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LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
LM3420 8.4-V Li-Ion Battery Charge Controller
1 Features
3 Description
•
•
•
•
The LM3420 series of controllers are monolithic
integrated circuits designed for charging and end-ofcharge control for lithium-ion rechargeable batteries.
The LM3420 is available in an 8.4-V version for one
through four cell charger applications.
1
•
•
•
•
Input Voltage Range: up to 20 V
Voltage Options for Charging 1, 2, 3, or 4 Cells
Output Current up to 15 mA
Precision (0.5%) End-of-Charge Control
– LM3420 ±1%
– LM3420A ±0.5%
Drive Capability for External Power Stage
Temperature Drift Curvature Correction for
Voltage Stability
Low Quiescent Current, 85 μA (Typ.)
Tiny SOT-23-5 package
2 Applications
•
•
Lithium-Ion Battery Charging
Suitable for Linear and Switching Regulator
Charger Designs
space
space
Included in a very small package is an (internally
compensated) op amp, a bandgap reference, an NPN
output transistor, and voltage setting resistors. The
amplifier's inverting input is externally accessible for
loop frequency compensation. The output is an openemitter NPN transistor capable of driving up to 15 mA
of output current into external circuitry.
A trimmed precision bandgap reference utilizes
temperature drift curvature correction for excellent
voltage stability over the operating temperature
range. The LM3420 series allows for precision endof-charge
voltage
threshold
for
lithium-ion
rechargeable batteries. The premium grade LM3420A
is available with an initial voltage threshold tolerance
of ±0.5%, while the standard grade LM3420 has an
initial voltage threshold tolerance of ±1%.
The LM3420 is available in a sub-miniature 5-lead
surface mount package thus allowing very compact
designs.
Device Information(1)
PART NUMBER
LM3420
PACKAGE
SOT-23 (5)
BODY SIZE (NOM)
2.90 mm x 1.60 mm
(1) For all available packages, see the orderable addendum at
the end of the datasheet.
Simplified Schematic
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
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Table of Contents
1
2
3
4
5
6
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
4
4
4
4
5
6
7
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
LM3420 Electrical Characteristics.............................
LM3420A Electrical Characteristics ..........................
Typical Characteristics ..............................................
7
Parameter Measurement Information .................. 9
8
Detailed Description ............................................ 10
7.1 Test Circuits .............................................................. 9
8.1 Overview ................................................................. 10
8.2 Functional Block Diagram ....................................... 10
8.3 Feature Description................................................. 10
8.4 Device Functional Modes........................................ 11
9
Application and Implementation ........................ 12
9.1 Application Information............................................ 12
9.2 Typical Application: Constant Current/Constant
Voltage Li-Ion Battery Charger ................................ 12
10 Power Supply Recommendations ..................... 17
11 Layout................................................................... 18
11.1 Layout Guidelines ................................................. 18
11.2 Layout Example .................................................... 18
12 Device and Documentation Support ................. 18
12.1 Trademarks ........................................................... 18
12.2 Electrostatic Discharge Caution ............................ 18
12.3 Glossary ................................................................ 18
13 Mechanical, Packaging, and Orderable
Information ........................................................... 19
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (May 2013) to Revision E
Page
•
Deleted info re: 4.2-V, 8.2-V, 12.6-V and 16.8 V versions .................................................................................................... 1
•
Added Device Information and ESD Ratings tables, Feature Description, Device Functional Modes, Application and
Implementation, Power Supply Recommendations, Layout, Device and Documentation Support, and Mechanical,
Packaging, and Orderable Information sections; moved some curves to Application Curves section .................................. 1
Changes from Revision C (April 2013) to Revision D
•
2
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 13
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5 Pin Configuration and Functions
SOT-23 (DBV) Package
5 Pins
Top
Pin Functions
PIN
NAME
NUMBER
I/O
DESCRIPTION
IN
1
I
GND
2
—
Input voltage supply
Ground
NC
3
—
No connection
OUT
4
O
Open emitter output capable of sourcing current
COMP
5
Compensation
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)
(1) (2)
MIN
MAX
UNIT
20
V
Output current
20
mA
Junction temperature
150
Input voltage VIN
Lead temperature
Vapor phase (60 seconds)
215
Infrared (15 seconds)
220
Power dissipation (TA = 25°C) (3)
−65
Storage temperature, Tstg
(1)
(2)
(3)
°C
300
mV
150
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
The maximum power dissipation must be derated at elevated temperatures and is dictated by TJmax (maximum junction temperature),
RθJA (junction-to-ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any
temperature is PDmax = (TJmax − TA)/RθJA or the number given in the Absolute Maximum Ratings, whichever is lower. The typical thermal
resistance (RθJA) when soldered to a printed circuit board is approximately 181.2°C/W for the DBV0005A package.
6.2 ESD Ratings
VALUE
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
V(ESD)
(1)
(2)
Electrostatic discharge
(1)
UNIT
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
V
±500
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted) (1) (2)
MIN
NOM
MAX
Ambient temperature
−40
85
Junction temperature
−40
125
Output current
(1)
(2)
15
UNIT
°C
mA
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
The maximum power dissipation must be derated at elevated temperatures and is dictated by TJmax (maximum junction temperature),
RθJA (junction to ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any
temperature is PDmax = (TJmax − TA)/RθJA or the number given in the Absolute Maximum Ratings, whichever is lower. The typical thermal
resistance (RθJA) when soldered to a printed circuit board is approximately 181.2°C/W for the DBV0005A package.
6.4 Thermal Information
LM3420
THERMAL METRIC (1)
SOT-23 (DBV)
UNIT
5 PINS
RθJA
Junction-to-ambient thermal resistance
RθJC(top)
Junction-to-case (top) thermal resistance
91.2
RθJB
Junction-to-board thermal resistance
38.2
ψJT
Junction-to-top characterization parameter
5.3
ψJB
Junction-to-board characterization parameter
37.7
(1)
4
181.2
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
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6.5 LM3420 Electrical Characteristics
Unless otherwise specified, specifications apply over full operating temperature range and VIN = VREG, VOUT = 1.5 V.
PARAMETER
Regulation voltage
VREG
Regulation voltage
tolerance
IQ
Quiescent current
Transconductance
ΔIOUT/ΔVREG
Gm
MIN (1)
TEST CONDITIONS
8.316
8.4
8.484
IOUT = 1 mA
8.232
8.4
8.568
IOUT = 1 mA, TJ = 25°C
–1%
1%
IOUT = 1 mA
–2%
2%
IOUT = 1 mA, TJ = 25°C
85
125
IOUT = 1 mA
85
150
20 μA ≤ IOUT ≤ 1 mA, VOUT = 6 V
TJ = 25°C
1
3.3
20 μA ≤ IOUT ≤ 1 mA, VOUT = 6 V
0.50
3.3
1 mA ≤ IOUT ≤ 15 mA, VOUT = 6 V
2.5
6
1.4
6
450
1000
1 V ≤ VOUT ≤ VREG − 1.3 V, RL = 470 Ω
200
1000
1 V ≤ VOUT ≤ VREG − 1.2 V, RL = 5 kΩ (3)
TJ = 25°C
1000
3500
700
3500
1 V ≤ VOUT ≤ VREG − 1.2V, RL = 470 Ω
TJ = 25°C
AV
(3)
1 V ≤ VOUT ≤ VREG − 1.3 V, RL = 5 kΩ
VSAT
Output saturation (4)
IL
Output leakage current
VIN = VREG + 100 mV
1
1.3
VIN = VREG − 100 mV, VOUT = 0 V
TJ = 25°C
0.1
0.5
VIN = VREG − 100 mV, VOUT = 0 V
0.1
1
181
227
En
Output noise voltage
IOUT = 1 mA, 10 Hz ≤ f ≤ 10 kHz
(2)
(3)
(4)
(5)
μA
V/V
1.2
TJ = 25°C
V
V/V
1
Internal feedback
resistor (5)
UNIT
mA/mV
VIN = VREG + 100 mV
TJ = 25°C
Rf
(1)
MAX (1)
IOUT = 1 mA, TJ = 25°C
1 mA ≤ IOUT ≤ 15 mA, VOUT = 6 V
Voltage gain
ΔVOUT/ΔVREG
TYP (2)
V
μA
135
140
kΩ
μVRMS
Limits are 100% production tested at 25°C. Limits over the operating temperature range are specified through correlation using
Statistical Quality Control (SQC) methods. The limits are used to calculate Averaging Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely parametric norm.
Actual test is done using equivalent current sink instead of a resistor load.
VSAT = V(IN) − VOUT, when the voltage at the IN pin is forced 100 mV above the nominal regulating voltage (VREG).
See Application and Implementation and Typical Characteristics sections for information on this resistor.
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6.6 LM3420A Electrical Characteristics
Unless otherwise specified, specifications apply over full operating temperature range and VIN = VREG, VOUT = 1.5 V.
PARAMETER
Regulation voltage
VREG
Regulation voltage
tolerance
IQ
Quiescent current
Transconductance
ΔIOUT/ΔVREG
Gm
MIN (1)
TEST CONDITIONS
8.358
8.4
8.442
IOUT = 1 mA
8.316
8.4
8.484
IOUT = 1 mA, TJ = 25°C
IOUT = 1 mA
Output saturation (4)
IL
Output leakage current
110
85
115
20 μA ≤ IOUT ≤ 1 mA, VOUT = 6 V
TJ = 25°C
1.3
3.3
20 μA ≤ IOUT ≤ 1 mA, VOUT = 6 V
0.75
3.3
1 mA ≤ IOUT ≤ 15 mA, VOUT = 6 V
3
6
1.5
6
550
1000
1 V ≤ VOUT ≤ VREG − 1.3 V, RL = 470 Ω
250
1000
1 V ≤ VOUT ≤ VREG − 1.2 V, RL = 5 kΩ (3)
TJ = 25°C
1500
3500
900
3500
(3)
VIN = VREG + 100 mV
1
1.3
VIN = VREG − 100 mV, VOUT = 0 V
TJ = 25°C
0.1
0.5
VIN = VREG − 100 mV, VOUT = 0 V
0.1
1
181
227
Output noise voltage
IOUT = 1 mA, 10 Hz ≤ f ≤ 10 kHz
6
μA
V/V
1.2
En
V
V/V
1
TJ = 25°C
UNIT
mA/mV
VIN = VREG + 100 mV
TJ = 25°C
Internal feedback
resistor (5)
(2)
(3)
(4)
(5)
1%
85
Rf
(1)
0.5%
–1%
IOUT = 1 mA
1 V ≤ VOUT ≤ VREG − 1.3 V, RL = 5 kΩ
VSAT
–0.5%
IOUT = 1 mA, TJ = 25°C
1 V ≤ VOUT ≤ VREG − 1.2 V, RL = 470 Ω
TJ = 25°C
AV
MAX (1)
IOUT = 1 mA, TJ = 25°C
1 mA ≤ IOUT ≤ 15 mA, VOUT = 6 V
Voltage gain
ΔVOUT/ΔVREG
TYP (2)
135
140
V
μA
kΩ
μVRMS
Limits are 100% production tested at 25°C. Limits over the operating temperature range are specified through correlation using
Statistical Quality Control (SQC) methods. The limits are used to calculate Averaging Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely parametric norm.
Actual test is done using equivalent current sink instead of a resistor load.
VSAT = VIN − VOUT, when the voltage at the IN pin is forced 100 mV above the nominal regulating voltage (VREG).
See Application and Implementation and Typical Characteristics sections for information on this resistor.
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6.7 Typical Characteristics
Figure 1. Bode Plot
Figure 2. Response Time
Figure 3. Response Time
Figure 4. Quiescent Current
Figure 5. Internal Feedback Resistor (Rf) Tempco
Figure 6. Normalized Temperature Drift
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Typical Characteristics (continued)
Figure 7. Output Saturation Voltage (VSAT)
8
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7 Parameter Measurement Information
7.1 Test Circuits
The test circuits shown in Figure 8, Figure 9 and Figure 10 can be used to measure and verify various LM3420
parameters. Test conditions are set by forcing the appropriate voltage at the VOUT Set test point and selecting the
appropriate RL or IOUT as specified in LM3420 Electrical Characteristics. Use a DVM at the “measure” test points
to read the data.
Figure 8. Circuit Used For Bode Plots
Figure 9. Circuit Used For Response Time
Figure 10. LM3420 Test Circuit
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8 Detailed Description
8.1 Overview
The LM3420 is a shunt regulator specifically designed to be the reference and control section in an overall
feedback loop of a lithium-ion battery charger. The regulated output voltage is sensed between the IN pin and
GROUND pin of the LM3420. If the voltage at the IN pin is less than the LM3420 regulating voltage (VREG), the
OUT pin sources no current. As the voltage at the IN pin approaches the VREG voltage, the OUT pin begins
sourcing current. This current is then used to drive a feedback device (opto-coupler), or a power device (linear
regulator, switching regulator, etc.), which servos the output voltage to be the same value as VREG.
In some applications, (even under normal operating conditions) the voltage on the IN pin can be forced above
the VREG voltage. In these instances, the maximum voltage applied to the IN pin should not exceed 20 V. In
addition, an external resistor may be required on the OUT pin to limit the maximum current to 20 mA.
8.2 Functional Block Diagram
IN
1.23 V
31 k
+
COMP
OUT
Rf
GND
8.3 Feature Description
8.3.1 Compensation
The inverting input of the error amplifier is brought out to allow overall closed-loop compensation. In many of the
applications circuits shown here, compensation is provided by a single capacitor (CC) connected from the
compensation pin to the out pin of the LM3420. The capacitor values shown in the schematics are adequate
under most conditions, but they can be increased or decreased depending on the desired loop response.
Applying a load pulse to the output of a regulator circuit and observing the resultant output voltage response is
an easy method of determining the stability of the control loop.
Analyzing more complex feedback loops requires additional information.
The formula for AC gain at a frequency (f) is shown in Equation 1:
and where
10
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Feature Description (continued)
•
Rf ≈ 181 kΩ
(1)
The resistor (Rf) in the formula is an internal resistor located on the die. Since this resistor value affects the
phase margin, the worst case maximum and minimum values are important when analyzing closed loop stability.
The minimum and maximum room temperature values of this resistor are specified in the LM3420 Electrical
Characteristics section of this data sheet, and a curve showing the temperature coefficient is shown in the curves
section. Minimum values of Rf result in lower phase margins.
8.3.2 VREG External Voltage Trim
The regulation voltage (VREG) of the LM3420 can be externally trimmed by adding a single resistor from the
COMP pin to the +IN pin or from the COMP pin to the GND pin, depending on the desired trim direction. Trim
adjustments up to ±10% of VREG can be realized, with only a small increase in the temperature coefficient. (See
temperature coefficient curve shown in Figure 11.)
Figure 11. Normalized Temperature Drift With Output Externally Trimmed
Decreasing VREG
Figure 12. Increasing VREG
Figure 13. Changing VREG
Formula for selecting trim resistor values is shown in Equation 2 and Equation 3, based on the percent of
increase (%incr) or percent of decrease (%decr) of the output voltage from the nominal voltage.
Rincrease = 26 × 105/%incr
Rdecrease = (154 × 105/%decr) − 181 × 103
(2)
(3)
8.4 Device Functional Modes
8.4.1 Operation as Control Section
The LM3420 is monolithic integrated circuits, which is suitable for charging and end-of-charge control for LithiumIon rechargeable batteries. In this application, LM3420 is the reference and control section in the overall
feedback loop. The regulated voltage is sensed between the IN pin and GND pin. If the voltage at the IN pin is
less than the regulating voltage (VREG), the OUT pin sources no current. As the voltage at the IN pin approaches
the VREG, the OUT pin begins sourcing current, which can drive a feedback device or a power device.
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9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The LM3420 regulator/driver provides the reference and feedback drive functions for a Lithium-Ion battery
charger. It can be used in many different charger configurations using both linear and switching topologies to
provide the precision needed for charging lithium-ion batteries safely and efficiently. Output voltage tolerances
better than 0.5% are possible without using trim pots or precision resistors. The circuits shown are designed for
2-cell operation, but they can readily be changed for either 1-, 3-, or 4-cell charging applications.
One item to keep in mind when designing with the LM3420 is that there are parasitic diodes present. In some
designs, under special electrical conditions, unwanted currents may flow. Parasitic diodes exist from OUT to IN,
as well as from GROUND to IN. In both instances the diode arrow is pointed toward the IN pin.
9.2 Typical Application: Constant Current/Constant Voltage Li-Ion Battery Charger
The circuit shown in Figure 14 performs constant-current, constant-voltage charging of two Li-Ion cells. At the
beginning of the charge cycle, when the battery voltage is less than 8.4 V, the LM3420 sources no current from
the OUT pin, keeping Q2 off, thus allowing the LM317 Adjustable voltage regulator to operate as a constantcurrent source. (The LM317 is rated for currents up to 1.5 A, and the LM350 and LM338 can be used for higher
currents.) The LM317 forces a constant 1.25 V across RLIM, thus generating a constant current of
ILIM = 1.25V/RLIM
(4)
Transistor Q1 provides a disconnect between the battery and the LM3420 when the input voltage is removed.
This prevents the 85-μA quiescent current of the LM3420 from eventually discharging the battery. In this
application Q1 is used as a low offset saturated switch, with the majority of the base drive current flowing through
the collector and crossing over to the emitter as the battery becomes fully charged. It provides a very low
collector to emitter saturation voltage (approximately 5 mV). Diode D1 is also used to prevent the battery current
from flowing through the LM317 regulator from the output to the input when the DC input voltage is removed.
As the battery charges, its voltage begins to rise, and is sensed at the IN pin of the LM3420. Once the battery
voltage reaches 8.4 V, the LM3420 begins to regulate and starts sourcing current to the base of Q2. Transistor
Q2 begins controlling the ADJ pin of the LM317 which begins to regulate the voltage across the battery and the
constant voltage portion of the charging cycle starts. Once the charger is in the constant voltage mode, the
charger maintains a regulated 8.4 V across the battery and the charging current is dependent on the state of
charge of the battery. As the cells approach a fully charged condition, the charge current falls to a very low value.
Figure 14. Constant Current/Constant Voltage Li-Ion Battery Charger
12
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Typical Application: Constant Current/Constant Voltage Li-Ion Battery Charger (continued)
9.2.1 Design Requirements
DESIGN PARAMETER
EXAMPLE VALUE
Input voltage
13 V - 20 V
Output voltage
8.4 V
Output current
1A
9.2.2 Detailed Design Procedure
9.2.2.1 Compensation Capacitor
The capacitor between OUT pin and COMP pin can be increase or decreased depending on the desired loop
response. Functional Block Diagram can be referred as different capacitance selection. In this case, 0.01-µF
capacitor is used.
9.2.3 Application Curve
Figure 15. Regulation Voltage vs Output Voltage and Load Resistance
9.2.4 Other Application Circuits
NOTE
Although the application circuits shown here have been built and tested, they should be
thoroughly evaluated with the same type of battery the charger will eventually be used
with.
Different battery manufacturers may use a slightly different battery chemistry which may
require different charging characteristics. Always consult the battery manufacturer for
information on charging specifications and battery details, and always observe the
manufacturers precautions when using their batteries. Avoid overcharging or shorting
Lithium-Ion batteries.
9.2.4.1 Low Dropout Constant Current/Constant Voltage 2-Cell Charger
Figure 16 shows a Li-Ion battery charger that features a dropout voltage of less than one volt. This charger is a
constant-current, constant-voltage charger (it operates in constant-current mode at the beginning of the charge
cycle and switches over to a constant-voltage mode near the end of the charging cycle). The circuit consists of
two basic feedback loops. The first loop controls the constant charge current delivered to the battery, and the
second determines the final voltage across the battery.
With a discharged battery connected to the charger, (battery voltage is less than 8.4 V) the circuit begins the
charge cycle with a constant charge current. The value of this current is set by using the reference section of the
LM10C to force 200 mV across R7 thus causing approximately 100 μA of emitter current to flow through Q1, and
approximately 1 mA of emitter current to flow through Q2. The collector current of Q1 is also approximately 100
μA, and this current flows through R2 developing 50 mV across it. This 50 mV is used as a reference to develop
the constant charge current through the current sense resistor R1.
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The constant current feedback loop operates as follows. Initially, the emitter and collector current of Q2 are both
approximately 1 mA, thus providing gate drive to the MOSFET Q3, turning it on. The output of the LM301A opamp is low. As the Q3 current reaches 1 A, the voltage across R1 approaches 50 mV, thus canceling the 50-mV
drop across R2, and causing the op-amp's output to start going positive, and begin sourcing current into R8. As
more current is forced into R8 from the op-amp, the collector current of Q2 is reduced by the same amount,
which decreases the gate drive to Q3, to maintain a constant 50 mV across the 0.05-Ω current sensing resistor,
thus maintaining a constant 1 A of charge current.
The current limit loop is stabilized by compensating the LM301A with C1 (the standard frequency compensation
used with this op-amp) and C2, which is additional compensation needed when D3 is forward biased. This helps
speed up the response time during the reverse bias of D3. When the LM301A output is low, diode D3 reverse
biases and prevents the op-amp from pulling more current through the emitter of Q2. This is important when the
battery voltage reaches 8.4 V, and the 1A charge current is no longer needed. Resistor R5 isolates the LM301A
feedback node at the emitter of Q2.
The battery voltage is sensed and buffered by the op-amp section of the LM10C, connected as a voltage follower
driving the LM3420. When the battery voltage reaches 8.4 V, the LM3420 begins regulating by sourcing current
into R8, which controls the collector current of Q2, which in turn reduces the gate voltage of Q3 and becomes a
constant voltage regulator for charging the battery. Resistor R6 isolates the LM3420 from the common feedback
node at the emitter of Q2. If R5 and R6 are omitted, oscillations could occur during the transition from the
constant-current to the constant-voltage mode. D2 and the PNP transistor input stage of the LM10C disconnects
the battery from the charger circuit when the input supply voltage is removed to prevent the battery from
discharging.
Figure 16. Low Dropout Constant Current/Constant Voltage 2-Cell Charger
9.2.4.2 High-Efficiency Switching Regulator Constant Current/Constant Voltage 2-Cell Charger
A switching regulator, constant-current, constant-voltage two-cell Li-Ion battery charging circuit is shown in
Figure 17. This circuit provides much better efficiency, especially over a wide input voltage range than the linear
topologies. For a 1-A charger an LM2575-ADJ. switching regulator IC is used in a standard buck topology. For
other currents, or other packages, other members of the SIMPLE SWITCHER® buck regulator family may be
used.
Circuit operation is as follows. With a discharged battery connected to the charger, the circuit operates as a
constant current source. The constant-current portion of the charger is formed by the loop consisting of one half
of the LM358 op amp along with gain setting resistors R3 and R4, current sensing resistor R5, and the feedback
reference voltage of 1.23 V. Initially the LM358 output is low causing the output of the LM2575-ADJ to rise thus
causing some charging current to flow into the battery. When the current reaches 1 A, it is sensed by resistor R5
(50 mΩ), and produces 50 mV. This 50 mV is amplified by the op-amps gain of 25 to produce 1.23V, which is
applied to the feedback pin of the LM2575-ADJ to satisfy the feedback loop.
Once the battery voltage reaches 8.4 V, the LM3420 takes over and begins to control the feedback pin of the
LM2575-ADJ. The LM3420 now regulates the voltage across the battery, and the charger becomes a constantvoltage charger. Loop compensation network R6 and C3 ensure stable operation of the charger circuit under
both constant-current and constant-voltage conditions. If the input supply voltage is removed, diode D2 and the
PNP input stage of the LM358 become reversed biased and disconnects the battery to ensure that the battery is
not discharged. Diode D3 reverse biases to prevent the op-amp from sinking current when the charger changes
to constant voltage mode.
14
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The minimum supply voltage for this charger is approximately 11 V, and the maximum is around 30 V (limited by
the 32-V maximum operating voltage of the LM358). If another op-amp is substituted for the LM358, make sure
that the input common-mode range of the op-amp extends down to ground so that it can accurately sense 50
mV. R1 is included to provide a minimum load for the switching regulator to assure that switch leakage current
does not cause the output to rise when the battery is removed.
Figure 17. High-Efficiency Switching Regulator Constant Current/Constant Voltage 2-Cell Charger
9.2.4.3 Low Dropout Constant Current/Constant Voltage Li-Ion Battery Charger
The circuit in Figure 18 is very similar to Figure 17, except the switching regulator has been replaced with a low
dropout linear regulator, allowing the input voltage to be as low as 10 V. The constant current and constant
voltage control loops are the same as the previous circuit. Diode D2 has been changed to a Schottky diode to
provide a reduction in the overall dropout voltage of this circuit, but Schottky diodes typically have higher leakage
currents than a standard silicon diode. This leakage current could discharge the battery if the input voltage is
removed for an extended period of time.
Figure 18. Low Dropout Constant Current/Constant Voltage Li-Ion Battery Charger
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9.2.4.4 High-Efficiency Switching Charger With High Side Current Sensing
Another variation of a constant current/constant voltage switch mode charger is shown in Figure 19. The basic
feedback loops for current and voltage are similar to the previous circuits. This circuit has the current sensing
resistor, for the constant current part of the feedback loop, on the positive side of the battery, thus allowing a
common ground between the input supply and the battery. Also, the LMC7101 op-amp is available in a very
small SOT-23-5 package thus allowing a very compact PC board design. Diode D4 prevents the battery from
discharging through the charger circuitry if the input voltage is removed, although the quiescent current of the
LM3420 is still present (approximately 85 μA).
Figure 19. High Efficiency Switching Charger
With High Side Current Sensing
9.2.4.5 Fast-Pulsed Constant Current 2-Cell Charger
A rapid charge Lithium-Ion battery charging circuit is shown in Figure 20. This configuration uses a switching
regulator to deliver the charging current in a series of constant current pulses. At the beginning of the charge
cycle (constant-current mode), this circuit performs identically to the previous LM2575 charger by charging the
battery at a constant current of 1 A. As the battery voltage reaches 8.4 V, this charger changes from a constant
continuous current of 1 A to a 5-second pulsed 1 A. This allows the total battery charge time to be reduced
considerably. This is different from the other charging circuits that switch from a constant current charge to a
constant voltage charge once the battery voltage reaches 8.4 V. After charging the battery with 1 A for 5
seconds, the charge stops, and the battery voltage begins to drop. When it drops below 8.4 V, the LM555 timer
again starts the timing cycle and charges the battery with 1 A for another 5 seconds. This cycling continues with
a constant 5-second charge time, and a variable off time. In this manner, the battery is charged with 1 A for 5
seconds, followed by an off period (determined by the battery's state of charge), setting up a periodic 1-A charge
current. The off time is determined by how long it takes the battery voltage to decrease back down to 8.4 V.
When the battery first reaches 8.4 V, the off time is very short (1 ms or less), but when the battery approaches
full charge, the off time begins increasing to tens of seconds, then minutes, and eventually hours.
The constant-current loop for this charger and the method used for programming the 1-A constant current is
identical to the previous LM2575-ADJ charger. In this circuit, a second LM3420-8.4 has its VREG increased by
approximately 400 mV (via R2), and is used to limit the output voltage of the charger to 8.8V in the event of a
bad battery connection, or the battery is removed or possibly damaged.
The LM555 timer is connected as a one-shot, and is used to provide the 5-second charging pulses. As long as
the battery voltage is less than the 8.4 V, the output of IC3 is held low, and the LM555 one-shot never fires (the
output of the LM555 is held high) and the one-shot has no effect on the charger. Once the battery voltage
exceeds the 8.4-V regulation voltage of IC3, the trigger pin of the LM555 is pulled high, enabling the one shot to
begin timing. The charge current is now pulsed into the battery at a 5-second rate, with the off time determined
by the battery's state of charge. The LM555 output goes high for 5 seconds (pulling down the collector of Q1)
which allows the 1-A constant-current loop to control the circuit.
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Figure 20. (Fast) Pulsed Constant Current 2-Cell Charger
9.2.4.6 MOSFET Low Dropout Charger
Figure 21 shows a low dropout constant voltage charger using a MOSFET as the pass element, but this circuit
does not include current limiting. This circuit uses Q3 and a Schottky diode to isolate the battery from the
charging circuitry when the input voltage is removed, to prevent the battery from discharging. Q2 should be a
high-current (0.2-Ω) FET, while Q3 can be a low-current (2-Ω) device.
Figure 21. MOSFET Low Dropout Charger
10 Power Supply Recommendations
The LM3420 is designed to operated from up to 20-V input voltage supply. This input supply must be well
regulated. If the input supply is noisy, additional input capacitors with low ESR can help to improve the output
noise performance.
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11 Layout
11.1 Layout Guidelines
For best overall performance, place all the circuit components on the same side of the circuit board and as near
as practical to the respective pin connections. Place ground return connections to the input and output capacitor,
and to the regulator ground pin as close to each other as possible, connected by a wide, component-side, copper
surface. The use of vias and long traces to create circuit connections is strongly discouraged and negatively
affects system performance. This grounding and layout scheme minimizes inductive parasitic, and thereby
reduces load-current transients, minimizes noise, and increases circuit stability. A ground reference plane is also
recommended and is either embedded in the PCB itself or located on the bottom side of the PCB opposite the
components. This reference plane serves to assure accuracy of the output voltage, shield noise, and behaves
similar to a thermal plane to spread heat from the device. In most applications, this ground plane is necessary to
meet thermal requirements.
11.2 Layout Example
VOUT
VIN
C
B
3
2
OUTPUT
E
2
3
GND
1
B
IN
COMP
OUT
C
4
1
ADJ
5
4 (TAB)
INPUT
E
GND
Figure 22. LM3420 Layout
12 Device and Documentation Support
12.1 Trademarks
SIMPLE SWITCHER is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
12.2 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
12.3 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
18
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13 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM3420AM5-8.4/NOPB
ACTIVE
SOT-23
DBV
5
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
D03A
LM3420M5X-8.4/NOPB
ACTIVE
SOT-23
DBV
5
3000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
D03B
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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