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19-1662; Rev 0; 4/00
KIT ATION EVALU LE B AVAILA
SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply
General Description Features
o Small 6-Pin SOT23 Package o Stand-Alone or P-Controlled Operation o 0.5% Voltage Set-Point Accuracy o Lowest Power Dissipation o Low 4.7V min Input Voltage o Top-Off Charging to Achieve Full Battery Capacity o No Inductor Required o Safely Precharges Near-Dead Cells o Automatic Power-Down when Input Power Removed
MAX1736
The MAX1736 is a simple, low-cost, single-cell lithiumion (Li+) battery charger for small hand-held applications. When accompanied by a current-limited voltage source (such as a wall cube), the MAX1736 provides simple, accurate charging and termination control for single-cell Li+ batteries. The MAX1736EUT42 is preset to a 4.2V battery regulation voltage, while the MAX1736EUT41 is preset to 4.1V. The MAX1736 initiates charging in one of four ways: battery insertion, charger power-up, battery voltage threshold, and by external manipulation of the EN pin. The device features an internal precharge current source that safely charges near-dead cells, as well as input-supply detection that shuts down the MAX1736 when the supply is removed to minimize battery current drain. The MAX1736 accepts input voltages up to 22V, making it compatible with a wide range of input supplies. It has a single control input yet offers stand-alone and microprocessor-controlled operation. The MAX1736 is packaged in a small SOT23-6 package. An evaluation kit (EV kit) is available to reduce design time.
Ordering Information
PART MAX1736EUT42-T MAX1736EUT41-T TEMP. RANGE PINPACKAGE SOT MARK AAHO AANC
________________________Applications
Single-Cell Li+ Portable Applications Wireless Handsets Personal Digital Assistants Digital Cameras Small Hand-Held Equipment Self-Charging Battery Packs Cradle Chargers
-40C to +85C 6 SOT23-6 -40C to +85C 6 SOT23-6
Typical Operating Circuit
CURRENT-LIMITED VOLTAGE SOURCE PFET IN 1
Pin Configuration
TOP VIEW
6 BATT
GATE IN BATT BATTERY MAX1736 ON OFF EN CT GND
GATE 2
MAX1736
5
CT
GND 3
4
EN
SOT23
________________________________________________________________ Maxim Integrated Products
1
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply MAX1736
ABSOLUTE MAXIMUM RATINGS
IN, GATE to GND....................................................-0.3V to +26V BATT, EN, CT to GND ..............................................-0.3V to +6V GATE to IN................................................................-6V to +0.3V GATE Continuous Current.................................-10mA to +10mA Continuous Power Dissipation (TA = +70C) 6-Pin SOT23 (derate 8.1mW/C above +70C) .............0.65W Operating Temperature Range ...........................-40C to +85C Storage Temperature Range ..............................-65C to + 150C Maximum Junction Temperature ......................................+ 150C Lead Temperature (soldering, 10s) .................................+300C
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
(VIN = 10V, VBATT = 4.2V for MAX1736EUT42 or 4.1V for MAX1736EUT41, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage (Note 2) Fast-Charge BATT Qualification Threshold Fast-Charge BATT Qualification Threshold Hysteresis BATT Regulation Voltage BATT Removal Detection Threshold BATT Removal Detection Threshold Hysteresis BATT Input Current, Input Power Removed BATT Input Current, Charger Disabled BATT Input Current, When Charging Precharge Source Current IN Input Current IN Detection Interval (Note 3) GATE Source/Sink Current GATE Drive Source Current at Battery Removal Minimum BATT Bypass Capacitance (Note 4) EN Logic High Threshold EN Logic Low Threshold CT Pulldown Current
2
CONDITIONS External P-MOSFET off BATT rising, transition from precharge to fast charge
MIN 4.7 2.4
TYP
MAX 22
UNITS V V mV
2.5 70
2.65
MAX1736EUT42 MAX1736EUT41 BATT rising
4.179 4.079 4.875
4.20 4.10 5.0 125
4.221 4.121 5.1
V V mV
VIN V BATT - 0.3V EN = GND, V BATT = 0 to 5V
0.1 2 0.4
1 6 0.75 8 1 125 60
A A mA mA mA s A mA F/A V
VBATT = 2V CCT = 0.33F
3.5
6 0.25 20
75 VBATT = 5.1V 15 1.5 2
100 30
0.7 1.6 2 2.4
V A
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 10V, VBATT = 4.2V for MAX1736EUT42 or 4.1V for MAX1736EUT41, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
MAX1736
PARAMETER CT Pullup Current Minimum On-Time Minimum Off-Time EN Pullup Resistance CCT = 0.33F CCT = 0.33F
CONDITIONS
MIN -12
TYP -10 165 33
MAX -8
UNITS A ms ms
175
350
725
k
ELECTRICAL CHARACTERISTICS
(VIN = 10V, VBATT = 4.2V for MAX1736EUT42 or 4.1V for MAX1736EUT41, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage (Note 2) Fast-Charge BATT Qualification Threshold BATT Regulation Voltage BATT Removal Detection Threshold BATT Input Current, Input Power Removed BATT Input Current, Charger Disabled BATT Input Current, When Charging Precharge Source Current IN Input Current GATE Source/Sink Current GATE Drive Source Current at Battery Removal EN Logic High Threshold EN Logic Low Threshold CT Pulldown Current CT Pullup Current EN Pullup Resistance VBATT = 5.1V VBATT = 2V
CONDITIONS External P-MOSFET off BATT rising, transition from precharge to fast charge MAX1736EUT42 MAX1736EUT41 BATT rising VIN V BATT - 0.3V EN = GND, V BATT = 0 to 5V
MIN 4.7 2.4 4.158 4.058 4.85
MAX 22 2.65 4.242 4.142 5.125 1 6 0.75
UNITS V V V
V A A mA mA mA A mA V
3 60 10 2
8 1 140 90
0.7 1.5 -12 170 2.5 -8 725
V A A k
Note 1: All devices are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design. Note 2: The input voltage range is specified with the external PFET off. When charging, the PFET turns on and the input voltage (the output voltage of the constant-current power source) drops to very near the battery voltage. When the PFET is on, IN may be as low as 2.5V. Note 3: Every 20s (for CT = 0.33F) the MAX1736 turns off the external P-channel MOSFET and samples IN to determine if input power is present. If input power is removed, the charger shuts down. Note 4: For design guidance, not tested.
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply MAX1736
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MINIMUM ON/OFF-TIMES vs. CCT
MAX1736 toc01
MAX1736-02
0.20 BATT VOLTAGE DEVIATION (%) 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15
5.7 PRECHARGE CURRENT (mA) PRECHARGE CURRENT (mA) 5.6 5.5 5.4 5.3 5.2
VBATT = 0
MINIMUM ON/OFF-TIME (ms)
1000
MINIMUM ON-TIME
VBATT = 1V VBATT = 2V
100
MINIMUM OFF-TIME
10 0.1 1 CCT (F) 10
-0.20 -40 -15 10 35 60 85 TEMPERATURE (C)
5.1 4 9 14 VIN (V) 19 24
INPUT DETECTION INTERVAL vs. CCT
MAX1736 toc04
TYPICAL OPERATING WAVEFORMS
IIN 5V/div OV VBATT 200mV/div AC-COUPLED PFET OFF PFET ON
MAX1736 toc05
1000 INPUT DETECTION INTERVAL (s)
100
10
IIN 1A/div OA VGATE 10V/div
1 0.1 1 CCT (F) 10 10ms/div
4
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MAX1736-03
10,000
BATT REGULATION VOLTAGE DEVIATION vs. TEMPERATURE
PRECHARGE CURRENT vs. INPUT VOLTAGE
5.8
SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply
Pin Description
PIN 1 2 3 4 NAME IN GATE GND EN DESCRIPTION Input Voltage from Current-Limited Voltage Source (22V max). Bypass to GND with a 0.1F capacitor. The charging current is set by the current limit of the external power supply. Gate Drive for External PMOS Pass Element. The PMOS device should have a VGS threshold of 2.5V or less (see Selecting External Components). Ground. Connect the battery's negative terminal to GND. Logic-Level Enable Input. Pull low to disable the MAX1736. EN is internally pulled up to VBATT + 100mV through 350k, but draws no current from BATT. Charge Time Control. Sets the minimum on-time, minimum off-time, and the IN detection interval. Place a 0.33F capacitor between CT and GND for most applications (see Selecting External Components). Cell Voltage Monitor Input, Precharge Current Output, and MAX1736 Power Source. Connect BATT to the positive terminal of a single Li+ cell. Bypass BATT with a capacitor to GND (1.5F per amp of charge current).
MAX1736
5
CT
6
BATT
Detailed Description
The MAX1736 provides a simple, safe, low-cost method of charging a single-cell Li+ battery with nearly no heat generation. Combined with a current-limited voltage source, the MAX1736 provides precharge, fast-charge, and top-off-charge capabilities. After constant-current fast charge, top-off safely finishes charging the battery by pulse-width modulating charge current. The top-off on-time is kept below the electrochemical time constant of the cell. The key advantage of this method is that the charge circuit is small and generates minimal heat while providing a safe method of charging to ensure maximum cell life. Figure 1 shows the MAX1736 functional diagram.
specified current limit that matches the desired fastcharge current for the Li+ cell. With the P-channel MOSFET on, V IN will be nearly equal to VBATT. To detect that an input supply is connected, the MAX1736 periodically turns the P-channel MOSFET off and checks the voltage at IN. During fast charge, this occurs once every input detection interval (20s with CCT = 0.33F). During pulsed top-off, input detection occurs more frequently and is continuous when the MOSFET is off (see Selecting External Components).
Pulsed Top-Off
When the battery approaches full charge, its instantaneous voltage reaches the BATT regulation voltage and pulsed top-off begins. The MAX1736 uses a hysteretic algorithm with a minimum on- and off-time. Cell voltage is sampled with no charging current to minimize errors due to battery and cell protection resistance. If the voltage is below the BATT regulation voltage, the P-channel MOSFET switches on for a minimum on-time. If, at the end of the minimum on-time, the cell voltage is still below the BATT regulation voltage, the switch remains on until the cell voltage reaches the BATT regulation voltage. At that point, the P-channel MOSFET then switches off for at least the minimum off-time. The minimum on-time is set by CT and should be set below the electrochemical time constant of the cell. A CCT value of 0.33F sets a minimum on-time of 165ms, which is adequate for most Li+ batteries.
5
Precharge
To protect Li+ cells from damage that may occur if fast charged from a dead state, the MAX1736 precharges the Li+ cell with 6mA at the start of a charging cycle when the cell voltage is below 2.5V. As soon as the cell voltage reaches 2.5V, the MAX1736 begins fast charging.
Fast Charge
In fast-charge mode, the MAX1736 turns on the external P-channel MOSFET. Charging current is set by the current limit of the external supply; current is not regulated by the MAX1736. The P-channel MOSFET is used only as a switch, not as a linear regulator. Therefore, the circuit's power dissipation is minimized, permitting rapid charge cycles with almost no heat generation. The external power supply should have a
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply MAX1736
IN
EN VBATT + 100mV
IN
GATE IN
6mA 350k
10A 1.4V STATE MACHINE CT 4.2V (4.1V) BATT
0.4V 5V REF
2A GND
MAX1736EUT42 (MAX1736EUT41)
Figure 1. Functional Diagram
Once the switch turns off, it remains off for at least the minimum off-time. After the minimum off-time, the Pchannel MOSFET turns on if the cell voltage is lower than the BATT regulation voltage. A C CT value of 0.33F sets a minimum off-time of 33ms. At the beginning of the pulsed top-off state, charge current is modulated at approximately an 83% duty cycle. Toward the end of top-off, charge current stays off for long periods of time between single "on" pulses. During these final pulses, the instantaneous cell voltage may exceed the BATT regulation voltage by several hundred millivolts, but these pulses are orders of magnitude shorter than the electrochemical time constant of the Li+ cell and do no harm. Pulsed top-off charge ends when the cell voltage no longer falls below the BATT regulation voltage. Figure 2 shows the state machine.
external power source, not by the MAX1736. The Pchannel MOSFET in Figure 3's application circuit is either on or off, allowing the source to be directly connected to the cell or disconnected. Therefore, it is important to choose a power supply with the correct current limit for the cell to be charged. In most applications, this will be a small wall cube with an open-circuit output voltage of 5V to 12V, which is specified as "current limited" or "constant current." Some low-cost wall cubes may have poor transient characteristics. For these wall cubes, output current may exceed the specified current limit by several times when the load is quickly connected. The MAX1736 limits this current peak by controlling the slew rate of the P-channel MOSFET. See C CT and C GATE for more information.
Selecting External Components
Input Power Supply
One reason the MAX1736 Li+ charger is so compact and simple is that the charging current is set by the
6
PMOS Switch
The P-channel MOSFET switches the current-limited source on and off. Because of the intentionally slow switching times and limited slew rate, the MAX1736 is not particular about the power FET it drives. Specifications to consider when choosing an appropriate
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply MAX1736
CHARGER POWER REMOVED (FROM ANY STATE) CHARGER POWER PRESENT SHUTDOWN FET: OFF DISABLED FET: OFF EN PRECHARGE FET: OFF IOUT = -6mA (FROM ANY STATE) EN
VBATT 2.5V
EVERY CYCLE
CHARGER POWER PRESENT
IN DETECTION INTERVAL
CHARGER POWER PRESENT
TOP-OFF FET: PULSED
VBATT > 4.2V
FAST-CHARGE FET: ON
Figure 2. State Machine
FET are the minimum drain-source breakdown voltage and the minimum turn-on threshold voltage (V GS ). Power dissipation during fast charge is approximately RDSON ICHG2, where ICHG is the current limit set by the input power source. The minimum breakdown voltage (BVDS) must typically be two times the wall cube's open-circuit voltage. An even larger margin may be necessary if the wall adapter has especially poor transient response. The MAX1736 can operate with input voltages up to 22V.
0.47F of the total capacitance should be low-ESR ceramic.
VBATT < 2.5V
TOP-OFF QUAL FET: OFF
FAST-CHARGE QUAL FET: OFF
VBATT < 2.5V
CCT and CGATE
Most applications will use the circuit of Figure 3 with CGATE = 0.22F, RGATE = 100k, and CCT = 0.33F. CGATE, RGATE, and the internal 100A pull-up and pulldown current sources act to slow the switching of the Pchannel MOSFET. This prevents a wall cube with poor transient response from subjecting VIN to excessive voltage when the P-channel MOSFET turns off, and prevents excessive current into the battery when the Pchannel MOSFET turns on. Excessive voltage at VIN can potentially damage the IC, input capacitor, and the PMOS switch. Excessive current into the battery can cause errors in the termination process of the MAX1736 (by raising the instantaneous battery voltage) and may trip the battery's protection circuitry. In applications utilizing a wall cube with poor transient response, increase the value of CGATE as needed to
7
BATT Capacitor
Bypass BATT with at least 1.5F per amp of charge current. If the battery is removed while the P-channel MOSFET is on, a BATT voltage over 5V is quickly sensed, and the FET is immediately turned off. In applications where the cell is removable, very large capacitance values may increase transient currents when the cell is replaced. Therefore, BATT capacitance in excess of 100F should be avoided. For best system performance with large output capacitance, at least
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SOT23, Single-Cell Li+ Battery Charger for Current-Limited Supply MAX1736
further slow switching edges and prevent transient spikes. CCT sets the minimum on-time and off-time according to the following equations: tON(MIN) = 5 105 x CCT tOFF(MIN) = 1 105 x CCT
GATE CURRENT-LIMITED WALL CUBE (800mA)
FDC638P PFET 0.22F
100k
Layout Guidelines
The MAX1736 controls the GATE slew rate; consequently, PC board layout is not as sensitive to noise as a high-frequency switching regulator. In addition, since cell voltage is sensed both during and between highcurrent pulses, the system is insensitive to ground errors. However, Maxim recommends maintaining large ground area and large traces for high-current paths. Refer to the MAX1736EVKIT for a recommended layout example.
0.1F
IN
BATT 2.2F MAX1736 GND
SINGLE Li+ CELL
CT 0.33F
Figure 3. Simple Application Circuit
Chip Information
TRANSISTOR COUNT: 1622
Package Information
6LSOT.EPS
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.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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