select battery B, or drive BATSEL low to select battery A.
Any change of BATSEL immediately stops charging. Charging begins again in approximately 10ms.
15
FBSA
Remote-Sense Input for the Output Voltage of Battery A. Connect a 100Ω resistor from FBSA to the
battery connector, and a 10nF capacitor from FBSA to PGND.
16
FBSB
Remote-Sense Input for the Output Voltage of Battery B. Connect a 100Ω resistor from FBSB to the
battery connector, and a 10nF capacitor from FBSB to PGND.
17
CSIN
Charge Current-Sense Negative Input
18
CSIP
Charge Current-Sense Positive Input. Connect a 10mΩ current-sense resistor between CSIP and CSIN.
19
PGND
20
DLO
Low-Side Power MOSFET Driver Output. Connect to low-side n-channel MOSFET. DLO drives
between LDO and PGND.
21
LDO
Linear-Regulator Output. LDO is the output of the 5.4V linear regulator supplied from DCIN. LDO also
directly supplies the DLO driver and the BST charge pump. Bypass with a 1µF ceramic capacitor
from LDO to PGND.
22
DCIN
Charger Bias Supply Input. Bypass DCIN with a 0.1µF capacitor to PGND.
23
LX
High-Side Power MOSFET Driver Source Connection. Connect to the source of the high-side n-channel
MOSFET.
24
DHI
High-Side Power MOSFET Driver Output. Connect to the high-side n-channel MOSFET gate.
25
BST
High-Side Power MOSFET Driver Power-Supply Connection. Connect a 0.1µF capacitor from BST to LX.
26
VCC
Device Power-Supply Input. Connect to LDO through an RC filter as shown in Figure 1.
27
CSSN
Input Current-Sense Negative Input
28
CSSP
Input Current-Sense Positive Input. Connect a 10mΩ current-sense resistor between CSSP and CSSN.
—
BP
Power Ground
Backside Paddle. Connect the backside paddle to analog ground.
______________________________________________________________________________________
13
MAX8731A
Pin Description
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
ADAPTER
INPUT
R2
100kΩ
R1
150kΩ
R13
100kΩ
D1
CSSP
RS1
10mΩ
DCIN
C1
1μF
ACIN
CSSN
VCC
R3
49.9kΩ
N
R12
33Ω
D2
ACOK
INPUT
BST
VDD
VDD
KBC
LDO
C11
1μF
LDO
R4
10kΩ
N1
DHI
R6
10kΩ
C2
0.1μF
DHI
SCL
LX
SDA
C9
220pF
N3
PGND
RS2
10mΩ
CSIP
C3
0.1μF
L1
4.3μH
L1: SUMIDA
CEP125-4R3MC-U
IINP
R7
10kΩ
N2
R11
1Ω
DLO
SDA
N1, N2,: SI4800BDY
N3: SI4810BDY
C10
0.1μF
R5
10kΩ
SCL
CIN2
10μF
CIN1
10μF
C12
1μF
MAX8731A
LDO
SYSTEM
LOAD
CSIN
VOUT
CCV
R8
10kΩ
C5
0.01μF
C4
0.01μF
COUT1
10μF
CCI
BATSEL
SELECTOR
R10
100Ω
FBSB
CCS
C6
0.01μF
COUT2
10μF
REF
DAC
C7
1μF
BP
C8
0.1μF
FBSA
GND
R9
100Ω
BATTERY
A
BATTERY
B
Figure 1. Typical Dual-Battery Application Circuit
Detailed Description
The typical application circuit is shown in Figure 1. The
MAX8731A includes all the functions necessary to
charge Li+, NiMH, and NiCd smart batteries. A highefficiency, synchronous-rectified, step-down DC-DC
converter is used to implement a precision constantcurrent, constant-voltage charger. The DC-DC converter drives a high-side n-channel MOSFET and provides
synchronous rectification with a low-side n-channel
MOSFET. The charge current and input current-sense
14
amplifiers have low input-offset error (±64µV typ),
allowing the use of small-valued sense resistors.
The MAX8731A features a voltage-regulation loop
(CCV) and two current-regulation loops (CCI and CCS).
The loops operate independently of each other. The
CCV voltage-regulation loop monitors either FBSA or
FBSB to ensure that its voltage never exceeds the voltage set by the ChargeVoltage() command. The CCI
battery current-regulation loop monitors the current
delivered to the selected battery to ensure that it never
exceeds the current limit set by the ChargeCurrent()
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
the set point. A third loop (CCS) takes control and
reduces the charge current when the adapter current
exceeds the input current limit set by the InputCurrent()
command.
A functional diagram is shown in Figure 2.
2V
(10A FOR RS2 = 10mΩ)
150mV
(750mA FOR RS2 = 10mΩ)
MAX8731A
BST
CSSP
POWER-FAIL
IMAX
ZCMP
CSIN
HIGHSIDE
DRIVER
ENABLE
100mV
ACIN
IMIN
VCC
LX
LEVEL
SHIFT
DC-DC
ACOK
DHI
CONVERTER
LDO
LVC
LOWSIDE
DRIVER
CCMP
REF/2
GND
DLO
OVP
PGND
LOWEST VOLTAGE CLAMP
CCV
CCI
5.4V
LINEAR
REGULATOR
GMV
DCIN
VCC
GMI
+
200mV
-
CCS
4.096V
REFERENCE
REF
CSI
CSA: CURRENT-SENSE
AMPLIFIER
SMBus LOGIC
GMS
11-BIT DAC
CHARGE VOLTAGE ( )
6-BIT DAC
CHARGE CURRENT ( )
6-BIT DAC
INPUT CURRENT ( )
SCL
A = 1V/V
CSA
CSS
SDA
A = 20V/V
GM
VDD
CSA
DAC
FBSA
FBSB
BATSEL
CSIN
CSIP
CSSP
CSSN
IINP
A = 20V/V
Figure 2. Functional Diagram
______________________________________________________________________________________
15
MAX8731A
command. The charge current-regulation loop is in control as long as the selected battery voltage is below the
charge voltage set point. When the selected battery
voltage reaches its set point, the voltage-regulation
loop takes control and maintains the battery voltage at
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
Setting Charge Voltage
To set the output voltage, use the SMBus to write a 16-bit
ChargeVoltage() command using the data format listed in
Table 1. The ChargeVoltage() command uses the WriteWord protocol (see Figure 3). The command code for
ChargeVoltage() is 0x15 (0b00010101). The MAX8731A
provides a 1.024V to 19.200V charge voltage range, with
16mV resolution. Set ChargeVoltage() below 1.024V to
terminate charging. Upon reset, the ChargeVoltage() and
ChargeCurrent() values are cleared and the charger
remains off until both the ChargeVoltage() and the
ChargeCurrent() command are sent. Both DHI and DLO
remain low until the charger is restarted.
Table 1. ChargeVoltage () (0x15)
BIT
BIT NAME
0
—
Not used. Normally a 1mV weight.
1
—
Not used. Normally a 2mV weight.
2
—
Not used. Normally a 4mV weight.
3
—
Not used. Normally a 8mV weight.
4
Charge voltage, DACV 0
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 16mV of charger voltage compliance.
5
Charge voltage, DACV 1
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 32mV of charger voltage compliance.
6
Charge voltage, DACV 2
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 64mV of charger voltage compliance.
7
Charge voltage, DACV 3
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 128mV of charger voltage compliance.
8
Charge voltage, DACV 4
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 256mV of charger voltage compliance.
9
Charge voltage, DACV 5
0 = Adds 0mV of charger voltage compliance, 1024mV min.
1 = Adds 512mV of charger voltage compliance.
10
Charge voltage, DACV 6
0 = Adds 0mA of charger voltage compliance.
1 = Adds 1024mV of charger voltage compliance.
11
Charge voltage, DACV 7
0 = Adds 0mV of charger voltage compliance.
1 = Adds 2048mV of charger voltage compliance.
12
Charge voltage, DACV 8
0 = Adds 0mV of charger voltage compliance.
1 = Adds 4096mV of charger voltage compliance.
13
Charge voltage, DACV 9
0 = Adds 0mV of charger voltage compliance.
1 = Adds 8192mV of charger voltage compliance.
14
Charge voltage, DACV 10
0 = Adds 0mV of charger voltage compliance.
1 = Adds 16,384mV of charger voltage compliance, 19,200mV max.
15
16
DESCRIPTION
—
Not used. Normally a 32,768mV weight.
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
Setting Input-Current Limit
System current normally fluctuates as portions of the
system are powered up or put to sleep. By using the
input-current-limit circuit, the output-current requirement of the AC wall adapter can be lowered, reducing
system cost.
The total input current, from a wall cube or other DC
source, is the sum of the system supply current and the
current required by the charger. When the input current
exceeds the set input current limit, the MAX8731A
decreases the charge current to provide priority to system load current. As the system supply rises, the available charge current drops linearly to zero. Thereafter,
the total input current can increase without limit.
The internal amplifier compares the differential voltage
between CSSP and CSSN to a scaled voltage set by
the InputCurrent() command (see Table 3). The total
input current is the sum of the device supply current,
the charger input current, and the system load current.
The total input current can be estimated as follows:
Table 2. ChargeCurrent() (0x14) (10mΩ Sense Resistor, RS2)
BIT
BIT NAME
0
—
Not used. Normally a 1mA weight.
DESCRIPTION
1
—
Not used. Normally a 2mA weight.
2
—
Not used. Normally a 4mA weight.
3
—
Not used. Normally an 8mA weight.
4
—
Not used. Normally a 16mA weight.
5
—
Not used. Normally a 32mA weight.
6
—
Not used. Normally a 64mA weight.
7
Charge Current, DACI 0
0 = Adds 0mA of charger current compliance.
1 = Adds 128mA of charger current compliance.
8
Charge Current, DACI 1
0 = Adds 0mA of charger current compliance.
1 = Adds 256mA of charger current compliance.
9
Charge Current, DACI 2
0 = Adds 0mA of charger current compliance.
1 = Adds 512mA of charger current compliance.
10
Charge Current, DACI 3
0 = Adds 0mA of charger current compliance.
1 = Adds 1024mA of charger current compliance.
11
Charge Current, DACI 4
0 = Adds 0mA of charger current compliance.
1 = Adds 2048mA of charger current compliance.
12
Charge Current, DACI 5
0 = Adds 0mA of charger current compliance.
1 = Adds 4096mA of charger current compliance, 8064mA max.
13
—
Not used. Normally a 8192mA weight.
14
—
Not used. Normally a 16,386mA weight.
15
—
Not used. Normally a 32,772mA weight.
______________________________________________________________________________________
17
MAX8731A
Setting Charge Current
To set the charge current, use the SMBus to write a 16bit ChargeCurrent() command using the data format
listed in Table 2. The ChargeCurrent() command uses
the Write-Word protocol (see Figure 3). The command
code for ChargeCurrent() is 0x14 (0b00010100). When
RS2 =10mΩ, the MAX8731A provides a charge current
range of 128mA to 8.064A, with 128mA resolution. Set
ChargeCurrent() to 0 to terminate charging. Upon reset,
the ChargeVoltage() and ChargeCurrent() values are
cleared and the charger remains off until both the
ChargeVoltage() and the ChargeCurrent() commands
are sent. Both DHI and DLO remain low until the charger
is restarted.
The MAX8731A includes a foldback current limit when
the battery voltage is low. If the battery voltage is less
than 2.5V, the charge current is temporarily set to
128mA. The ChargeCurrent() register is preserved and
becomes active again when the battery voltage is higher than 2.5V. This function effectively provides a foldback current limit, which protects the charger during
short circuit and overload.
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
⎡ (ICHARGE × VBATTERY ) ⎤
IINPUT = ILOAD + ⎢
⎥ + IBIAS
(VIN × η)
⎢⎣
⎥⎦
where η is the efficiency of the DC-DC converter (typically 85% to 95%).
To set the input current limit, use the SMBus to write a
16-bit InputCurrent() command using the data format
listed in Table 3. The InputCurrent() command uses the
Write-Word protocol (see Figure 3). The command
code for InputCurrent() is 0x3F (0b00111111). When
RS1 = 10mΩ, the MAX8731A provides an input-currentlimit range of 256mA to 11.004A, with 256mA resolution. InputCurrent() settings from 1mA to 256mA result
in a current limit of 256mA. Upon reset the input current
limit is 256mA.
Charger Timeout
The MAX8731A includes a timer to terminate charging
if the charger does not receive a ChargeVoltage() or
ChargeCurrent() command within 175s. If a timeout
occurs, both ChargeVoltage() and ChargeCurrent()
commands must be resent to reenable charging.
Remote Sense
The MAX8731A features dual remote sense, which
allows the rejection of board resistance and selector
resistance when used in either single- or dual-battery
systems. To fully utilize remote sensing, connect FBS_
directly to the battery interface through an unshared
battery sense trace in series with a 100Ω resistor, and
10nF capacitor (see Figure 1). In single-battery systems,
connect BATSEL directly to GND and use only FBSA.
Remote sensing cancels the effect of impedance in
series with the battery. This impedance normally causes the battery charger to prematurely enter constantvoltage mode with reducing charge current. The result
is that the last 20% of charging takes longer than necessary. When in constant-voltage mode, the remaining
charge time is proportional to the total resistance in
series with the battery. Remote sensing reduces
charge time according to the following equation:
t CVRS = t CV 0 ×
RPack
RPack + RBoard
Table 3. InputCurrent() (0x3F) (10mΩ Sense Resistor, RS1)
18
BIT
BIT NAME
DESCRIPTION
0
—
Not used. Normally a 2mA weight.
1
—
Not used. Normally a 4mA weight.
2
—
Not used. Normally an 8mA weight.
3
—
Not used. Normally a 16mA weight.
4
—
Not used. Normally a 32mA weight.
5
—
Not used. Normally a 64mA weight.
6
—
Not used. Normally a 128mA weight.
7
Input Current, DACS 0
0 = Adds 0mA of input current compliance.
1 = Adds 256mA of input current compliance.
8
Input Current, DACS 1
0 = Adds 0mA of input current compliance.
1 = Adds 512mA of input current compliance.
9
Input Current, DACS 2
0 = Adds 0mA of input current compliance.
1 = Adds 1024mA of input current compliance.
10
Input Current, DACS 3
0 = Adds 0mA of input current compliance.
1 = Adds 2048mA of input current compliance.
11
Input Current, DACS 4
0 = Adds 0mA of input current compliance.
1 = Adds 4096mA of input current compliance.
12
Input Current, DACS 5
0 = Adds 0mA of input current compliance.
1 = Adds 8192mA of input current compliance, 11,004mA max.
13
—
Not used. Normally a 16,384mA weight.
14
—
Not used. Normally a 32,768mA weight.
15
—
Not used. Normally a 65,536mA weight.
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
The MAX8731A includes a safety feature, which limits
the charge voltage when FBS_ or the selector is disconnected. The MAX8731A guarantees that CSIN does
not regulate more than 250mV above the selected
charging voltage. This also limits the extent to which
remote sense can cancel charge-path impedance.
Input Current Measurement
Use IINP to monitor the system-input current sensed
across CSSP and CSSN. The voltage at IINP is proportional to the input current by the equation:
VIINP = IINPUT x RS1 x GIINP x R8
where I INPUT is the DC current supplied by the AC
adapter, GIINP is the transconductance of IINP (3mA/V
typ), and R8 is the resistor connected between IINP
and ground. Typically, IINP has a 0 to 3.5V output voltage range. Leave IINP open if not used.
LDO Regulator
An integrated low-dropout (LDO) linear regulator provides a 5.4V supply derived from DCIN, and delivers over
30mA of load current. The LDO powers the gate drivers
of the n-channel MOSFETs. See the MOSFET Drivers
section. LDO has a minimum current limit of 35mA. This
allows the MAX8731A to work with 87nC of total gate
charge (both high-side and low-side MOSFETs). Bypass
LDO to PGND with a 1µF or greater ceramic capacitor.
AC Adapter Detection
The MAX8731A includes a hysteretic comparator that
detects the presence of an AC power adapter. When
ACIN is greater than 2.048V, the open-drain ACOK output becomes high impedance. Connect 10kΩ pullup
resistance between LDO and ACOK. Use a resistive
voltage-divider from the adapter’s output to the ACIN
pin to set the appropriate detection threshold. Select
the resistive voltage-divider not to exceed the 6V
absolute maximum rating of ACIN.
VDD Supply
The VDD input provides power to the SMBus interface.
Connect VDD to LDO, or apply an external supply to
VDD to keep the SMBus interface active while the supply to DCIN is removed. When VDD is biased the internal registers are maintained. Bypass VDD to GND with
a 0.1µF or greater ceramic capacitor.
Operating Conditions
The MAX8731A has the following operating states:
• Adapter Present: When DCIN is greater than 7.5V,
the adapter is considered to be present. In this condition, both the LDO and REF function properly and
battery charging is allowed:
a) Charging: The total MAX8731A quiescent current
when charging is 1mA (max) plus the current required
to drive the MOSFETs.
b) Not Charging: To disable charging, set either
ChargeCurrent() or ChargeVoltage() to zero. When the
adapter is present and charging is disabled, the total
adapter quiescent current is less than 1mA and the
total battery quiescent current is less than 5µA.
• Adapter Absent (Power Fail): When VCSSP is less
than VCSIN + 10mV, the MAX8731A is in the powerfail state, since the DC-DC converter is in dropout.
The charger does not attempt to charge in the
power-fail state. Typically, this occurs when the
adapter is absent. When the adapter is absent, the
total MAX8731A quiescent battery current is less
than 1µA (max).
• VDD Undervoltage (POR): When VDD is less than
2.5V, the VDD supply is in an undervoltage state and
the internal registers are in their POR state. The
SMBus interface does not respond to commands.
When VDD rises above 2.5V, the MAX8731A is in a
power-on reset state. Charging does not occur until
the ChargeVoltage() and ChargeCurrent() commands are sent. When V DD is greater than 2.5V,
SMBus registers are preserved.
The MAX8731A allows charging under the following
conditions:
1) DCIN > 7.5V, LDO > 4V, REF > 3.1V
2) VCSSP > VCSIN + 210mV (15mV falling threshold)
3) VDD > 2.5V
______________________________________________________________________________________
19
MAX8731A
where RPack is the total resistance in the battery pack,
RBoard is the board resistance in series with the battery
charge path, tCV0 is the constant-voltage charge time
without remote sense, and tCVRS is the constant-voltage charge time with remote sense.
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
SMBus Interface
The MAX8731A receives control inputs from the SMBus
interface. The MAX8731A uses a simplified subset of
the commands documented in System Management
Bus Specification V1.1, which can be downloaded from
www.smbus.org. The MAX8731A uses the SMBus
Read-Word and Write-Word protocols (Figure 3) to
communicate with the smart battery. The MAX8731A
performs only as an SMBus slave device with address
0b0001001_ (0x12) and does not initiate communication on the bus. In addition, the MAX8731A has two
identification (ID) registers (0xFE): a 16-bit device ID
register and a 16-bit manufacturer ID register (0xFF).
The data (SDA) and clock (SCL) pins have Schmitt-trigger inputs that can accommodate slow edges. Choose
pullup resistors (10kΩ) for SDA and SCL to achieve rise
times according to the SMBus specifications.
Communication starts when the master signals a
START condition, which is a high-to-low transition on
SDA, while SCL is high. When the master has finished
communicating, the master issues a STOP condition,
which is a low-to-high transition on SDA, while SCL is
high. The bus is then free for another transmission.
Figures 4 and 5 show the timing diagram for signals on
the SMBus interface. The address byte, command
byte, and data bytes are transmitted between the
START and STOP conditions. The SDA state changes
only while SCL is low, except for the START and STOP
conditions. Data is transmitted in 8-bit bytes and is
sampled on the rising edge of SCL. Nine clock cycles
are required to transfer each byte in or out of the
MAX8731A because either the master or the slave
acknowledges the receipt of the correct byte during the
ninth clock cycle. The MAX8731A supports the charger
commands as described in Table 4.
a) WRITE-WORD FORMAT
S
SLAVE
W
ADDRESS
7 BITS
1b
MSB
LSB 0
PRESET TO
0b0001001
ACK
1b
0
b) READ-WORD FORMAT
S
SLAVE
W
ADDRESS
7 BITS
1b
MSB
LSB 0
Preset to
0b0001001
ACK
1b
0
COMMAND
BYTE
8 BITS
MSB
LSB
ChargerMode() = 0x12
ChargeCurrent() = 0x14
ChargeVoltage() = 0x15
AlarmWarning() = 0x16
InputCurrent() = 0x3F
ACK
1b
0
COMMAND
ACK
BYTE
8 BITS
1b
MSB
LSB
0
ChargerSpecInfo() = 0x11
ChargerStatus() = 0x13
LEGEND:
S = START CONDITION OR REPEATED START CONDITION
ACK = ACKNOWLEDGE (LOGIC-LOW)
W = WRITE BIT (LOGIC-LOW)
LOW DATA
ACK
BYTE
8 BITS
1b
MSB
LSB
0
D7
D0
S
SLAVE
ADDRESS
7 BITS
MSB
LSB
PRESET TO
0b0001001
HIGH DATA
ACK P
BYTE
8 BITS
1b
MSB
LSB
0
D15
D8
R
ACK
1b
1
1b
0
LOW DATA
ACK
BYTE
8 BITS
1b
MSB
LSB
0
D7
D0
HIGH DATA
NACK P
BYTE
8 BITS
1b
MSB
LSB
1
D15
D8
P = STOP CONDITION
NACK = NOT ACKNOWLEDGE (LOGIC-HIGH)
R = READ BIT (LOGIC-HIGH)
MASTER TO SLAVE
SLAVE TO MASTER
Figure 3. SMBus Write-Word and Read-Word Protocols
20
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
tLOW
B
tHIGH
C
E
D
F
G
I
H
J
K
L
MAX8731A
A
M
SMBCLK
SMBDATA
tSU:STA
tHD:STA
tSU:DAT
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW
tHD:DAT
tHD:DAT
tSU:STO tBUF
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW START CONDITION
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW
Figure 4. SMBus Write Timing
A
B
tLOW
C
D
E
F
G
H
tHIGH
J
I
K
SMBCLK
SMBDATA
tSU:STA tHD:STA
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
tSU:DAT
tHD:DAT
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
tSU:DAT
tSU:STO
tBUF
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
Figure 5. SMBus Read Timing
______________________________________________________________________________________
21
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
Table 4. Battery-Charger Command Summary
COMMAND
COMMAND NAME
READ/WRITE
POR STATE
0x14
ChargeCurrent()
Write Only
6-Bit Charge-Current Setting
0x15
ChargeVoltage()
Write Only
11-Bit Charge-Voltage Setting
0x0000
0x3F
InputCurrent()
Write Only
6-Bit Charge-Current Setting
0x0080
0xFE
ManufacturerID()
Read Only
Manufacturer ID
0x004D
0xFF
DeviceID()
Read Only
Device ID
0x0008
Battery-Charger Commands
The MAX8731A supports four battery-charger commands that use either Write-Word or Read-Word protocols, as summarized in Table 4. ManufacturerID() and
DeviceID() can be used to identify the MAX8731A. On
the MAX8731A, the ManufacturerID() command always
returns 0x004D and the DeviceID() command always
returns 0x0008.
DC-DC Converter
The MAX8731A employs a synchronous step-down DCDC converter with an n-channel high-side MOSFET
switch and an n-channel low-side synchronous rectifier.
The MAX8731A features a pseudo-fixed-frequency, current-mode control scheme with cycle-by-cycle current
limit. The controller’s constant off-time (tOFF) is calculated based on VCSSP, VCSIN, and a time constant with a
minimum value of 300ns. The MAX8731A can also operate in discontinuous-conduction mode for improved
light-load efficiency. The operation of the DC-DC controller is determined by the following four comparators
as shown in the functional diagrams in Figures 2 and 6:
FBS_
ChargeVoltage ( )
+200mV
IMAX
2V
CCMP
R
Q
DH
DRIVER
IMIN
S
Q
ZCMP
150mV
DCIN
CSIN
OFF-TIME
COMPUTE
The IMIN comparator triggers a pulse in discontinuous
mode when the accumulated error is too high. IMIN
compares the control signal (LVC) against 100mV (typ).
When LVC is less than 100mV, DHI and DLO are both
forced low. Indirectly, IMIN sets the peak inductor current in discontinuous mode.
The CCMP comparator is used for current-mode regulation in continuous-conduction mode. CCMP compares LVC against the inductor current. The high-side
MOSFET on-time is terminated when the CSI voltage is
higher than LVC.
The IMAX comparator provides a secondary cycle-bycycle current limit. IMAX compares CSI to 2V (corresponding to 10A when RS2 = 10mΩ). The high-side
MOSFET on-time is terminated when the current-sense
signal exceeds 10A. A new cycle cannot start until the
IMAX comparator’s output goes low.
The ZCMP comparator provides zero-crossing detection during discontinuous conduction. ZCMP compares
the current-sense feedback signal to 750mA (RS2 =
10mΩ). When the inductor current is lower than the
750mA threshold, the comparator output is high and
DLO is turned off.
CCV, CCI, CCS, and LVC Control Blocks
LVC
100mV
0x0000
The OVP comparator is used to prevent overvoltage at
the output due to battery removal. OVP compares FBS_
against the set voltage (ChargeVoltage()). When FBS_
is 200mV above the set value, the OVP comparator output goes high and the high-side MOSFET on-time is terminated. DHI and DLO remain off until the OVP
condition is removed.
OVP
CSI
OFF-TIME
ONE-SHOT
Figure 6. DC-DC Converter Functional Diagram
22
DESCRIPTION
DL
DRIVER
The MAX8731A controls input current (CCS control
loop), charge current (CCI control loop), or charge voltage (CCV control loop), depending on the operating
condition. The three control loops—CCV, CCI, and
CCS—are brought together internally at the lowest voltage-clamp (LVC) amplifier. The output of the LVC
amplifier is the feedback control signal for the DC-DC
controller. The minimum voltage at the CCV, CCI, or
CCS appears at the output of the LVC amplifier and
clamps the other control loops to within 0.3V above the
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
Continuous-Conduction Mode
With sufficient charge current, the MAX8731A’s inductor current never crosses zero, which is defined as continuous-conduction mode. The regulator switches at
400kHz (nominal) if VCSIN < 0.88 x VCSSP. The controller starts a new cycle by turning on the high-side
MOSFET and turning off the low-side MOSFET. When
the charge-current feedback signal (CSI) is greater
than the control point (LVC), the CCMP comparator output goes high and the controller initiates the off-time by
turning off the high-side MOSFET and turning on the
low-side MOSFET. The operating frequency is governed by the off-time and is dependent upon VCSIN and
VCSSP. The off-time is set by the following equation:
−V
V
t OFF = 2.5μs × CSSP CSIN
VCSSP
The on-time can be determined using the following
equation:
t ON =
L × IRIPPLE
VCSSN − VBATT
where:
V
×t
IRIPPLE = BATT OFF
L
The switching frequency can then be calculated:
fSW =
1
t ON + t OFF
These equations describe the controller’s pseudofixed-frequency performance over the most common
operating conditions.
At the end of the fixed off-time, the controller initiates a
new cycle if the control point (LVC) is greater than
100mV and the peak charge current is less than the
cycle-by-cycle current limit. Restated another way,
IMIN must be high, IMAX must be low, and OVP must
be low for the controller to initiate a new cycle. If the
peak inductor current exceeds the IMAX comparator
threshold or the output voltage exceeds the OVP
threshold, then the on-time is terminated. The cycle-bycycle current limit effectively protects against overcurrent and short-circuit faults.
If during the off-time the inductor current goes to zero,
the ZCMP comparator output pulls high, turning off the
low-side MOSFET. Both the high- and low-side
MOSFETs are turned off until another cycle is ready to
begin. ZCOMP causes the MAX8731A to enter into discontinuous-conduction mode (see the Discontinuous
Conduction section).
There is a 0.3µs minimum off-time when the (VCSSP VCSIN) differential becomes too small. If VCSIN ≥ 0.88 x
VCSSP, the threshold for the 0.3µs minimum off-time is
reached. The switching frequency in this mode varies
according to the equation:
f=
1
L × IRIPPLE
+ 0.3μs
VCSSN − VBATT
Discontinuous Conduction
The MAX8731A can also operate in discontinuous-conduction mode to ensure that the inductor current is
always positive. The MAX8731A enters discontinuousconduction mode when the output of the LVC control
point falls below 100mV. This corresponds to peak
inductor current = 500mA:
100mV
ICHG = 1 ×
= 250mA
2 20 × RS2
charge current for RS2 = 10mΩ.
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 100mV. Discontinuousmode operation can occur during conditioning charge
of overdischarged battery packs, when the charge current has been reduced sufficiently by the CCS control
loop, or when the charger is in constant-voltage mode
with a nearly full battery pack.
______________________________________________________________________________________
23
MAX8731A
control point. Clamping the other two control loops
close to the lowest control loop ensures fast transition
with minimal overshoot when switching between different control loops (see the Compensation section).
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
Compensation
The charge-voltage and charge-current regulation
loops are independent and compensated separately at
the CCV, CCI, and CCS.
CCV Loop Compensation
The simplified schematic in Figure 7 is sufficient to
describe the operation of the MAX8731A when the voltage loop (CCV) is in control. The required compensation network is a pole-zero pair formed with CCV and
RCV. The zero is necessary to compensate the pole
formed by the output capacitor and the load. RESR is
the equivalent series resistance (ESR) of the charger
output capacitor (COUT). RL is the equivalent charger
output load, where RL = ΔVBATT / ΔICHG. The equivalent output impedance of the GMV amplifier, ROGMV, is
FBS_
RESR
COUT
GMV
RCV
ROGMV
ChargeVoltage( )
CCV
GMOUT =
1
A CSI × RS2
where ACSI = 20V/V, and RS2 = 10mΩ in the typical
application circuits, so GMOUT = 5A/V. The loop-transfer function is given by:
LTF = GMOUT × RL × GMV × ROGMV
(1 + sCOUT × RESR )(1 + sCCV × RCV )
×
(1 + sCCV × ROGMV )(1 + sCOUT × RL )
The poles and zeros of the voltage loop-transfer function
are listed from lowest frequency to highest frequency in
Table 5.
GMOUT
CCV
greater than 10MΩ. The voltage amplifier transconductance, GMV = 0.125µA/mV. The DC-DC converter
transconductance is dependent upon the charge-current sense resistor RS2:
RL
Near crossover CCV is much lower impedance than
ROGMV. Since CCV is in parallel with ROGMV, CCV dominates the parallel impedance near crossover.
Additionally, RCV is much higher impedance than CCV
and dominates the series combination of RCV and CCV,
so near crossover:
ROGMV × (1+ sCCV × RCV )
≅ RCV
(1+ sCCV × ROGMV )
Figure 7. CCV Loop Diagram
Table 5. CCV Loop Poles and Zeros
NAME
CCV Pole
EQUATION
f P _ CV =
1
2πROGMV × CCV
DESCRIPTION
Lowest frequency pole created by CCV and GMV’s finite output resistance.
Voltage-loop compensation zero. If this zero is at the same frequency or
lower than the output pole fP_OUT, then the loop-transfer function
approximates a single-pole response near the crossover frequency. Choose
CCV to place this zero at least 1 decade below crossover to ensure
adequate phase margin.
CCV Zero
f Z _ CV =
1
2πRCV × CCV
Output
Pole
f P _ OUT =
1
2πRL × COUT
Output pole formed with the effective load resistance RL and the output
capacitance COUT. RL influences the DC gain but does not affect the
stability of the system or the crossover frequency.
Output
Zero
f P _ OUT =
1
2πRL × COUT
Output ESR Zero. This zero can keep the loop from crossing unity gain if
fZ_OUT is less than the desired crossover frequency; therefore, choose a
capacitor with an ESR zero greater than the crossover frequency.
24
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
1
RL ⋅
≅
(1+ sCOUT × RL ) sCOUT
If RESR is small enough, its associated output zero has
a negligible effect near crossover and the loop-transfer
function can be simplified as follows:
RCV
LTF = GMOUT ×
GMV
sCOUT
Setting LTF = 1 to solve for the unity-gain frequency
yields:
fCO _ CV = GMOUT × GMV ×
RCV
2π × COUT
For stability, choose a crossover frequency lower than
1/10 the switching frequency. For example, choose a
crossover frequency of 50kHz and solve for RVC using
the component values listed in Figure 1 to yield RCV =
10kΩ:
RCV =
2π × COUT × fCO _ CV
GMV × GMOUT
≅ 10kΩ
GMV = 0.125µA/mV
80
GMOUT = 5A/V
COUT = 2 x 10µF
FOSC = 400kHz
RL = 0.2Ω
FCO_CV = 50kHz
To ensure that the compensation zero adequately cancels the output pole, select fZ_CV ≤ fP_OUT:
CCV ≥ (RL / RCV) COUT
C CV ≥ 400pF (assuming 2 cells and 2A maximum
charge current.)
Figure 8 shows the Bode plot of the voltage-loop frequency response using the values calculated above.
CCI Loop Compensation
The simplified schematic in Figure 9 is sufficient to
describe the operation of the MAX8731A when the battery current loop (CCI) is in control. Since the output
capacitor’s impedance has little effect on the response
of the current loop, only a simple single pole is required
to compensate this loop. ACSI is the internal gain of the
current-sense amplifier. RS2 is the charge currentsense resistor (10mΩ). ROGMI is the equivalent output
impedance of the GMI amplifier, which is greater than
10MΩ. GMI is the charge-current amplifier transconductance = 1µA/mV. GMOUT is the DC-DC converter
transconductance = 5A/V.
0
CSIP
60
CSIN
GMOUT
-45
20
0
-90
PHASE (DEGREES)
MAGNITUDE (dB)
RS2
40
CSI
CCI
-20
GMI
MAG
PHASE
-40
0.1
1
10
100
1k
10k
100k
-135
1M
CCI
ROGMI
ChargeCurrent( )
FREQUENCY (Hz)
Figure 8. CCV Loop Response
Figure 9. CCI Loop Diagram
______________________________________________________________________________________
25
MAX8731A
C OUT is also much lower impedance than RL near
crossover so the parallel impedance is mostly capacitive and:
The loop-transfer function is given by:
LTF = GMOUT × A CSI × RS2 × GMI
ROGMI
1+ sROGMI × CCI
This describes a single-pole system. Since:
1
GMOUT =
A CSI × RS2
the loop-transfer function simplifies to:
LTF = GMI
ROGMI
1+ sROGMI × CCI
The crossover frequency is given by:
GMI
fCO _ CI =
2πCCI
CCS Loop Compensation
The simplified schematic in Figure 11 is sufficient to
describe the operation of the MAX8731A when the
input current-limit loop (CCS) is in control. Since the
output capacitor’s impedance has little effect on the
response of the input current-limit loop, only a single
pole is required to compensate this loop. ACSS is the
internal gain of the current-sense resistor; RS1 = 10mΩ
in the typical application circuits. ROGMS is the equivalent output impedance of the GMS amplifier, which is
greater than 10MΩ. GMS is the charge-current amplifier
transconductance = 1µA/mV. GMIN is the DC-DC converter’s input-referred transconductance = (1/D) x
GMOUT = (1 / D) x 5A/V.
The loop-transfer function is given by:
LTF = GMIN × ACSS × RSI × GMS
ROGMS
1 + SROGMS × CCS
For stability, choose a crossover frequency lower than
1/10 the switching frequency:
Since:
CCI > 10 × GMI / (2π fOSC) = 4nF, for a 400kHz switching frequency.
the loop-transfer function simplifies to:
Values for CCI greater than 10 times the minimum value
can slow down the current-loop response. Choosing CCI
= 10nF yields a crossover frequency of 15.9kHz. Figure
10 shows the Bode plot of the current-loop frequency
response using the values calculated above.
100
GMIN =
LTF = GMS
1
A CSS × RS2
ROGMS
1 + SROGMS × CCS
ADAPTER
INPUT
0
MAG
PHASE
80
MAGNITUDE (dB)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
CSSP
60
InputCurrent( )
CSS
40
RS1
CSSI
-45
GMS
20
0
CCS
-20
-40
-90
0.1
10
1k
GMIN
CCS
ROGMS
100k
FREQUENCY (Hz)
Figure 10. CCI Loop Response
26
Figure 11. CCS Loop Diagram
______________________________________________________________________________________
SYSTEM
LOAD
SMBus Level 2 Battery Charger
with Remote Sense
fCO _ CS =
the device). Unlike the DLO output, the DHI output uses
a 50ns (typ) delay time to prevent the low-side MOSFET
from turning on until DHI is fully off. The same considerations should be used for routing the DHI signal to the
high-side MOSFET.
GMS
2πCCS
For stability, choose a crossover frequency lower than
1/10 the switching frequency:
CCS = 5 × GMS /(2πfOSC )
Choosing a crossover frequency of 30kHz and using
the component values listed in Figure 1 yields CCS >
5.4nF. Values for CCS greater than 10 times the minimum value may slow down the current-loop response
excessively. Figure 12 shows the Bode plot of the input
current-limit-loop frequency response using the values
calculated above.
MOSFET Drivers
The DHI and DLO outputs are optimized for driving
moderate-sized power MOSFETs. The MOSFET drive
capability is the same for both the low-side and highsides switches. This is consistent with the variable duty
factor that occurs in the notebook computer environment where the battery voltage changes over a wide
range. There must be a low-resistance, low-inductance
path from the DLO driver to the MOSFET gate to prevent shoot-through. Otherwise, the sense circuitry in the
MAX8731A interprets the MOSFET gate as “off” while
there is still charge left on the gate. Use very short,
wide traces measuring 10 to 20 squares or less
(1.25mm to 2.5mm wide if the MOSFET is 25mm from
100
0
MAGNITUDE (dB)
80
60
40
-45
20
0
-20
-40
0.1
10
1k
FREQUENCY (Hz)
100k
-90
10M
PHASE (DEGREES)
MAG
PHASE
The high-side driver (DHI) swings from LX to 5V above
LX (BST) and has a typical impedance of 3Ω sourcing
and 1Ω sinking. The low-side driver (DLO) swings from
DLOV to ground and has a typical impedance of 1Ω
sinking and 3Ω sourcing. This helps prevent DLO from
being pulled up when the high-side switch turns on, due
to capacitive coupling from the drain to the gate of the
low-side MOSFET. This places some restrictions on the
MOSFETs that can be used. Using a low-side MOSFET
with smaller gate-to-drain capacitance can prevent
these problems.
Design Procedure
MOSFET Selection
Choose the n-channel MOSFETs according to the maximum required charge current. The MOSFETs must be
able to dissipate the resistive losses plus the switching
losses at both VDCIN(MIN) and VDCIN(MAX).
For the high-side MOSFET, the worst-case resistive
power losses occur at the maximum battery voltage
and minimum supply voltage:
PDCONDUCTION(HighSide) =
VFBS _
VCSSP
× ICHG2 × RDS(ON)
Generally a low-gate charge high-side MOSFET is preferred to minimize switching losses. However, the
RDS(ON) required to stay within package power-dissipation limits often limits how small the MOSFET can be.
The optimum occurs when the switching (AC) losses
equal the conduction (RDS(ON)) losses. Calculating the
power dissipation in N1 due to switching losses is difficult since it must allow for difficult quantifying factors
that influence the turn-on and turn-off times. These factors include the internal gate resistance, gate charge,
threshold voltage, source inductance, and PCB layout
characteristics. The following switching-loss calculation
provides a rough estimate and is no substitute for
breadboard evaluation, preferably including a verification using a thermocouple mounted on N1:
PDSWITCHING (High Side) =
1
× t Trans × VDCIN × ICHG × fSW
2
Figure 12. CCS Loop Response
______________________________________________________________________________________
27
MAX8731A
The crossover frequency is given by:
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
where tTRANS is the driver’s transition time and can be
calculated as follows:
⎛ 1
1 ⎞ 2QG
t TRANS = ⎜
+
, and fSW ≈ 400kHz
⎟×
⎝ IGsrc IGsnk ⎠ IGATE
IGATE is the peak gate-drive current.
The following is the power dissipated due to the highside n-channel MOSFET’s output capacitance (CRSS):
V2
× CRSS × fSW
PDCOSS (HighSide) ≈ DCIN
2
The total power low-side MOSFET dissipation is:
PDTOTAL (Low Side) ≈ PDCONDUCTION(Low Side)
+ PDBDY (Low Side)
These calculations provide an estimate and are not a substitute for breadboard evaluation, preferably including a
verification using a thermocouple mounted on the MOSFET.
Inductor Selection
The charge current, ripple, and operating frequency
(off-time) determine the inductor characteristics. For
optimum efficiency, choose the inductance according
to the following equation:
The following high-side MOSFET’s loss is due to the
reverse-recovery charge of the low-side MOSFET’s
body diode:
PDQRR(HighSide) = QRR2 x VDCIN x fSW x 0.5
Ignore PDQRR (HighSide) if a Schottky diode is used
parallel to the low-side MOSFET.
The total high-side MOSFET power dissipation is:
PDTOTAL (HighSide) ≈ PDCONDUCTION(HighSide)
+ PDSWITCHING (HighSide) + PDCOSS (HighSide)
+PDQRR (HighSide)
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied. If the high-side MOSFET chosen
for adequate RDS(ON) at low-battery voltages becomes
hot when biased from V IN(MAX), consider choosing
another MOSFET with lower parasitic capacitance. For
the low-side MOSFET (N2), the worst-case power dissipation always occurs at maximum input voltage:
⎛ VFBS _ ⎞
PDCONDUCTION(Low Side) = ⎜1−
⎟
⎝ VCSSP ⎠
× ICHG2 × RDS(ON)
The following additional loss occurs in the low-side
MOSFET due to the body diode conduction losses:
V
×t
L = BATT OFF
0.3 × ICHG
This sets the ripple current to 1/3 the charge current
and results in a good balance between inductor size
and efficiency. Higher inductor values decrease the ripple current. Smaller inductor values save cost but
require higher saturation current capabilities and
degrade efficiency.
Inductor L1 must have a saturation current rating of at
least the maximum charge current plus 1/2 the ripple
current (ΔIL):
ISAT = ICHG + (1/2) ΔIL
The ripple current is determined by:
ΔIL = VBATT × tOFF / L
where:
tOFF = 2.5µs (VDCIN - VBATT) / VDCIN for VBATT < 0.88
VDCIN
or during dropout:
PDBDY (Low Side) = 0.05 × IPEAK × 0.4V
tOFF = 0.3µs for VBATT > 0.88 VDCIN
28
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
⎛ V
⎞
BATT ( VDCIN − VBATT )
⎟
IRMS = ICHG ⎜
⎜
⎟
VDCIN
⎝
⎠
The input capacitors should be sized so that the temperature rise due to ripple current in continuous conduction
does not exceed approximately +10°C. The maximum
ripple current occurs at 50% duty factor or VDCIN = 2 x
VBATT, which equates to 0.5 x ICHG. If the application of
interest does not achieve the maximum value, size the
input capacitors according to the worst-case conditions.
SYSTEM
POWER
SUPPLIES
AC-TO-DC
CONVERTER
(ADAPTER)
MAX8731A
SMART-BATTERY
CHARGER/
POWER-SOURCE
SELECTOR
MAX8731A
Input-Capacitor Selection
The input capacitor must meet the ripple current
requirement (IRMS) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or
OS-CON) are preferred due to their resilience to powerup surge currents:
BATT+
BATT-
SMART
BATTERY
SMBus
CONTROL
SIGNALS
FOR
BATTERY
SMBus
CONTROL
SIGNALS
FOR
BATTERY
SYSTEM HOST
(KEYBOARD CONTROLLER)
Output-Capacitor Selection
The output capacitor absorbs the inductor ripple current
and must tolerate the surge current delivered from the
battery when it is initially plugged into the charger. As
such, both capacitance and ESR are important parameters in specifying the output capacitor as a filter and to
ensure stability of the DC-DC converter (see the
Compensation section). Beyond the stability requirements, it is often sufficient to make sure that the output
capacitor’s ESR is much lower than the battery’s ESR.
Either tantalum or ceramic capacitors can be used on the
output. Ceramic devices are preferable because of their
good voltage ratings and resilience to surge currents.
Applications Information
Smart-Battery System
Background Information
Smart-battery systems have evolved since the conception of the smart-battery system (SBS) specifications.
Originally, such systems consisted of a smart battery
and smart-battery charger that were compatible with the
SBS specifications and communicated directly with one
another using SMBus protocols. Modern systems still
employ the original commands and protocols, but often
use a keyboard controller or similar digital intelligence to
mediate the communication between the battery and the
charger (Figure 13). This arrangement permits considerable freedom in the implementation of charging algorithms at the expense of standardization. Algorithms can
vary from the simple detection of the battery with a fixed
set of instructions for charging the battery to highly complex programs that can accommodate multiple battery
Figure 13. Typical Smart-Battery System
configurations and chemistries. Microcontroller programs can perform frequent tests on the battery’s state
of charge and dynamically change the voltage and current applied to enhance safety. Multiple batteries can
also be utilized with a selector that is programmable over
the SMBus.
Setting Input-Current Limit
The input-current limit should be set based on the current capability of the AC adapter and the tolerance of
the input-current limit. The upper limit of the input current threshold should never exceed the adapter’s minimum available output current. For example, if the
adapter’s output current rating is 5A ±10%, the input
current limit should be selected so that its upper limit is
less than 5A × 0.9 = 4.5A. Since the input current-limit
accuracy of the MAX8731A is ±3%, the typical value of
the input-current limit should be set at 4.5A / 1.03 ≈
4.36A. The lower limit for input current must also be
considered. For chargers at the low end of the spec,
the input-current limit for this example could be 4.36A ×
0.95, or approximately 4.14A.
Layout and Bypassing
Bypass DCIN with a 1µF ceramic to ground (Figure 1).
D1 protects the MAX8731A when the DC power source
input is reversed. Bypass VDD, DCIN, LDO, VCC, DAC,
and REF as shown in Figure 1.
______________________________________________________________________________________
29
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
Good PCB layout is required to achieve specified noise
immunity, efficiency, and stable performance. The PCB
layout artist must be given explicit instructions—preferably, a sketch showing the placement of the powerswitching components and high-current routing. Refer to
the PCB layout in the MAX8731A evaluation kit for examples. A ground plane is essential for optimum performance. In most applications, the circuit will be located
on a multilayer board, and full use of the four or more
copper layers is recommended. Use the top layer for
high-current connections, the bottom layer for quiet connections, and the inner layers for uninterrupted ground
planes.
Use the following step-by-step guide:
1) Place the high-power connections first, with their
grounds adjacent:
a) Minimize the current-sense resistor trace
lengths, and ensure accurate current sensing
with Kelvin connections.
b) Minimize ground trace lengths in the high-current paths.
c) Minimize other trace lengths in the high-current
paths.
Use > 5mm wide traces in the high-current
paths.
d) Connect C1 and C2 to high-side MOSFET
(10mm max length). Place the input capacitor
between the input current-sense resistor and
drain of the high-side MOSFET.
e) Minimize the LX node (MOSFETs, rectifier cathode, inductor (15mm max length)). Keep LX on
one side of the PCB to reduce EMI radiation.
f)
Since the return path of DHI is LX, route DHI near
LX. Optimally, LX and DHI should overlap. The
same principle is applied to DLO and PGND.
g) Ideally, surface-mount power components are
flush against one another with their ground terminals almost touching. These high-current
grounds are then connected to each other with a
wide, filled zone of top-layer copper, so they do
30
not go through vias. The resulting top-layer subground plane is connected to the normal innerlayer ground plane at the paddle. Other
high-current paths should also be minimized, but
focusing primarily on short ground and currentsense connections eliminates approximately 90%
of all PCB layout problems.
2) Place the IC and signal components. Keep the
main switching node (LX node) away from sensitive
analog components (current-sense traces and REF
capacitor).
Important: The IC must be no further than 10mm
from the current-sense resistors. Quiet connections
to REF, CCS, DAC, CCV, CCI, ACIN, and VCC
should be returned to a separate ground (GND)
island. The analog ground is separately worked
from power ground in Figure 1. There is very little
current flowing in these traces, so the ground island
need not be very large. When placed on an inner
layer, a sizable ground island can help simplify the
layout because the low-current connections can be
made through vias. The ground pad on the backside of the package should also be connected to
this quiet ground island.
3) Keep the gate-drive traces (DHI and DLO) as short
as possible (L < 20mm), and route them away from
the current-sense lines and REF. These traces
should also be relatively wide (W > 1.25mm).
4) Place ceramic bypass capacitors close to the IC.
The bulk capacitors can be placed further away.
Place the current-sense input filter capacitors under
the part, connected directly to the GND pin.
5) Use a single-point star ground placed directly
below the part at the PGND pin. Connect the power
ground (ground plane) and the quiet ground island
at this location.
Chip Information
TRANSISTOR COUNT: 10,234
PROCESS: BiCMOS
______________________________________________________________________________________
SMBus Level 2 Battery Charger
with Remote Sense
QFN THIN.EPS
PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
21-0140
K
1
2
______________________________________________________________________________________
31
MAX8731A
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
MAX8731A
SMBus Level 2 Battery Charger
with Remote Sense
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE,
16, 20, 28, 32, 40L THIN QFN, 5x5x0.8mm
21-0140
K
2
2
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.
32 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2007 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products. Inc.
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