MAX1898EUB41+T [MAXIM]
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MAXIM INTEGRATED PRODUCTS |
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19-2295; Rev 0; 1/02
Linear Charger for Single-Cell Li+ Battery
General Description
Features
The MAX1898, with an external PNP or PMOS transistor,
forms a complete charger for 1-cell lithium-ion (Li+) bat-
teries. The MAX1898 provides accurate constant-cur-
rent/constant-voltage charging. Battery regulation voltage
accuracy is ±±.ꢀ75 for optimum battery performance
and cycle life. Charging current is user set, but is sensed
internally so no external current-sense resistor is needed.
The MAX1898 also provides outputs to monitor charge
status, presence-of-input power source, and charge cur-
rent. Additional features include shutdown, optional
charge-cycle restart without input-power cycling, selec-
table charge termination safety timer, and low-current
preconditioning for deeply discharged cells.
ꢀ Simple, Safe Linear Charger
ꢀ Uses Low-Cost PNP or PMOS Pass Element
ꢀ 4.5V to 12V Input Range
ꢀ Internal Current-Sense Resistor
ꢀ ±±.ꢀ5ꢁ Voltage ꢂAAuraAc
ꢀ Programmable Charging Current
ꢀ ꢂutomatiA Input Power-SourAe DeteAtion
ꢀ LED Charge Status IndiAator
ꢀ Programmable Safetc Timer
ꢀ Current-Sense Monitor Output
ꢀ Optional/ꢂdjustable ꢂuto-Restart
ꢀ Small µMꢂX PaAkage
The MAX1898 offers the maximum integration and func-
tionality in the smallest, most basic application circuit
possible. Very few external components are required.
The MAX1898 offers two versions to safely charge all
Li+ battery chemistries. The battery regulation voltage
is 4.2V for the MAX1898EUB42 and 4.1V for the
MAX1898EUB41. Both are available in a 1±-pin ultra-
thin µMAX package.
Ordering Information
PꢂRT
TEMP RꢂNGE
-4±°C to +87°C
-4±°C to +87°C
PIN-PꢂCKꢂGE
1± µMAX
MAX1898EUB42
MAX1898EUB41
1± µMAX
Applications
Single-Cell Li+ Powered Portables
Self-Charging Battery Packs
PDAs
Typical Operating Circuit
Cell Phones
INPUT
4.5V TO 12V
Cradle Chargers
OUTPUT
IN
CS
LED
Li+
BATTERY
MAX1898
Pin Configuration
CHG
DRV
ON
TOP VIEW
EN/OK
CT
BATT
ISET
OFF
IN
CHG
EN/OK
ISET
1
2
3
4
5
10 CS
9
8
7
6
DRV
CT SETS
CHARGE
TIME
MAX1898
GND
SETS CHARGE
CURRENT
BATT
RSTRT
RSTRT
CT
GND
µMꢂX
________________________________________________________________ Maxim Integrated Products
1
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.
Linear Charger for Single-Cell Li+ Battery
ꢂBSOLUTE MꢂXIMUM RꢂTINGS
Continuous Power Dissipation (T = +ꢀ±°C)
A
IN, DRV, CS, CHG to GND.....................................-±.3V to +14V
BATT, EN, ISET, RSTRT, CT to GND........................-±.3V to +6V
DRV to IN................................................................-14V to +±.3V
IN to CS Current ....................................................................±2A
DRV Continuous Current ..................................................±1±mA
1±-Pin µMAX (derate 7.6mW/°C above +ꢀ±°C)..........444mW
Operating Temperature Range ...........................-4±°C to +87°C
Junction Temperature......................................................+17±°C
Storage Temperature Range.............................-67°C to +17±°C
Lead Temperature (soldering, 1±s) ....................………..+3±±°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.
ELECTRICꢂL CHꢂRꢂCTERISTICS
(V = 8V, V
= 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, T = ±°C to +85°C, unless otherwise noted. Typical values are
ꢂ
IN
BATT
at T = +27°C.)
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Voltage Range
4.5
12
V
Input OK Trip Point, IN Rising
45
30
70
55
100
85
mV
mV
(V - V
)
IN
BATT
Input OK Trip Point, IN Falling
(V - V
)
BATT
IN
V
V
Input OK Trip Point, IN Rising
Input OK Trip Point, IN Falling
4.0
3.9
4.25
4.15
4.5
4.4
V
V
IN
IN
EN/OK Sink Current (When Input
Power Not OK)
V
V
= 3.9V, V
= 4V,
IN
BATT
100
µA
µA
= 0.8V
EN/OK
EN/OK Source Current (When
Input Power OK)
V
= 2V
5
2
10
20
EN/OK
EN/OK Logic Input High
EN/OK Logic Input Low
4.5V ≤ V ≤ 12V
V
V
IN
4.5V ≤ V ≤ 12V
0.6
IN
Done, RSTRT = GND, V
MAX1898EUB42 and 4.0V for MAX1898EUB41
= 4.1V for
BATT
40
4
100
Done, RSTRT = open, V = 4.1V for
MAX1898EUB42 and 4.0V for MAX1898EUB41
BATT
10
BATT Input Current
µA
EN/OK = GND
4
3
2
2
10
10
10
5
V
V
V
= 4V
IN
IN
IN
= V
= 4V (I + I )
CS
µA
BATT
IN
IN, CS Input Current
VOLTAGE LOOP
= 12V, (I + I ), done, RSTRT = open
mA
IN
CS
4.168
4.069
4.232
4.131
MAX1898EUB42
MAX1898EUB41
4.2
4.1
Voltage-Loop Set Point
Voltage-Loop Gain
V
dB
V
∆V
/∆V
DRV
51
BATT
MAX1898EUB42
MAX1898EUB41
2.394
2.337
4
2.520
2.460
2.646
2.583
V
Prequal Voltage Threshold
Timers running
= V - 1V, V = 3.5V
BATT
BATT
DRV Sink Current
V
mA
DRV
IN
DRV Output Resistance
470
Ω
2
_______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery
ELECTRICAL CHARACTERISTICS (continued)
(V = 8V, V
= 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are
A
IN
BATT
at T = +25°C.)
A
PARAMETER
Maximum Voltage (V - V
CONDITIONS
MIN
TYP
MAX
UNITS
)
V = 3.5V
BATT
3.5
7
V
IN
DRV
V
Restart Threshold (As a
BATT
Percentage of Voltage-Loop Set
Point)
RSTRT = GND
92
50
95.25
100
98
%
RSTRT Internal Resistance
CURRENT LOOP
kΩ
Internal Current-Sense Resistance
RMS Charge Current
IN to CS pin-to-pin resistance
105
200
1.4
mΩ
A
Current-Sense Amplifier Gain, I
to I Fast Charge
IN
SET
I
I
= 500mA, V
= 1.4V
0.95
0.73
450
1
1
1.05
1.30
550
mA/A
mA/A
CS
CS
SET
Current-Sense Amplifier Gain, I
= 50mA, V
= 0.14V,
SET
ISET
to I Prequal
V
= 2V
BATT
IN
RSET = 2.8kΩ, V
performance
= 3.5V, closed-loop
BATT
Current-Loop Regulation Set Point
Current-Loop Gain
500
mA
dB
%
∆V
/∆V
26
20
22
DRV
LOAD
LOAD
ISET
I
I
falling, RSET = 2.8kΩ
rising, RSET = 2.8kΩ
15
17
25
27
CHG Trip Threshold (As a
Percentage of Full Load Current)
TIMER FUNCTION
Timer Source/Sink Current
Timer Accuracy
V
= 0.9V
5
5
µA
%
CT
C
V
V
= 100nF for 2 hours, 54 minutes nominal
-10
4
+10
6
CT
CHG Output Low Current
CHG Output High Leakage
CHG Blink Rate
= 1V
mA
µA
Hz
CHG
CHG
= 12V
1
Fault state (50% duty cycle), CT = 100nF
1
1.5
2
ELECTRICAL CHARACTERISTICS
(V = 8V, V
IN
= 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, T = -40°C to +85°C, unless otherwise noted.)
A
BATT
PARAMETER
CONDITIONS
MIN
MAX
UNITS
Supply Voltage Range
4.5
12
V
Input OK Trip Point, IN Rising
45
30
100
mV
mV
(V - V
)
IN
BATT
Input OK Trip Point, IN Falling
(V - V
85
)
BATT
IN
V
V
Input OK Trip Point, IN Rising
Input OK Trip Point, IN Falling
4.0
3.9
4.5
4.4
V
V
IN
IN
EN/OK Sink Current (When Input
Power Not OK)
V
V
= 3.9V, V
= 4V, V = 0.8V
EN/OK
100
µA
µA
IN
BATT
EN/OK Source Current (When Input
Power OK)
= 2V
5
2
20
EN/OK
EN/OK Logic Input High
EN/OK Logic Input Low
4.5V ≤ V ≤12V
V
V
IN
4.5V ≤ V ≤12V
0.6
IN
_______________________________________________________________________________________
3
Linear Charger for Single-Cell Li+ Battery
ELECTRICAL CHARACTERISTICS (continued)
(V = 8V, V
IN
= 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, T = -40°C to +85°C, unless otherwise noted.)
A
BATT
PARAMETER
CONDITIONS
MIN
MAX
UNITS
Done, RSTRT = GND, V
MAX1898EUB42 and 4.0V for MAX1898EUB41
= 4.1V for
BATT
100
Done, RSTRT = open, V
MAX1898EUB42 and 4.0V for MAX1898EUB41
= 4.1V for
BATT
10
BATT Input Current
µA
EN/OK = GND
10
10
10
5
V
V
V
= 4V
IN
IN
IN
= V
= 4V (I + I )
CS
µA
BATT
IN
IN, CS Input Current
VOLTAGE LOOP
= 12V, (I + I ), done, RSTRT = open
mA
IN
CS
MAX1898EUB42
MAX1898EUB41
4.137
4.038
2.394
2.337
3.5
4.263
4.162
2.646
2.583
Voltage-Loop Set Point
V
V
MAX1898EUB42
MAX1898EUB41
V
Prequal Voltage Threshold
Timers running
BATT
DRV Sink Current
Maximum Voltage (V - V
V
V
= V - 1V, V
= 3.5V
mA
V
DRV
IN
BATT
)
= 3.5V
3
7
IN
DRV
BATT
V
Restart Threshold (As a
BATT
Percentage of Voltage-Loop Set
Point)
RSTRT = GND
92
50
98
%
CURRENT LOOP
Internal Current-Sense Resistance
RMS Charge Current
IN to CS pin-to-pin resistance
200
1.4
mΩ
A
Current-Sense Amplifier Gain, I
to I Fast Charge
IN
SET
I
I
= 500mA, V
= 1.4V
0.93
0.6
1.07
1.50
560
mA/A
mA/A
mA
CS
CS
SET
Current-Sense Amplifier Gain, I
= 50mA, V
= 0.14V,
SET
ISET
to I Prequal
V
= 2V
BATT
IN
RSET = 2.8kΩ, V
performance
= 3.5V, closed-loop
BATT
Current-Loop Regulation Set Point
440
I
I
falling, RSET = 2.8kΩ
15
17
25
27
+13
7
LOAD
LOAD
CHG Trip Threshold (As a
Percentage of Full Load Current)
%
rising, RSET = 2.8kΩ
Timer Accuracy
C
V
V
= 100nF for 2 hours, 54 minutes nominal
-13
3.5
%
mA
µA
Hz
CT
CHG Output Low Current
CHG Output High Leakage
CHG Blink Rate
= 1V
CHG
CHG
= 12V
1
Fault state (50% duty cycle), CT = 100nF
1
2
Note 1: Specifications to -40°C are guaranteed by design and not production tested.
4
_______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery
Typical Operating Characteristics
(V = 5V, V
IN
= 4V, EN/OK = open, RSET = 2.8kΩ, RSTRT = GND, T = +25°C, unless otherwise noted.)
A
BATT
NORMALIZED TIMER PERIOD
vs. TEMPERATURE
PRECHARGE CURRENT
vs. INPUT VOLTAGE
NORMALIZED BATT REGULATION
VOLTAGE vs. TEMPERATURE
1.02
60
55
50
45
40
1.002
1.001
1.000
0.999
0.998
0.997
0.996
NORMALIZED TO +25°C
NORMALIZED TO +25°C
ISET = 2.8kΩ
CHARGING CURRENT SET
FOR 500mA
1.00
0.98
0.96
0.94
0.92
0.90
-40
-15
10
35
60
85
-40
-15
10
35
60
85
5
6
7
8
9
10
11
12
TEMPERATURE (°C)
TEMPERATURE (°C)
V
(V)
IN
NORMALIZED BATTERY CURRENT
vs. BATTERY VOLTAGE
VOLTAGE-LOOP GAIN
MAX1898 toc05
1.2
1.0
0.8
0.6
0.4
0.2
0
60
40
200
150
100
50
GAIN
PHASE
20
0
-20
-40
0
-50
3.9
4.0
4.1
4.2
4.3
0.01
0.1
1
10
100
1000
BATTERY VOLTAGE (V)
FREQUENCY (kHz)
_______________________________________________________________________________________
5
Linear Charger for Single-Cell Li+ Battery
Pin Description
PIN
1
NAME
IN
FUNCTION
Sense Input. Detects power source; positive side of internal current-sense resistor.
2
CHG
Open-Drain LED Driver. See Table 1 for CHG output states.
Logic-Level Enable Input and Input Power OK Output. Hold low to disable charger. Float for normal
operation, drive with an open-drain device to utilize both pin functions, or connect through 10kΩ for logic-
controlled ON/OFF. See the EN/OK (EN Input, OK Output) section for details.
3
EN/OK
ISET serves two functions. It provides an analog output proportional to the actual charge current. Output
current from ISET is 1mA per amp of battery charging current. The charging current is set by connecting a
resistor from ISET to ground. See the Current-Limit Mode section.
4
5
ISET
CT
Safety Charge Timer Control. Connect timer capacitor to program safety time-out interval; 100nF for
approximately three hours. Connect to GND to disable the timer function.
Automatic-Restart Control Pin. When tied to GND, a new charging cycle starts if the cell drops 200mV below
the battery regulation threshold. The restart threshold may be lowered by connecting a resistor from RSTRT
to ground. See the Reinitiating a Charging Cycle section. Automatic restart is disabled when RSTRT is
unconnected or when CT is grounded (timer disabled).
6
RSTRT
7
8
9
BATT
GND
DRV
Battery Sense Input. Positive terminal of single Li+ cell.
Ground
External Transistor Driver. This pin drives the gate/base of an external PMOS/PNP pass transistor.
Current-Sense Input. Negative side of internal current-sense resistor. Connect to source/emitter of
PMOS/PNP pass transistor.
10
CS
• The prequalification cycle ends with the cell reaching
Detailed Description
2.5V.
The MAX1898 forms a complete charger for 1-cell Li+
batteries. It includes precision voltage control ( 0.75%)
to optimize both cell performance and cycle life.
Factory-set 4.2V (MAX1898EUB42) and 4.1V
(MAX1898EUB41) versions charge all common Li+
chemistries. Externally selectable charge current is
sensed internally, eliminating the need for a current-
sense resistor. The charger also supplies outputs that
indicate charge status (CHG), the presence of input
power (EN/OK), and charge-current magnitude (ISET).
Other features include a shutdown control input
(EN/OK), selectable threshold for charge-cycle restart
without cycling input power (RSTRT), and a selectable
charge termination safety timer. See Figure 1.
Li+ cells can be damaged when fast-charged from a
completely dead state. A fully discharged cell may indi-
cate an abnormal cell condition. As a built-in safety fea-
ture, the MAX1898 prequalifies the cell with 10 percent
of the user-programmed fast-charge current at the start
of a charge cycle. When the cell voltage rises to 2.5V,
the MAX1898 begins fast-charging.
Current-Limit Mode
The MAX1898 regulates charging current by linearly
controlling an external PMOS or PNP transistor. The
maximum charging current is programmed by an exter-
nal RSET resistor connected from ISET to GND. Select
the RSET value based on the following formula:
The MAX1898 initiates fast charge when any of these
conditions are met (see Figure 6):
1400V
RSET
IFASTCHG
=
• An external power source is connected AND the cell
voltage is above 2.5V.
where:
• The cell voltage falls to the restart threshold, 4.0V in
the MAX1898EUB42, or 3.9V for the MAX1898EUB41.
I
is in amps and RSET is in ohms.
FASTCHG
• The IC is reset by driving the EN/OK pin low and
The ISET pin may also be used to monitor the actual
charging current at any time. The output current from
then high again.
6
_______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery
DC IN
0.1µF
LED
IN
4.5V TO 12V
5mA
CHG
84Ω
84mΩ
20mΩ
CT
CHARGING FOR
3hr,
OSC
÷218
C
= 100nF
CT
CSA
CS
CHG
LOGIC
TIMEOUT
0.1µF
470Ω
ISET
RSET
HV
DRV
(2.8kΩ FOR
500mA CHARGE
CURRENT)
ICHARGE < 20%
DRV
280mV ISET (FALLING)
305mV ISET (RISING)
CA
REF
1.4V
BATT
PREQUAL
CONTROL
LOGIC
2.5V
VA
69.275kΩ
1.4V
1.4V
10µF
DONE
SHUTDOWN
3V
V REG
L
34.637kΩ
RESTART
100kΩ
BATT
IN
EN/OK
2.653kΩ
DONE
UVLO
SHUTDOWN
RSTRT
SHUTDOWN
GND
Figure 1. Functional Diagram
_______________________________________________________________________________________
7
Linear Charger for Single-Cell Li+ Battery
Reinitiating a Charging Cycle
The MAX1898 can be configured so that a charging
cycle automatically restarts when the cell voltage drops
200mV below the battery regulation voltage (connect
RSTRT to GND). The restart threshold can also be low-
ered by connecting an external resistor between RSTRT
and GND. If automatic charging restart is not desired,
this feature may be disabled by floating the RSTRT pin.
With restart disabled, charging can only be reinitiated
by pulling EN/OK low, or by removing and reapplying
the input power source.
4.2
4.0
3.8
3.6
3.4
3.2
1.4
BATTERY VOLTAGE
AMP HOURS
1.2
1.0
0.8
BATTERY CURRENT
0.6
0.4
0.2
0
POWER DISSIPATION
EN/OK (EN Input, OK Output)
The EN/OK pin serves two functions. It primarily serves as
a logic input (active high) to enable the charger. In addi-
tion to ON/OFF control, EN/OK also indicates when input
power is connected. When input power is applied to IN
0
30
60
90
120
150
180
TIME (MINUTES)
(V > V
and V > 4.25V) EN/OK pulls high (to 3V)
IN
IN
BATT
Figure 2. Li+ Charging Profile
with a 100kΩ internal pullup resistance. Consequently,
EN/OK may be output to a logic input, to indicate AC
adapter presence, but at the same time may be driven by
open-drain logic to turn the charger ON and OFF. An
open-drain logic-low input will over-ride the internal pullup
resistor and turn off the charger (see Figure 3). When the
open drain goes high impedance, EN/OK will go high if
the ISET pin is 1mA per amp of charging current. The
output voltage at ISET is proportional to charging cur-
rent as follows:
ICHG × RSET
V
=
ISET
1000
The voltage at ISET is nominally 1.4V at the selected
fast-charge current, and falls with charging current as
the cell becomes fully charged. See Figure 2 for typical
Li+ charging profile.
POWER OK
IN
3V
OPEN-DRAIN
LOGIC
CHG Status Output
CHG is an open-drain output that monitors the cell’s
charging status. CHG has a 5mA current limit so an
LED can be connected directly from IN to CHG for a
visible indication. Alternatively, a pullup resistor (typi-
cally 100kΩ) to a logic supply provides a logic-level
output. Table 1 outlines the status of CHG for various
charger states.
100kΩ
OFF
EN/OK
ON
MAX1898
GND
Figure 3. Driving EN/OK with Open-Drain Logic to Retain
ON/OFF Control with Power OK Indication
Table 1. CHG Output States
CONDITION
CHG
No battery or no charger input.
High impedance (LED off)
The cell voltage is less than 2.5V and charge current is regulated to 10% of the selected fast-
charge current (prequalification state).
Low impedance (LED on)
Low impedance (LED on)
High impedance (LED off)
Cell voltage is greater than 2.5V (fast-charge state).
Charge cycle complete—charge current is less then 20% of the selected fast charge current or
safety timer expires (3 hours for C = 100nF).
CT
Charge cycle fault—the cell voltage is less than 2.5V and the prequalification timer (45min for C
= 100nF) has expired.
Blinking, LED on 50% duty
factor, 1.5Hz
CT
8
_______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery
power is present at IN. If there is no (or insufficient)
voltage at IN, EN/OK will remain low and the charger
will remain off.
220nF
5V TO 12V
10
9
CS
Battery Drain Current
The MAX1898 uses advanced CMOS circuitry to com-
pletely monitor battery state, while drawing minimal current
from the cell itself. When the input voltage is less than the
battery, battery drain is typically 3µA. When input power is
present and charging is complete (the charge timer
expires), battery drain is then typically 40µA. This current
may be reduced to 4µA by disabling the restart feature.
DRV
IN
1
7
5
BATT
CT
Li+ CELL
10µF
LED
CHARGE
INDICATOR
MAX1898
C , 100nF CAPACITOR
CT
2
3
6
SETS CHARGE TIME
TO 3hr
CHG
Applications Information
The MAX1898 basic application circuit is shown in
Figure 4.
INPUT POWER
OK OUTPUT OR
ENABLE INPUT
EN/OK
OUTPUT VOLTAGE
PROPORTIONAL
TO CHARGE CURRENT
4
ISET
RSTRT
GND
8
R
SETS
SET
CHARGE
100nF
5V TO 12V
CURRENT
10
CS
Figure 5. Application Circuit with PNP Pass Transistor
9
DRV
IN
1
C
nF = 34.33× t
hours
[
[
]
]
CT
CHG
7
BATT
where:
Li+ CELL
10µF
LED
CHARGE
t
= nominal charge time limit in hours.
INDICATOR
CHG
MAX1898
5
4
CT
The maximum charge timer is intended as a safety timer.
It is normally not intended to be part of the charge-con-
trol loop. When charging a Li+ cell at a 1C rate, normal
charge time is approximately 1.5 hours, but this may
have wide variations that depend on temperature and
cell type. In most cases, 3 hours is recommended for the
maximum charge time limit when fast-charging at 1C so
that normal-charging is never interrupted by the charge
timer. Consult the battery manufacturer for details on rec-
ommended timer settings.
C
, 100nF CAPACITOR
CT
2
3
6
SETS CHARGE TIME
TO 3hr
CHG
INPUT POWER
OK OUTPUT OR
ENABLE INPUT
EN/OK
OUTPUT VOLTAGE
PROPORTIONAL
TO CHARGE CURRENT
ISET
RSTRT
GND
8
R
SETS
SET
CHARGE
CURRENT
Figure 4. Basic Application Circuit
The charge safety timer may also be completely dis-
abled by connecting CT to ground. Note: If the charge
timer is disabled, both the restart function and prequali-
fication fault indication are also disabled. Only the indi-
cation (blinking LED) is disabled, the prequalification
function itself is not modified.
An LED can be added as a charge-state indicator. A
10µF bypass capacitor must be added to the BATT pin.
A Schottky diode should be added between the exter-
nal transistor drain/collector and BATT, to avoid battery
discharge in the event of an input source short circuit.
Controlling Automatic Restart
The MAX1898 features automatic restart that resumes
charging when the cell voltage drops to a predetermined
level. In most timed chargers, once charging times out,
subsequent loading of the battery can discharge the bat-
tery and charging will not resume unless the charger is
restarted by an outside signal. By automatically resuming
charging when the battery voltage drops, the MAX1898
ensures that the cell does not remain partially charged
Figure 5 shows a charger circuit using an external PNP
pass transistor.
Selecting Maximum Charge Time
The maximum charge time can be set with an external
capacitor connected from CT to GND. Select the timing
capacitor using:
_______________________________________________________________________________________
9
Linear Charger for Single-Cell Li+ Battery
ASYNCHRONOUS FROM
ANYWHERE
V
< 4.5V OR
> V
IN
SHUTDOWN
CHARGER = OFF
LED = OFF
V
< 2.5V
V
BATT
BATT
IN
PREQUALIFICATION
10% CHARGE CURRENT
LED = ON
V
< 4.5V OR
IN
V
TIMEOUT/4
> V
BATT
IN
EXPIRES AND
V
< 2.5V
BATT
V
> 2.5V,
BATT
FAULT
LED BLINKING AT 1.5Hz
3hr TIMER STARTS
RSTRT = GND AND
V
V
< 4.0V (MAX1898EUB42)
< 3.9V (MAX1898EUB41)
BATT
BATT
FAST CHARGE
100% CHARGER CURRENT
LED = ON
TIMEOUT = 3hr
ICHARGE > 22%
OF ISET
ICHARGE < 20%
OF ISET
DONE
TIMEOUT = 3hr
TOP OFF
LED = OFF
(CHARGE COMPLETE)
CHARGER = OFF
LED = OFF
DOTTED LINE INDICATES THE STATES THAT
ARE NOT AVAILABLE WHEN CT = 0 (TIMER DISABLED)
Figure 6. Charger State Diagram
after use when charger power is available. The restart
feature can be configured in the following ways:
resistor using the following equations:
For MAX1898EUB42:
• Floating the RSTRT pin disables restart. Once the
battery is charged and the charge timer expires,
charging can only be started by cycling input power
to IN, or by toggling EN/OK. When the restart func-
tion is disabled, battery drain after charge comple-
tion is reduced to 4µA. With the restart function
enabled, battery drain current after charge comple-
tion is 40µA.
69.275kΩ
R
kΩ =
− 37.301kΩ
[
]
RSTRT
V
RSTRT
−1
1.4V
for V
values between 3V and 4V.
RSTRT
For MAX1898EUB41:
• Ground the RSTRT pin to enable the default restart
threshold of 4V for the MAX1898EUB42 and 3.9V for
the MAX1898EUB41. Once the battery is charged
and the charge timer expires, charging restarts when
the cell voltage drops to the restart-threshold voltage.
66.822kΩ
R
kΩ =
− 37.301kΩ
[
]
RSTRT
V
RSTRT
−1
1.4V
• If desired, lower the restart-threshold voltage by con-
for V
values between 3V and 3.9V.
RSTRT
necting a resistor from RSTRT to GND. Select the
Note that if CT is grounded (charge timer disabled), the
restart function is also disabled.
Table 2. External Transistors
EXTERNAL
TRANSISTORS
RESISTANCE
RATING
VOLTAGE LIMIT
(V)
POWER
DISSIPATION (W)
PACKAGE
SOT223
COMMENTS
PMOS
Fairchild NDT454P
50mΩ
-30
-15
3
—
PNP
Zetex FZT968
0.17V, V
0.32V, V
3
1
SOT223
h
h
= 300min
CE(sat)
CE(sat)
FE
FE
= 160min, 1W
PNP
FCX717
-12
SOT89
rating suitable for
0.5A charge or less
10 ______________________________________________________________________________________
Linear Charger for Single-Cell Li+ Battery
A PNP bipolar transistor may also be used as a pass
device. DRV can sink up to 4mA, so reasonable high
Selecting External Components
The MAX1898 drives an external transistor to control
charging current. The most important specifications for the
pass transistor (MOSFET or bipolar) are current rating and
package power dissipation. Since the MAX1898 operates
as a linear charger, heat will dissipate in the external tran-
sistor. The worst-case power dissipation will be:
current gain (h or Beta) is required to allow DRV to
FE
drive the base of a bipolar pass transistor. For a 500mA
fast-charge current, an h of 125 is required. Although
FE
their current gain is high, Darlington PNP transistors are
not recommended due to stability limitations.
If V is less than V
, the body diode inherent in the
P
= I (V - 2.5V)
FASTCHG IN
IN
BATT
DISS(MAX)
external enhancement-mode FET allows current to flow
from the cell back to the source. To prevent this, add a
Schottky diode between the drain and BATT, as shown
in Figure 4.
The pass transistor and heatsinking (typically an expand-
ed PC board plane) must be rated for P
.
DISS(MAX)
The pass device will most commonly be a P-channel
MOSFET. Its operating voltage must exceed the
expected input voltage, but MOSFET on-resistance is
important only in low-dropout designs where V is only
IN
slightly higher than the battery regulation voltage. In
most charger designs, a fairly low-cost FET (around
Chip Information
TRANSISTOR COUNT: 1816
PROCESS: BiCMOS
100mΩ to 200mΩ R
) performs more than ade-
quately. Lower resistance FETs perform well also, but
are not worth the added cost in this case.
DS(ON)
______________________________________________________________________________________ 11
Linear Charger for Single-Cell Li+ Battery
Package Information
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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