Max 1538

General DescriptionThe MAX1538 selector provides power-source controlfor dual-battery systems. The device selects betweenan AC adapter and dual batteries based on the pres-ence of the three power sources and the state ofcharge of each battery. The MAX1538 includes analogcomparators to detect AC/airline-adapter presence anddetermine battery undervoltage. Fast analog circuitryallows the device to switch between power sources toimplement a break-before-make time, which allows hotswapping of battery packs. The MAX1538 indepen-dently performs power-source monitoring and selec-tion, freeing the system power-management P forother tasks. This simplifies the development of Ppower-management firmware and allows the P to enterstandby, reducing system power consumption.

The MAX1538 supports relearn mode, which allowsthe system to measure and fully utilize battery capacity.In this state, the part allows the selected battery to bedischarged even when an AC adapter is present. TheMAX1538 can also be used to power the system in anaircraft. On detecting an airline adapter, the MAX1538automatically disables charging or discharging of bat-tery packs and only allows the system to be poweredfrom the adapter.

The MAX1538 is available in a space-saving 28-pin thinQFN package with a maximum footprint of 5mm x 5mm.

ApplicationsNotebook and Subnotebook Computers

Internet Tablets

Dual-Battery Portable Equipment

Features Automatically Detects and Responds to

Low-Battery Voltage ConditionBattery Insertion and RemovalAC-Adapter PresenceAirline-Adapter Presence

Step-Down and Step-Up Charger Compatibility Fast Break-Before-Make Selection

Allows Hot Swapping of Power SourcesNo External Schottky Diodes Needed

50A Maximum Battery Quiescent Current Implements Battery Capacity Relearning Allows Usage of Aircraft Supply Direct Drive of P-Channel MOSFETs Simplifies Power-Management P Firmware 4.75V to 28V AC-Adapter Input Voltage Range Small 28-Pin Thin QFN Package (5mm x 5mm)

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Power-Source Selector for Dual-Battery Systems

________________________________________________________________ Maxim Integrated Products 1

19-3169; Rev 0; 1/04

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxims website at www.maxim-ic.com.

MAX1538

REVBLK

DISA

DISB

BATABATB

CHGB

CHGA

CHGIN

ADPIN

EXTLD

ADPBLK

AIRDET

ACDET

MINVA

MINVB

VDD

CHRGBATSELRELRNOUT2OUT1OUT0

GND

BATSUP

BATTERYCHARGER

CHG_OUT

SYST

EM LO

AD

ADAP

TER

BATT

ERY

B

BATT

ERY

APART TEMP RANGE PIN-PACKAGE

MAX1538ETI -40C to +85C 28 Thin QFN

Ordering Information

Pin Configuration appears at end of data sheet.

Typical Operating Circuit

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2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS(VBATA = VBATB = VCHGIN = 16.8V, CVDD= 1F, VMINVA = VMINVB = 0.93V, VEXTLD = VADPIN = 28V, VCHRG = VBATSEL = VRELRN = 0,CADPPWR = CREVBLK = CADPBLK = CDISBAT = CDISA = CDISB = CCHGA = CCHGB = 4.7nF, TA = 0C to +85C, unless otherwise noted.Typical values are at TA = +25C.)

Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.

VEXTLD, VBATSUP, VADPIN, VBATA, VBATB, VCHGIN to GND .................................................-0.3V to +30V

VADPPWR to GND...................................-0.3V to (VADPIN + 0.3V)VREVBLK, VADPBLK to GND ...................-0.3V to (VEXTLD + 0.3V)VCHGA, VCHGB, VDISBAT to GND ..........-0.3V to (VCHGIN + 0.3V)VDISA to GND..........................................-0.3V to (VBATA + 0.3V)VDISB to GND..........................................-0.3V to (VBATB + 0.3V)VDD, VCHRG, VBATSEL, VRELRN, VOUT0, VOUT1, VOUT2,

VMINVA, VMINVB, VAIRDET, VACDET to GND..........-0.3V to +6V

Continuous Power Dissipation (TA = +70C)28-Pin Thin QFN 5mm x 5mm(derate 20.8mW/C above +70C)..........................1666.7mW

Operating Temperature RangeMAX1538ETI ....................................................-40C to +85C

Junction Temperature ......................................................+150CStorage Temperature Range .............................-65C to +150CLead Temperature (soldering, 10s) .................................+300C

PARAMETER CONDITIONS MIN TYP MAX UNITSADPIN, EXTLD Supply VoltageRange

4.75 28.00 V

CHGIN, BATA, BATB andBATSUP Supply Voltage Range

4.75 19.00 V

VADPIN = highest,VADPPWR = high

21 50

VADPIN = highest,VADPPWR = low

23 54

VBATA = highest,VDISA = high

21 42

VBATA = highest, VDISA = low 24 50

VBATB = highest,VDISB = high

21 42

VBATB = highest, VDISB = low 24 50

ADPIN, BATA, BATB, BATSUPQuiescent Current (Current fromthe Highest Voltage Supply)

VBATA = 4.75V to 19V,VBATB = 4.75V to 19V,VBATSUP = 4.75V to 19V,VADPIN = 4.75V to 28V,no external load at VDD

VBATSUP = highest 18 40

A

VADPPWR = high 0.01 0.5ADPIN Quiescent Current (ADPINCurrent When Not the HighestVoltage)

VADPIN = 4.75V to 18V,no external load at VDD VADPPWR = low 2.6 6

A

VDISA = high 3.9 6.0BATA Quiescent Current (BATACurrent When Not the HighestVoltage)

VBATA = 4.75V to 19V,no external load at VDD VDISA = low 7.0 12

A

VDISB = high 3.9 6.0BATB Quiescent Current (BATBCurrent When Not the HighestVoltage)

VBATB = 4.75V to 19V,no external load at VDD VDISB = low 7.0 12

A

Adapter selected (REVBLK or ADPBLK pins low) 3.0 6.1EXTLD Quiescent Current

Adapter not selected (REVBLK and ADPBLK pins high) 0.02 1.0A

AC or ai r l i ne state ( C H G A, C H GB, and D IS BAT p i ns hi g h) 0.03 1.5

Charge state (CHGA or CHGB pin low, DISBAT pin high) 3.1 6.2CHGIN Quiescent Current

Discharge or relearn state (CHGA or CHGB pin low,DISBAT pin low)

6.1 12.1

A

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ELECTRICAL CHARACTERISTICS (continued)(VBATA = VBATB = VCHGIN = 16.8V, CVDD= 1F, VMINVA = VMINVB = 0.93V, VEXTLD = VADPIN = 28V, VCHRG = VBATSEL = VRELRN = 0,CADPPWR = CREVBLK = CADPBLK = CDISBAT = CDISA = CDISB = CCHGA = CCHGB = 4.7nF, TA = 0C to +85C, unless otherwise noted.Typical values are at TA = +25C.)

PARAMETER CONDITIONS MIN TYP MAX UNITSLINEAR REGULATORVDD Output Voltage IVDD = 0 to 100A 3.270 3.3 3.330 V

VBATA or VBATB = 5V to 19V, VADPIN = 5V 1.0

VBATA = VBATB = 5V, VADPIN = 5V to 28V 1.0VDD Power-Supply RejectionRatio VBATA, VBATB, or VADPIN = 5V to 19V, sawtooth at

10V/s, other supplies = 12V1

mV / V

VDD Undervoltage Lockout Rising edge, relative to regulation point -55 -10 mV

COMPARATORSACDET, AIRDET Input VoltageRange

0 5.5 V

ACDET, AIRDET Input BiasCurrent

VAIRDET = VACDET = 3V 0.1 1 A

ACDET, AIRDET Trip Threshold Input falling 1.97 2.0 2.03 V

ACDET, AIRDET Hysteresis 20 mV

MINV_ Operating Voltage Range 0.93 2.60 V

MINV_ Input Bias Current VMINV_ = 0.93V to 2.6V -50 +50 nA

VMINV_ = 0.93V 4.605 4.65 4.695

VMINV_ = 1.5V 7.455 7.5 7.545BAT_ Minimum Voltage TripThreshold

VBAT_ falling

VMINV_ = 2.6V 12.93 13 13.07

V

BAT_ Minimum VoltageHysteresis

125 mV

BAT_ Pack Removal DetectionThreshold

VBAT_ falling 1.90 2.0 2.10 V

BAT_ Pack Removal Hysteresis 85 mV

GATE DRIVERS (Note 1)

VSOURCE = 15V, VPIN = 7.5V 18 60ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Source Current (PMOSTurn-Off) VSOURCE = 15V, VPIN = 13V 3 15

mA

VSOURCE = 15V, VPIN = 15V 20 70ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Sink Current (PMOSTurn-On) VSOURCE = 15V, VPIN = 9.5V 10 55

mA

VSOURCE = 8V to 19V (ADPPWR, REVBLK, and AOPBLK,VSOURCE = 8V to 28V)

-11.0 -9.0 -7.0ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-On Clamp Voltage(VPIN to VSOURCE) VSOURCE = 4.75V to 8V -8.00 -3.65

V

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ELECTRICAL CHARACTERISTICS (continued)(VBATA = VBATB = VCHGIN = 16.8V, CVDD= 1F, VMINVA = VMINVB = 0.93V, VEXTLD = VADPIN = 28V, VCHRG = VBATSEL = VRELRN = 0,CADPPWR = CREVBLK = CADPBLK = CDISBAT = CDISA = CDISB = CCHGA = CCHGB = 4.7nF, TA = 0C to +85C, unless otherwise noted.Typical values are at TA = +25C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITSADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-On Time

VSOURCE = 15V, VPIN = 13V to VPIN = 9V 0.3 0.88 s

ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-Off Time

VSOURCE = 15V, VPIN = 9V to VPIN = 13V 0.3 0.88 s

STATE SELECTION INPUTSCHRG, BATSEL, RELRN InputLow Voltage

0.8 V

CHRG, BATSEL, RELRN InputHigh Voltage

2.1 V

CHRG, BATSEL, RELRN InputLeakage Current

VCHRG = VBATSEL = VRELRN = 5.5V 0.1 1 A

STATE OUTPUTSVOUT_ = 0.4V 1

OUT0, OUT1, OUT2 Sink CurrentVOUT_ = 5.5V 25

mA

OUT0, OUT1, OUT2 LeakageCurrent

V OU T _ = 5.5V 0.1 1 A

TRANSITION TIMESMINV_ Comparator Delay tMINV VBAT_ = 5.5V to VBAT_ = 4.45V 5.5 11 s

AIRDET and ACDET ComparatorDelay

tADP Falling edge with -20mV overdrive 2.7 6.0 s

BAT_ Removal Comparator Delay Falling edge with -20mV overdrive 10 s

Battery-Insertion Blanking Time tBBLANK 13 21 31 ms

State-Machine Delay 50 ns

MOSFET Turn-On Delay tTRANS 5 7.5 10 s

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_______________________________________________________________________________________ 5

ELECTRICAL CHARACTERISTICS (VBATA = VBATB = VCHGIN = 16.8V, CVDD = 1F, VMINVA = VMINVB = 0.93V, VEXTLD = VADPIN = 28V, VCHRG = VBATSEL = VRELRN = 0,CADPPWR = CREVBLK = CADPBLK = CDISBAT = CDISA = CDISB = CCHGA = CCHGB = 4.7nF, TA = -40C to +85C, unless otherwise noted.)(Note 2)

PARAMETER CONDITIONS MIN MAX UNITSADPIN, EXTLD Supply VoltageRange

4.75 28.00 V

CHGIN, BATA, BATB, andBATSUP Supply Voltage Range

4.75 19.00 V

VADPIN = highest,VADPPWR = high

50

VADPIN = highest,VADPPWR = low

54

VBATA = highest, VDISA = high 42

VBATA = highest, VDISA = low 50

VBATB = highest, VDISB = high 42

VBATB = highest, VDISB = low 50

ADPIN, BATA, BATB, BATSUPQuiescent Current (Current fromthe Highest Voltage Supply)

V B AT A = 4.75V to 19V ,V B AT B = 4.75V to 19V ,V B AT S U P = 4.75V to 19V ,V A D P IN = 4.75V to 28V ,no exter nal l oad at V D D

VBATSUP = highest 40

A

VADPPWR = high 1ADPIN Quiescent Current (ADPINCurrent When Not the HighestVoltage)

VADPIN = 4.75V to 18V,no external load at VDD VADPPWR = low 9

A

VDISA = high 7.5BATA Quiescent Current (BATACurrent When Not the HighestVoltage)

VBATA = 4.75V to 19V,no external load at VDD VDISA = low 16

A

VDISB = high 7.5BATB Quiescent Current (BATBCurrent When Not the HighestVoltage)

VBATB = 4.75V to 19V,no external load at VDD VDISB = low 16

A

Adapter selected (REVBLK or ADPBLK pins low) 9.5EXTLD Quiescent Current

Adapter not selected (REVBLK and ADPBLK pins high) 1.0A

AC or ai r l i ne state ( C H G A, C H GB, and D IS BAT p i ns hi g h) 1.5

Charge state (CHGA or CHGB pin low, DISBAT pin high) 10CHGIN Quiescent Current

Discharge or relearn state (CHGA or CHGB pin low,DISBAT pin low)

18.5

A

LINEAR REGULATORVDD Output Voltage IVDD = 0 to 100A 3.270 3.330 V

VDD Undervoltage Lockout Rising edge, relative to regulation point -60 -10 mV

COMPARATORSACDET, AIRDET Input VoltageRange

0 5.5 V

ACDET, AIRDET Trip Threshold Input falling 1.94 2.06 V

MINV_ Operating Voltage Range 0.93 2.60 V

VMINV_ = 0.93V 4.59 4.72

VMINV_ = 1.5V 7.4 7.6BAT_ Minimum Voltage TripThreshold

VBAT_ falling

VMINV_ = 2.6V 12.86 13.14

V

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PARAMETER SYMBOL CONDITIONS MIN MAX UNITSBAT_ Pack Removal DetectionThreshold

VBAT_ falling 1.88 2.12 V

GATE DRIVERS (Note 1)

VSOURCE = 15V, VPIN = 7.5V 18ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Source Current (PMOSTurn-Off) VSOURCE = 15V, VPIN = 13V 3

mA

VSOURCE = 15V, VPIN = 15V 20ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Sink Current (PMOSTurn-On) VSOURCE = 15V, VPIN = 9.5V 10

mA

V S OU RC E = 8V to 19V ( AD P P W R, RE V BLK,and AD P BLK, V S OU RC E = 8V to 28V )

-11.7 -6.5ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-On Clamp Voltage(VPIN to VSOURCE) VSOURCE = 4.75V to 8V -8.00 -3.50

V

ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-On Time

VSOURCE = 15V, VPIN = 13V to VPIN = 9V 0.88 s

ADPPWR, REVBLK, ADPBLK,DISBAT, DISA, DISB, CHGA,CHGB Turn-Off Time

VSOURCE = 15V, VPIN = 9V to VPIN = 13V 0.88 s

STATE SELECTION INPUTSCHRG, BATSEL, RELRN InputLow Voltage

0.8 V

CHRG, BATSEL, RELRN InputHigh Voltage

2.1 V

STATE OUTPUTSV OU T _ = 0.4V 1

OUT0, OUT1, OUT2 Sink CurrentV OU T _ = 5.5V 25

mA

TRANSITION TIMESMINV_ Comparator Delay tMINV VBAT_ = 5.5V to VBAT_ = 4.45V 11 s

AIRDET and ACDET ComparatorDelay

tADP Falling edge with -20mV overdrive 6 s

Battery-Insertion Blanking Time tBBLANK 12 31 ms

MOSFET Turn-On Delay tTRANS 5 10 s

Note 1: VPIN refers to the voltage of the driver output. VSOURCE refers to the power source for the driver. ADPPWR, REVBLK, ADP-BLK, DISBAT, DISA, DISB, CHGA, and CHGB gate drivers correspond to sources at ADPIN, EXTLD, EXTLD, CHGIN, BATA,BATB, CHGIN, and CHGIN, respectively.

Note 2: Guaranteed by design. Not production tested.

ELECTRICAL CHARACTERISTICS (continued)(VBATA = VBATB = VCHGIN = 16.8V, CVDD = 1F, VMINVA = VMINVB = 0.93V, VEXTLD = VADPIN = 28V, VCHRG = VBATSEL = VRELRN = 0,CADPPWR = CREVBLK = CADPBLK = CDISBAT = CDISA = CDISB = CCHGA = CCHGB = 4.7nF, TA = -40C to +85C, unless otherwise noted.)(Note 2)

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VDD LOAD REGULATION

MAX

1538

toc0

1

VDD LOAD CURRENT (mA)

V DD

(V)

0.150.100.05

3.291

3.292

3.293

3.294

3.295

3.296

3.297

3.298

3.299

3.2900 0.20

VDD vs. TEMPERATURE

MAX

1538

toc0

2

TEMPERATURE (C)

V DD

(V)

6040200-20

3.290

3.295

3.300

3.305

3.310

3.285-40 80

IBAT_ vs. VBAT_

MAX

1538

toc0

4

BATTERY VOLTAGE (V)

BATT

ERY

INPU

T CU

RREN

T (

A)

15105

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

00 20

BAT_ NOT HIGHEST SUPPLY

IBAT_ vs. VBAT_

MAX

1538

toc0

3

BATTERY VOLTAGE (V)

BATT

ERY

INPU

T CU

RREN

T (

A)

14121086

5

10

15

20

25

30

35

04 16

BAT_ HIGHEST SUPPLY

Typical Operating Characteristics(Circuit of Figure 1. TA = +25C, unless otherwise noted.)

ADAPTER INSERTIONMAX1538 toc05

10.0s/div

20V

10V

VADPIN ANDVEXTLD

VADPBLK

VREVBLK

VOUT1

0V

20V

10V

0V

5V

0V

VADPIN

VEXTLDVREVBLK

VADPBLK

tADP

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Typical Operating Characteristics (continued)(Circuit of Figure 1. TA = +25C, unless otherwise noted.)

SOURCE SELECTION CHANGEMAX1538 toc10

2.00s/div

0V

10V

0V5V

VDISB(10V/div)

VDISA(10V/div)

VBATBAC-COUPLED(5V/div)

VBATSEL

VOUT0

10V

20V

10VtTRANSINDUCTIVE KICK

NO CAPACITORAT BATB

BATTERY INSERTIONMAX1538 toc06

5.00ms/div

10V

0V

VDISB(10V/div)

VOUT0(10V/div)

VDISA

VBATA

VEXTLD

0V

10V

10V

0V

20V

10V

SYSTEM LOAD = 3A

VBATB = 16.8VVBATA = 10V

A

B

A: CONTACT BOUNCEB: BATTERY INSERTION BLANKING TIME = 22ms

BATTERY REMOVAL TIMINGMAX1538 toc08

4.00s/div

10V

10.2V

9.8V

VOUT0(10V/div)

VDISB

VDISA

VEXTLD

9.6V

10V

0V

10V

0V

0V

5 x MINV

tMINV

(tADP FORADAPTERREMOVALTIMING)

tTRANS

VBATA = 16.8VBATTERY BREMOVED

SYSTEM LOAD = 3A

SOURCE SELECTION CHANGEMAX1538 toc09

2.00s/div

0V

10V

0V5V

VDISB(10V/div)

VDISA(10V/div)

VBATBAC-COUPLED(5V/div)

VBATSEL

VOUT0

10V

20V

10VtTRANS

INDUCTIVE KICKCBATB = 1F

SYSTEM LOAD = 3A

BATTERY REMOVALMAX1538 toc07

5.00ms/div

10V

0V

VDISB(10V/div)

VOUT0(10V/div)

VDISA

VBATA

VEXTLD

0V

10V

10V

0V

20V

10V

CONTACT BOUNCE

VBATB = 16.8VVBATA = 10V

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PIN NAME FUNCTION

1 MINVAMinimum Battery A Voltage Set Point. Battery A discharge is prevented if VBATA has fallen below 5 xVMINVA.

2 MINVBMinimum Battery B Voltage Set Point. Battery B discharge is prevented if VBATB has fallen below 5 xVMINVB.

3 BATSELBattery-Selection Input. Drive to logic low to charge battery A or give discharge preference to battery A.Drive to logic high to charge battery B or give discharge preference to battery B.

4 RELRN Battery-Relearn Logic-Level Input. Drive RELRN high to enable battery-relearn mode.

5 CHRGCharge-Enable Logic-Level Input. Drive CHRG high to enable the charging path from the charger to thebattery selected by BATSEL.

6 OUT0

7 OUT1

8 OUT2

Selector-State Output. This open-drain output indicates the state of the MAX1538. See Table 1 forinformation on decoding.

9 ACDETAC-Adapter Detection Input. When VACDET is greater than the ACDET trip threshold (2V typ), adapterpresence is detected.

10 AIRDETAirline-Adapter Detection Input. When VAIRDET > 2V and VACDET < 2V, the airline-adapter presence isdetected. Charging is disabled when an airline adapter is detected.

11 ADPINAdapter Input. When VADPIN > VBATSUP, the MAX1538 is powered by ADPIN. ADPIN is the supply rail forthe ADPPWR MOSFET driver.

12 ADPPWR

Adapter-Power P-Channel MOSFET Driver. Connect ADPPWR to the gate of P1 (Figure 1). P1 disconnectsthe adapter from the system during relearn mode. Exclude P1 and leave ADPPWR disconnected if relearnis not used. ADPPWR is driven relative to ADPIN. ADPPWR and REVBLK are driven with the same controlsignal.

13 REVBLKGate Drive for the Reverse-Blocking P-Channel MOSFET. Connect REVBLK to the gate of P2 (Figure 1). P2enables and disables the AC adapters power path. REVBLK is driven relative to EXTLD. REVBLK andADPPWR are driven with the same control signal.

BREAK-BEFORE-MAKE TIMINGMAX1538 toc11

1.00s/div

14V

16V

MOSFETDRIVERS

12V

10V

8V

tTRANS

MOSFETTURN-OFF

TIME MOSFETTURN-ON

TIMEMOSFET FOR INITIAL

DISCHARGE PATH

MOSFET FOR FINALDISCHARGE PATH

FIRST SOURCE INSERTIONMAX1538 toc12

200s/div

0V

0V

0V

5V

VADPIN

VREVBLK

VEXTLD

OUT1OUT2, OUT0

10V

20V

20V

10V

POWER-UP TIME

Typical Operating Characteristics (continued)(Circuit of Figure 1. TA = +25C, unless otherwise noted.)

Pin Description

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10 ______________________________________________________________________________________

PIN NAME FUNCTION

14 ADPBLKGate Drive for the Adapter-Blocking P-Channel MOSFET. Connect ADPBLK to the gate of P3 (Figure 1). P3enables and disables the battery discharge path. ADPBLK is driven relative to EXTLD. ADPBLK andDISBAT are driven with the same control signal.

15, 21 N.C. Not Internally Connected

16 EXTLD External Load. EXTLD is the supply rail for REVBLK and ADPBLK.

17 CHGIN Charger Node Input. CHGIN is the supply rail for DISBAT, CHGA, and CHGB.

18 DISBAT

Gate Drive for the Battery-Discharge P-Channel MOSFET. Connect DISBAT to the gate of P4 (Figure 2). P4disconnects the battery from the system load when charging from a step-up converter. Exclude P4 andleave DISBAT disconnected if using a step-down charger. DISBAT is driven relative to CHGIN. DISBAT andADPBLK are driven by the same control signal.

19 CHGAGate Drive for the Charge Battery A P-Channel MOSFET. Connect CHGA to the gate of P6 (Figure 1). P6enables and disables the charge path into battery A. CHGA is driven relative to CHGIN. CHGA and DISAare driven by the same control signal.

20 CHGBGate Drive for the Charge Battery B P-Channel MOSFET. Connect CHGB to the gate of P7 (Figure 1). P7enables and disables the charge path into battery B. CHGB is driven relative to CHGIN. CHGB and DISBare driven by the same control signal.

22 BATBBattery B Voltage Input. Battery undervoltage and absence is determined by measuring BATB. BATB is thesupply rail for DISB.

23 DISBGate Drive for the Discharge from Battery B P-Channel MOSFET. Connect DISB to the gate of P8 (Figure 1).P8 enables and disables the discharge path from battery B. DISB is driven relative to BATB. DISB andCHGB are driven by the same control signal.

24 DISAGate Drive for the Discharge from Battery A P-Channel MOSFET. Connect DISA to the gate of P5 (Figure 1).P5 enables and disables the discharge path from battery A. DISA is driven relative to BATA. DISA andCHGA are driven by the same control signal.

25 BATABattery A Voltage Input. Battery undervoltage and absence is determined by measuring BATA. BATA is thesupply rail for DISA.

26 BATSUPBATSUP powers the MAX1538. Diode OR BATA and BATB to BATSUP externally. ADPIN is diodeconnected to BATSUP internally. Bypass with a 0.1F capacitor from BATSUP to GND.

27 GND Ground

28 VDD Linear-Regulator Output. Bypass with a 1F capacitor from VDD to GND.

Pin Description (continued)

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______________________________________________________________________________________ 11

MAX1538REVBLK

DISA

DISB

BATA

BATB

CHGB

CHGA

CHGIN

ADPIN

EXTLD

ADPBLK

BATTERY ABATTERY B

AIRDET

ACDET

MINVA

MINVB

VDD

CHRG

BATSEL

RELRN

OUT2

OUT1

OUT0

GNDBATSUP

ADAPTER

ADPPWR

R1 R2 R3

P1

IN

P2

P3

P5

P6P7

P8

STEP-DOWN CHARGER

CSYS

CCHG

C2

FOR RELEARNMODE ONLY

CBATB

CBATA

D1 D2

R10

R11

R12

R13

C10.1F

LOGIC SUPPLY

RSNSSYSTEM LOAD

CHARGER OUTPUT

CADAPTER

C30.1F

CHARGER INPUT

OUT

Figure 1. Step-Down Typical Application Circuit

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MAX1538

REVBLK

DISA

DISB

BATB

CHGB

CHGA

CHGIN

ADPIN

DISBAT

BATTERY ABATTERY B

AIRDET

ACDET

OUT

MINVA

MINVB

GND

VDD

CHRG

BATSEL

RELRN

OUT2

OUT1

OUT0

BATA BATSUP

ADAPTER

ADPPWR

R1 R2 + R3

P1

IN

P2

P4

P5

P6P7

P8

STEP-UP CHARGER

CSYS

C2

FOR RELEARNMODE ONLY

CBATB

CBATA

D1

D2

R10

R11

R12

R13

C11F

LOGIC SUPPLY

CHARGER OUTPUT

CADAPTER

C30.1F

CHARGER INPUT

CCHG

EXTLD

ADPBLKP3

EXTERNAL AC/AIR-DETECTION CIRCUIT

OUTSYSTEM LOAD

Figure 2. Typical Application Circuit for Step-Up/Step-Down Charger

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ACDET

AIRDET

VDD

GND

ADPIN

LDO

REF

BATSUP

2V

MINVA

BATA

R

4R

0.4V

MINVB

BATB

R

4R

0.4V

BATSELRELRN

CHRG

ADPIN

ADPPWR

EXTLD

REVBLK

CHGIN

DISBAT

ADPBLK

CHGA

CHGB

BATA

DISA

BATB

DISB

OUT1OUT2

OUT0

STATEMACHINE

R

S Q

BATTERY BUNDERVOLTAGE

LATCH

MAX1538N N N

Q

R

S Q

BATTERY AUNDERVOLTAGE

LATCH

Q

Figure 3. Functional Diagram


14 ______________________________________________________________________________________

Detailed DescriptionThe MAX1538 performs power path selection betweenan adapter input and two batteries, relieving the hostsystem from the burden of real-time response to power-source changes. The integrated selector implements afixed break-before-make timer to ensure that powersources are not connected together and yet the load isnot left unserviced. The MAX1538 monitors battery andadapter state and presence to determine which sourceto select and whether to charge the battery. Logicinputs CHRG, BATSEL, and RELRN allow the host toenable/disable charging, select which battery to use,and impose battery discharge even with adapter pres-ence. The MAX1538 automatically detects airlineadapters and prevents charging when an airlineadapter is detected. Open-drain logic outputs OUT2,

OUT1, and OUT0 indicate the state of the selector sothe host can properly respond.

The MAX1538 can be configured for use with a step-down battery charger, as shown in Figure 1, or with astep-up/step-down battery charger, as shown in Figure2. The minimum MAX1538 system requires only sixMOSFETs. The MAX1538 provides relearn-mode sup-port with the addition of P1. Relearn mode allows thesystem to relearn the batterys capacity without userintervention.

Table 1 summarizes the possible states and configura-tions of the MAX1538.

SOURCE STATE LOGIC INPUTS MOSFET STATE (See Figure 4)Battery

AdapterA B C

HG

RE

LRN

BA

TSE

L System(ADPPWR

and REVBLK)

Battery(ADPBLK

and DISBAT)

BATT A(CHGA and

DISA)

BATT B(CHGB and

DISB)

OUT

2

OUT

1

OUT

0

STATE

AC X X 1 0 0 On Off On Off 1 1 0 Charge AAC X X 1 0 1 On Off Off On 1 1 1 Charge BAC N X X 1 0 Off On On Off 1 0 0 Relearn AAC X N X 1 1 Off On Off On 1 0 1 Relearn BAC Otherwise On Off Off Off 0 1 0 AC adapter

AIR X X X X X On Off Off Off 0 1 1 Airline

Absent N X X X 0Absent N U X X X

Off On On Off 0 0 0 Discharge A

Absent X N X X 1Absent U N X X X

Off On Off On 0 0 1 Discharge B

Absent U U X X X Off Off Off Off 0 0 0 IdleLegend

AC AC adapter is present. VACDET and VAIRDET are both above 2V.

AIR Airline adapter is present. VACDET is below 2V and VAIRDET is above 2V.

Absent External adapter is absent. VACDET and VAIRDET are both below 2V.

N N indicates the battery is normal. The battery is normal when it has not tripped the undervoltage latch (5 xVMINV_). See the Battery Presence and Undervoltage Detection section.

U U indicates the battery has tripped the undervoltage comparator. An undervoltage battery is detectedwhen VBAT_ goes below 5 x VMINV_. See the Battery Presence and Undervoltage Detection section.

Otherwise Otherwise covers all cases not explicitly shown elsewhere in the table.X X X X X X indicates dont care. The output does not depend on any inputs labeled X.

Table 1. MAX1538 State Table

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Battery Presence and Undervoltage Detection

The MAX1538 determines battery absence and under-voltage and does not allow discharge from an under-voltage battery. A battery is considered undervoltagewhen VBAT_ < 5 x VMINV_, and remains classified asundervoltage until VBAT_ falls below 2V and again risesabove 5 x VMINV. The undervoltage latch is alsocleared when the charge path is enabled. Set the bat-tery undervoltage threshold using resistive voltage-dividers R10, R11, R12, and R13, as shown in Figure 1.The corresponding undervoltage threshold is:

To minimize error, use 1% or better accuracy dividerresistors, and ensure that the impedance of the dividerresults in a current about 100 times the MINV_ inputbias current at the MINV_ threshold voltage. To opti-mize error due to 50nA input bias current at MINV_ andminimize current consumption, typically choose resis-tors (R10 + R11) or (R12 + R13) smaller than 600k.Since batteries often exhibit large changes in their ter-minal voltage when a load current is removed, furtherdischarge after the undervoltage latch has been set is

not allowed until the battery is removed or the chargepath to the battery is selected. Battery removal isdetected when VBAT_ falls below 2V. For correct detec-tion of battery removal, ensure that the leakage currentinto BAT_ is lower than the leakage current out of BAT_so that BAT_ falls below 2V when the battery isremoved. The contributors to leakage current into BAT_are D1, D2, P6, and P7.

Battery Relearn ModeThe MAX1538 implements a battery relearn mode,which allows for host-device manufacturers to imple-ment a mode for coulomb-counting fuel gauges (suchas the MAX1781) to measure battery capacity withoutuser intervention. In battery relearn mode, the ACadapter is switched off and battery discharge is select-ed. In this implementation, the host system couldprompt users when their battery capacity becomesinaccurate, use the host system as a load to dischargethe battery, and then recharge the battery fully.Coulomb-counting fuel-gauge accuracy is increasedafter a relearning cycle.

Battery relearn mode requires the addition of MOSFETP1, which blocks current from the adapter to the sys-tem. To enable relearn mode, drive RELRN high anddrive BATSEL low to relearn battery A or high to relearnbattery B. Relearn mode overrides the functionality ofthe CHG pin. Battery relearn mode does not occurwhen the selected batterys undervoltage latch hasbeen set, or when the selector is in airline mode (seethe Airline Mode and AC Adapter section.) The RELRNpin only applies when an AC adapter is present. If theAC adapter is absent and RELRN is ignored, OUT[2:1]= 10 when the MAX1538 is in battery relearn mode. IfCHG = 0, only OUT2 is needed to indicate that theMAX1538 was properly placed in relearn mode.

If the selected battery trips the undervoltage latch whenin relearn mode, the AC adapter is switched in withoutcausing a crash to the system. OUT2 can indicate thatthe relearn cycle is terminated due to battery undervolt-age. Typically, after the host system performs a batteryrelearn cycle, it either charges the discharged batteryor begins a relearn cycle on the other battery. To switchto charge mode, drive RELRN low and CHG high.Since RELRN overrides CHG, in many applications it isbest to permanently keep CHG high and reduce the IOneeded to control the selector.

When the AC adapter is available, it is used as thepower source for EXTLD unless the RELRN pin is high.In this state, the charger can be enabled and a battery charged.

V VR

R R

V VR

R R

BATA Undervoltage DD

BATB Undervoltage DD

_

_

= +

= +

511

10 11

513

12 13

ADAPTER

ADAPTERSWITCH

SYSTEM

BATTERYSWITCH

CHARGER

"A"SWITCH

"B"SWITCH

BATTERY A

BATTERY B

ADAPTER

ADAPTERSWITCH

SYSTEM

BATTERYSWITCH

CHARGER

"A"SWITCH

"B"SWITCH

BATTERY A

BATTERY B

ADAPTER

ADAPTERSWITCH

SYSTEM

BATTERYSWITCH

CHARGER

"A"SWITCH

"B"SWITCH

BATTERY A

BATTERY B

CHARGE DISCHARGE/RELEARN

AC/AIR

Figure 4. MAX1538 Selection States

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Airline Mode and AC AdapterThe MAX1538 provides compatibility with airlineadapters. For airplane safety, the use of an airlineadapter requires that the battery charger or chargepath is disabled. The MAX1538 disables the chargepath when an airline adapter is detected. In airlinemode, ADPPWR and REVBLK drive P1 and P2 on, andall other MOSFETs are off, regardless of the state ofRELRN, CHG, BATSEL, or the batteries. If the ACthreshold is above the airline threshold, select a resis-tive voltage-divider (as shown in Figure 1) according tothe following equations:

where VACDET_Threshold and VAIRDET_Threshold are typ-ically 2.0V (see the Electrical Characteristics). An ACadapter is detected when the adapter voltage is aboveVAC_Threshold, and an airline adapter is detected when

the adapter voltage is between VAC_Threshold andVAIR_Threshold.

To minimize error, use 1% accuracy or better dividerresistors, and ensure that the impedance of the dividerresults in a current about 100 times the ACDET andAIRDET input bias current. To optimize error due to 1Ainput bias current at ACDET/AIRDET and minimize cur-rent consumption, typically choose R3 less than 20k.See the Adapter Removal Debouncing section for moreinformation regarding R1, R2, and R3. Short R2 to dis-able airline-adapter mode.

Optionally, an external circuit can be implemented todetermine the presence of an AC/airline adapter. Thecircuit in Figure 5 provides fast detection of an airlineadapter, yet allows external circuitry to discriminatebetween airline and AC adapters. If VAC_Threshold

V t t t

I

CV

MINV MINV TRANS ON

SYS MAX

SYSSYS MIN

__

tQV

VI

VI

QV

k

tQV

VI

QV

k

ONG

G OFF OFF

G

G

ONG

G ON

G

G

=

+

=

= =

1

1

2

20 93

50 25

.

.

MAX1538

MAX1908MAX1909 OR

MAX1535 REVBLK

CHGIN

ADPINDCIN

P2

CSYS C2EXTLD

ADPBLKP3

SYSTEM LOAD CSSPCSSN

1F

BATT

ADAPTER

CADAPTER

Figure 7. Combining the MAX1538 with a Charger

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crash, and CSYS is the total system holdup capaci-tance, which does not need to be near the MAX1538.The timing related to the system holdup capacitance isshown in Figure 8.

Charger output capacitance contributes to CSYS for thestep-down charger topology (Figure 1), but not for thestep-up/step-down charger topology (Figure 2).

Leakage Current into BAT_Leakage current into BATA or BATB can interfere withproper battery-removal detection. D1 and D2 must below leakage to ensure that battery removal is properlydetected. Choose MOSFETs P6 and P7 with low off-leakage current. Board leakage current can also be aproblem. For example, neighbor pins BATA and BATSUP should have greater than 50M impedancebetween each other. Proper battery-removal detectionrequires that:

where IBoard is board leakage current, IDS_OFF is theoff-leakage current of MOSFETs P6 and P7, ID_Leakageis the reverse leakage current of the diodes, andIBAT_Sink@2V is the BAT_ leakage current at 2V (0.4A;see the Typical Operating Characteristics).

Inductive KickWhen the adapter or a battery is delivering a significantcurrent to the system and that path is disabled (typical-ly to enable another path), a voltage spike is generatedat the source. This is due to a parasitic inductanceshown in Figure 9. When the adapter is disconnected, apositive voltage spike occurs at ADPIN. When a dis-charging battery is disconnected, a positive voltagespike occurs at BAT_. Connect a capacitor from BAT_or ADPIN to GND to limit this inductive kick. Choose thesource capacitance according to the following equation:

where VSOURCE is the maximum DC voltage of thesource in question, ISYS_MAX is the maximum systemload, and LSOURCE (parasit ic inductance) andCSOURCE are shown in Figure 9.

During battery charge, the voltage spike during batterydisconnect is negative. To ensure that this negativevoltage spike does not go below 0V, choose CBAT_according to the following equation:

CL I

VBAT

BAT CHG MAX

BAT MIN

__ _

_ _

>

2

2

CL I

VSOURCE

SOURCE SYS MAX

SOURCE

_>

2

2 230

I I I I

I IBoard DS OFF P DS OFF P D leakage

D leakage BAT Sink V

+ + + +

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Max 1538