LT1510 Design Manual

Application Note 68
December 1996
LT1510 Design Manual
Applications Engineering Staff
INTRODUCTION
The ever-growing popularity of portable equipment in
recent years has pushed battery technologists to search
for battery types that store more energy in a smaller
volume, weigh less and are safer. Also, the power source
selection for charging the batteries has diversified. For
example, a notebook computer can be connected to a
car battery, a power adapter, a docking station or even to
solar cells.
The variety of input voltages, coupled with the need for
high efficiency and the need for accurate constant voltage
and constant current, as in the case of Li-Ion batteries (see
below), have led to the introduction of a switching type
constant-voltage, constant-current battery charger IC, the
LT®1510.
Being a switching regulator, the LT1510 can operate over
a large range of input voltages, up to 28V, with efficiency
in the 90% range. Because the LT1510 operates in current
mode, its output performance is not affected by input
voltage changes.
An important feature of the LT1510 is its constant-current
output (see Figure 1). Although other switching regulators
offer current limiting, the LT1510 offers constant current
with 5% accuracy. In addition, the transition from constant current to constant voltage and back is very smooth.
Only a few basic calculations are required to design with
the LT1510. As the reader will see later, a voltage divider
(two resistors) and a current programming resistor need
to be selected for the constant voltage and constant
current, respectively.
In constant-current/constant-voltage operation, the
LT1510 can charge lithium-ion (Li-Ion) and sealed-leadacid (SLA) batteries. In constant-current only operation,
the LT1510 can charge nickel-metal-hydride (NiMH) and
nickel-cadmium (NiCd) batteries.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TABLE OF CONTENTS
ON BATTERIES AND CHARGERS ............................................................................................................................................. AN68-2
LT1510 OPERATION/BLOCK DIAGRAM.................................................................................................................................... AN68-5
COMPONENT SELECTION ........................................................................................................................................................ AN68-8
INTEGRATING THE LT1510 INTO A SYSTEM ......................................................................................................................... AN68-13
CHARGING BATTERIES/TERMINATION METHODS ................................................................................................................ AN68-16
APPENDIX A: TEST RESULTS ................................................................................................................................................ AN68-28
APPENDIX B: AUXILIARY CIRCUITS FOR TESTING BATTERIES AND CHARGERS ................................................................. AN68-35
AN68-1
Application Note 68
ADAPTER
VCC
ADAPTER
LT1510
GND
BAT
R1
OVP
PROG
+
+
BATTERY
Another product in the Linear Technology Corporation line
of constant-current /constant-voltage battery charger ICs
is the LT1511. The LT1511 offers higher charge current
(3A) and total system current control. The LT1511 can be
configured so that when the system current requirement
increases, the charge current decreases and the total
system current matches the power adapter’s maximum
current.
R2
R3
OUTPUT/CHARGE
CURRENT
CONSTANT CURRENT
CONSTANT VOLTAGE
ICONST =
2.465 (2000/R3)
VCONST =
2.465 (1 + R1/R2)
battery charger circuits developed by Linear Technology
Corporation. These examples are intended to serve as
starting points in your design process. Most of them can
be adapted to your specific application and battery chemistry needs.
ON BATTERIES AND CHARGERS
OUTPUT/
BATTERY
VOLTAGE
AN68 F01
Figure 1. Constant-Current and Constant-Voltage Modes
of the LT1510 with Simplified Schematic (Above)
Other advantages of designing with the LT1510 include:
• Efficiency in the 90% range
• High constant-voltage and constant-current accuracy
• An internal sense resistor that can be connected to either
terminal of the battery
• Wide range of battery voltages (2V to above 20V)
Developing a stand-alone or embedded battery charger for
today’s portable products, incorporating the latest battery
technology, requires careful consideration of how the
system elements, including battery, charger, system controller and system load, work together.
An understanding of the charging characteristics of the
battery and the application’s requirements is necessary in
order to design a reliable battery charger. A fast battery
charger must quickly recharge a battery to full charge. Fast
charging batteries requires accurate charge termination
when the battery is fully charged in order to prevent
damage or reduced battery life. Similarly, excessive discharging can damage any battery type.
• Small inductor
Battery Charger Checklist
• Internal power switch
In order to organize your approach, start with the following
checklist of design considerations:
• Low drain in sleep mode
• Soft start
• Select an appropriate battery for your application’s
needs
• Shutdown control
• Know your battery charging needs
• Up to 1.5A charge current
This application note will show how to design with the
LT1510 and will describe various methods of terminating
the charge. Test results and test methods are included in
the appendices. The circuits in this application note are
constant-current and constant-current/constant-voltage
AN68-2
✓
Charging current
✓
Charging voltage and its temperature dependence
✓
Primary charge termination method for the battery
✓
Secondary (safety) charge termination method
Application Note 68
✓
✓
✓
Can you include a temperature sensor in your battery
pack?
Table 1. Battery Type Characteristics
SLA
NiCd
NiMH
Li-Ion
Recommended charging method to achieve longest
battery life
Energy Density (W-Hr/kg)
30
40
60
90
Energy Density (W-Hr/l)
60
100
140
210
Effect of cell temperature on charging method and
charge level
Operating Cell Voltage (V)
Discharge Profile
2.0
1.2
1.2
3.6**
Sloping
Flat
Flat
Sloping
800
1000
Life in Cycles*
500
1000
• Stand-alone or embedded charger?
Self-Discharge
3%/mo
15%/mo
20%/mo
6%/mo
• What charger power source is available?
Internal Resistance
Low
Lowest
Moderate
Highest
Discharge Rate
<5C
<10C
<3C
<2C
• Single or multiple battery pack charging capability?
• Environmental operating conditions
• Charging interval: standard (overnight) or fast
(< 4 hours)
• Will your application draw current during battery
charging?
• What kind of control do you need over your battery
charger? (shutdown, fault signals, charge level signals,
charge completion signals)
• What will your charger do when the battery is removed?
• What happens when a fully charged battery is “hot
plugged” or “cold plugged” into the charger?
• What happens when the adapter is plugged live into the
system?
Comparing Four Rechargeable Battery Chemistries
The major rechargeable batteries readily available today
are nickel-cadmium (NiCd), nickel-metal-hydride (NiMH),
sealed-lead-acid (SLA) and lithium-ion (Li-Ion). These
batteries serve the common function of supplying renewable energy to user applications, but not every battery type
is appropriate for every application. Different battery technologies have distinctive characteristics that determine
their suitability for a particular use. These characteristics
include energy density, cell voltage, battery internal resistance, maximum charge rate, discharge profile, life (number of charge/discharge cycles), self-discharge rate and
discharge rate. Table 1 shows typical characteristics of
batteries of each chemistry.
* Until only 80% of initial charge capacity is achievable upon recharge.
** The operating cell voltage drops during discharge. This is an average
voltage.
Understand the Charging Requirements for Your Battery: Different battery chemistries have different charge
requirements. The descriptions below indicate the most
common charging methods for these battery types. Additional methods, such as pulse charging, are not covered
here.
System reliability may require a primary and a secondary
termination method for preventing overcharge.
Battery capacity is described by the bold letter C, which
represents the capacity in ampere-hours. Charging at C
rate means charging at a current of C amps (for instance,
charging a 1.3AH battery at 1.3A rate). Batteries are not
100% efficient in converting charge current into stored
charge. It therefore takes longer than one hour to charge
a battery to full capacity when charging at the C rate.
Consult your battery manufacturers for their recommended
charging rates and methods.
• Nickel-Cadmium: NiCd batteries are charged with a
constant-current profile. NiCd batteries can be continuously charged at the standard C/10 trickle rate indefinitely without excessive temperature rise or damage.
Fast charging NiCd batteries requires a charge termination method. Primary termination can be based upon
∆T/∆t (rate of temperature rise) or – ∆V (cell voltage
decrease at full charge) sensing. It is recommended that
the secondary termination be a ∆TCO (temperature rise
over ambient) termination. Many manufacturers’ NiCd
batteries can be charged at significantly greater than the
C/1 rate, reducing the charge time to as little as fifteen
minutes.
AN68-3
Application Note 68
• Nickel-Metal-Hydride: NiMH batteries are charged with
a constant-current profile. The standard C/10 rate charging works well for overnight charges. NiMH batteries are
more susceptible to damage from overcharging than
NiCd batteries; the charge must therefore be reduced to
C/40 or switched to a pulsed trickle charge after 16
hours. This can be implemented with a timer.
Fast charging NiMH cells also require charge termination. NiMH cells exhibit a slower increase in cell voltage
during charge than NiCd cells and a flatter peak. Primary
termination can be based on sensing the zero dV/dt
condition of the battery voltage characteristic (voltage
peak) or ∆TCO. Following this, the charging circuit
should reduce the current to a maintenance charge of
C/40 or a pulsed trickle charge to counteract the batteries’ self-discharge characteristic. Secondary termination can be temperature related or controlled by a timer.
• Lithium-Ion: Li-Ion batteries are charged with a constant-voltage, current-limited supply. These batteries
require special attention due to their susceptibility to
damage in overcharge, deep-discharge and short-circuit conditions. Constant current is supplied until the
cell voltage reaches 4.1V or 4.2V per cell (depending on
the manufacturer), followed by constant-voltage charging with the required accuracy of ±50mV per cell. The
charging current then tapers down naturally. Increased
battery cycle life may be achieved by terminating the
charge 30 to 90 minutes after the charging current drops
below some current threshold.
Several manufacturers include fault-sensing and current-balancing circuits within the battery pack. The
fault-sensing circuit will open a series connection within
the battery pack in the event of excessive cell voltage,
discharge current or temperature, or in the event of an
undervoltage condition. The use of battery packs containing appropriate cell monitor/control devices is recommended. The current-balancing circuit diverts charge
current from fully charged cells to partially charged
cells.
• Sealed-Lead-Acid: SLA batteries can be charged with a
constant-voltage, current-limited supply or with a constant-current supply. For standby power applications,
constant-voltage (float) charging is the traditional
method. Charge is delivered at the current limit until the
“float” voltage across the battery is reached and the
voltage is then held constant while the current into the
battery decreases naturally as the cells reach full charge.
The float voltage of approximately 2.25V per cell can be
maintained indefinitely. For longer battery life, the float
voltage should change by – 1mV to – 5mV per °C per cell
to match the cell voltage temperature dependence.
Charge Termination Techniques: To prevent battery damage and extend battery cycle life with fast charging, it is
necessary to terminate charging after NiCd and NiMH cells
have reached full charge. In addition, it is a recommended
practice to provide a secondary charge termination method
as a safety measure. Some charge algorithms include
“top-off” charge stages before completing the charge.
Fast charging with the constant-voltage method is
achieved by increasing the charging voltage to approximately 2.45V per cell, which extends the charging time
at the current limit and reduces total charge time. When
the battery voltage reaches the constant charging voltage, the current decreases naturally as the cells reach
full charge. After a minimum charge-current level is
reached, the charging voltage is reduced to a nominal
float voltage or stopped.
A number of methods have been used to detect the fully
charged condition of a battery and terminate the charge.
Several reliable termination methods are based on the
thermal release of energy in the battery near full charge.
The voltage characteristic of a battery during constantcurrent charge is also an indicator of the electrochemical
process of battery cell recharging, and several of the
following methods are based on battery voltage. Not all
termination methods are good for all battery types.
Fast charging with the constant-current method requires monitoring the battery voltage. Consult manufacturers’ data sheets for battery characteristic details.
The most common termination techniques are discussed
below. The best suited battery for the termination is given
in parenthesis.
AN68-4
Application Note 68
• IMIN: After the charger has reached a constant-voltage
state, the charge current tapers off. Termination is
triggered when the current drops below a set current
threshold. (Li-Ion, SLA)
• dT/dt detects the rate of change in temperature with
time. Termination can be based on a maximum dT/dt,
such as 1°C/min for NiCd. (NiCd)
• ∆TCO (delta temperature cutoff) detects temperature
rise over ambient temperature. Terminates on preset
threshold temperature differential. (NiCd, SLA)
• TCO (temperature cutoff) represents an absolute battery
temperature at which the charge is terminated.
(NiCd, NiMH)
• – ∆V (negative delta V): At constant current, NiCd and
NiMH batteries exhibit a temperature rise toward the end
of charge. Since they have a negative temperature
coefficient, their voltage drops. Termination is activated
when a decrease in battery voltage is detected. (NiCd,
NiMH)
• dV/dt (slope of voltage time curve) detects the rate of
change of battery voltage with time at constant-current
charge. (NiCd)
• Zero dV/dt (zero voltage change) detects the actual peak
voltage at constant-current charge. (NiMH)
• Slope inflection method (using the second derivative of
VBAT versus time) detects the negative going, zerocrossing rate of change in slope of the voltage/time
curve just before charge completion. (NiCd)
• Threshold voltage detection terminates charge or reduces charge current significantly when the battery
reaches a certain maximum voltage.
• Timer sets the maximum charging time limit and terminates when the limit is reached. (Li-Ion, NiCd, NiMH,
SLA)
Table 2 summarizes the standard charge and fast charge
information.
Table 2. Battery Charging Characteristics
SLA
NiCd
NiMH
Li-Ion
Current
Limit
Float
Voltage
Constant
Current
Constant
Current
Current
Limit
Constant
Voltage
Constant Current (A)
0.25C
0.1C
0.1C
0.1C
Constant Voltage
(V/Cell)
2.25
1.50
1.50
4.1V or
4.2V±50mV
24
16
16
0°/45°C
5°/40°C
5°/40°C
5°/40°C
None
None
Timer
Timer
Standard Charge
Time (hours)
Temperature Range
Termination
Fast Charge
Constant Current
>1.5C
>1C
>1C
1C
Constant Voltage
(V/Cell)
2.45
1.50
1.50
4.1 or
4.2V±50mV
Typical Time (hours)
<1.5
<3
<3
<2.5
Temperature Range
0°/30°C
15°/40°C
15°/40°C
10°/40°C
Primary Termination
IMIN*
dT/dt
∆TCO
– ∆V
Zero dV/dt IMIN*+ Timer
– ∆V
dT/dt
∆TCO
Slope
Inflection
∆TCO
Secondary Termination
Timer
TCO
TCO
TCO
∆TCO
Timer
Timer
Timer
*IMIN is minimum current threshold termination.
LT1510 OPERATION/BLOCK DIAGRAM
The LT1510 is a current mode, PWM (pulse-width modulated), positive buck switcher that operates in one of two
states: constant current or constant voltage. Figure 2
shows a block diagram of the LT1510 along with a typical
charger circuit. The main functions in the diagram can be
divided into three major groups: the PWM switcher function, the constant-current function and the constantvoltage function. Only constant-current or constant-voltage components can be active at any given time. The PWM
switcher function is active as long as the charger is not
disabled; it includes a 200kHz oscillator, set-reset flipflop, switch QSW, PWM comparator C1, buffer B1 and
current amplifier CA2. The constant-current components
include RS1 and CA1. The constant-voltage components
include RS1, CA1 and voltage amplifier VA.
AN68-5
Application Note 68
VIN
200kHz
OSCILLATOR
+
D1
VCC
SHUTDOWN
0.7V
+
–
VSW
S
CIN
BOOST
–
VCC
R
CB
QSW
+
R
+
SW
1.5V
DB
D2
SLOPE COMPENSATION
VBAT
L1
GND
–
PWM
C1
R2
SENSE
+
IPROG
+
+
–
B1
IBAT
CA1
RS1
R1
1k
R3
IPROG
= 500µA/A
IBAT
RV1
OVP*
+
–
VC
60k
+
CC
VA
CA2
gm = 0.64
VREF
Ω
ICHRG
BAT
–
–
VREF
2.465V
+
COUT
B
RV2
RC
PROG
RAVE
AN68 F02
*NOT AVAILABLE IN 8-PIN SO PACKAGE
RPROG
CAVE
Figure 2. LT1510 Block Diagram Showing Basic Charger Circuit
In constant-current operation, the 200kHz oscillator sets
the set-reset flip-flop (SR-FF), which turns QSW on. The
current rise through L1, RS1 and B is amplified by CA1,
converted from current to voltage by R1, buffered by B1
and compared to the steady-state voltage at the VC pin by
C1. When the VC level is reached at the positive input of C1,
the SR-FF resets and waits for the next 200kHz oscillator
set signal. Current regulation is achieved by a slow loop
containing RAVE, CAVE, CA2, CC and RC. Since CA2 and the
60k resistor constitute a high gain voltage amplifier, the
voltages at its negative input (or the PROG pin) and its
positive input are equal. In other words, in current mode
the voltage at the PROG pin is equal to VREF (2.465V).
AN68-6
Since the current gain of RS1 and CA1 is 2000, RPROG’s
value prescribes the constant-current current level. The
charge current level can be calculated from:
 2000 
ICONST = 2.465 

 RPROG
(01)
In constant-voltage operation, the 200kHz loop operates
similarly to that in constant-current, but the voltage regulation is achieved by overriding RS1 and CA1 with voltage
divider RV1, RV2 and voltage amplifier VA. The loop regulates the voltage at the OVP pin to equal VREF. The voltage
at the BAT pin in a constant-voltage state is:
Application Note 68
R +R 
VCONST = 2.465  V1 V2 
 R V2 
(02)
RAVE and CAVE smooth the output of CA1 in the constantcurrent state or VA in the constant-voltage state. CC and RC
compensate the regulation loop.
CIN bypasses the input and COUT smoothes the charge
current into the battery B.
Pin Descriptions
GND: This is the ground pin. There are two kinds of ground
pins for the LT1510: electrical and fused. The function of
the electrical ground pin is to serve as the analog ground
reference for the LT1510. For best regulation in constantvoltage operation, connect the bottom side of RV2 as
close as possible to this pin or run a separate trace from
the resistor to this pin. The four pins at the corners of the
16-pin package are fused to the internal die for heat
sinking. Connect these pins to expanded printed circuit
board copper areas for proper heat removal.
SW: The LT1510 topology is positive buck. The NPN
switch (QSW) in the positive buck topology is connected
between the input supply and the inductor/catch diode
node. Inside the LT1510, the bipolar switch is connected
between the VCC and SW pins. Keep the trace from the SW
pin to D2 short and wide. To minimize generated electromagnetic interference (EMI), keep the trace from the SW
pin to L1 as short and wide as possible.
BOOST: The monolithic NPN transistor selected for the
switch QSW is superior to a PNP in terms of speed and
collector resistance. However, its saturation voltage is
limited by its base-emitter drop. The LT1510’s BOOST pin
provides a means of bootstrapping the drive to QSW,
thereby allowing it to saturate against the input rail.
Capacitor CB, which is charged and refreshed during the
off time through diode DB and SENSE pin, acts as a
bootstrap. CB delivers base drive to QSW through the
BOOST pin. For best switch performance and, consequently, best efficiency and highest switching duty cycle,
connect the anode of DB to a source of 3V to 6V that is
active when the charger is.
OVP: The OVP (overvoltage protection) pin is used to
program the output voltage in the constant-voltage state.
The output voltage is sensed through a voltage divider
comprising RV1 and RV2 and fed back to the OVP pin. The
OVP feedback sense voltage is 2.465V.
SENSE: Inductor current and average charging current
are controlled by monitoring an on-chip 0.08Ω sense
resistor RS1. This resistor is internally connected between
the SENSE pin and the BAT pin. Inductor current information is used to control the buck regulator and the average
current information is used to control the battery charging
current. In most applications the sense resistor appears in
series with the output side of the buck regulator inductor,
but it is also possible to sense current at ground in the
negative terminal of the battery. Do not bypass the SENSE
pin. Note that the total pin-to-pin resistance is higher
(0.2Ω) than the value of the sense resistor itself.
BAT: The “downstream” side of the sense resistor (see
SENSE pin description) is connected to the BAT pin. In
most applications the BAT pin is connected directly to the
positive terminal of the battery. The BAT pin constitutes
the output of the buck regulator and must therefore be
bypassed. If the sense resistor is used to measure current
in the negative terminal of the battery, the BAT pin can be
grounded.
VC: The VC pin is used for frequency compensation of the
buck regulator in both constant-current and constantvoltage operation. The VC pin is also used for soft start and
shutdown. A CC capacitor of at least 0.1µF filters out noise
and controls the rate of soft start. Switching starts at 0.7V;
higher VC corresponds to higher charging current in
normal operation. For switching shutdown pull VC to
ground with a transistor.
PROG: The PROG pin is used to program the constant
current by selecting the RPROG value. CAVE and RAVE must
be connected to the PROG pin. CAVE averages the current
monitored through RS1 so the constant-current control
loop regulates the average current into the battery. During
normal operation, the PROG pin voltage stays close to
2.465V. If the PROG pin is shorted to ground, the switching will stop. When a current sinking device is connected
to PROG pin for the purpose of changing the constant
current charge, the device has to be able to sink current at
a compliance of up to 2.465V.
VCC: This is the input supply to the LT1510. Short VCC1 and
VCC2 together when using the 16-pin package. The
AN68-7
Application Note 68
operating voltage range is 8V to 28V. Below 8V the
undervoltage lockout may be activated and switching may
stop; 28V is the absolute maximum value. VCC must be at
least 2V above the highest battery voltage. VCC should not
be forced to > 0.7V below the SW pin because there is a
parasitic diode connected from the SW pin to the VCC pin.
For good bypassing, a low ESR capacitor of 10µF or higher
is required. The trace from the bypass capacitor to the VCC
pin should be as short as possible.
COMPONENT SELECTION
This section provides the user with the criteria for selecting the components of a typical circuit with 2-level constant-current (ICONST) charge, constant-voltage (VCONST)
charge and charge disable, as shown in Figure 3.
When selecting components, keep the following points in
mind:
• The topology of the LT1510 is positive buck.
• The average output voltage is regulated in the constantvoltage state.
2
C1
0.22µF
CR1
1N5819
The critical parameters for parts selection are discussed in
the following paragraphs. The designer must apply safety
margins as necessary for the system.
CA1 (internal to the LT1510) has 700µA of input bias
current typically. (This current flows into the BAT pin or
SENSE pin.) This current is supplied by the step-down
converter (through the SENSE pin) when the charger is
active. When the charger is in shutdown mode (VC < 0.3V),
there is an inherent BAT pin source current of 375µA
maximum that flows out of the BAT pin. When the battery
is present, it will absorb this current regardless of the
values of R1 and R2. However, if the battery is removed
and the charger is in shutdown mode, the output voltage
at BAT pin can rise to a voltage as high as:
If the output voltage must stay below battery voltage when
the battery is removed, the divider current must be at least
375µA. Adding a switch transistor, Q1, as shown in Figure
3, takes care of the increased battery drain. When VIN is
CR3
1N5819
1
+
–
• The recommended parts will operate over the full input
and output ranges at a constant current of up to 1.3A.
[(R1 + R2)(375µA)] volts
• The average output current is regulated in the constantcurrent state.
VIN
11V TO 25V
• The switching frequency is 200kHz.
L1***
33µH
CR2
1N914
3
GND*
GND*
SW
VCC2
BOOST
VCC1
16
15
14
13
PROG
U1
12
5
LT1510
OVP
VC
4
6
7
8
GND
SENSE
GND*
GND*
BAT
GND*
GND*
C3
0.1µF
11
B1**
10
9
+
C2
22µF
25V
TANT
+ Li-Ion
4.1V
+
4.1V
Q1
VN2222
R2
4.87k
0.25%
R1
11.3k
0.25%
R5
300Ω
C5†
10µF
25V
C4
1µF
R6
37.9k
R7
4.22k
NO_CHARGE
R3
100k
Q2
VN2222
+
HI_CHARGE
R4
1k
Q3
VN2222
R8
100k
AN68 F03
* SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
** MOLI ENERGY ICR-18650
*** COILTRONICS CTX33-2
† TOKIN OR MARCOM CERAMIC SURFACE MOUNT
Figure 3. A Typical LT1510 Based Li-Ion Charger Circuit
AN68-8
Application Note 68
removed, Q1 is off and battery drain is prevented. If there
are high frequency components at VIN, they may pass
through the gate to source capacitance of Q1 to the OVP
pin. An added 220k resistor between VIN and the gate of
Q1 attenuates the high frequency signal.
For better efficiency, it is recommended that the anode of
CR2 be connected to a voltage source between 3V and 6V.
A 10µF ceramic bypass capacitor should be connected to
the anode with a short lead.
Care must be taken when selecting input and output
capacitors. They should be rated at 200kHz and have
adequate ripple current rating. Poor choice of input or
output capacitors may be detected too late. The input
capacitor should be able to withstand the high surge
current that occurs when a power adaptor is hot-plugged
to the charger. The output capacitor should be able to
withstand the high surge current that occurs when a
battery is replaced. Tantalum capacitors have a known
failure mechanism when subjected to high surge currents.
The surge current ratings of tantalum capacitors are
proportional to their voltage ratings. Consult the manufacturer. The actual capacitance is not critical in most cases.
25V, tantalum, SMT) withstands transient current equal to
the operating voltage in amps (in other words, 25A for 25V
operating voltage).
The RMS ripple current can be calculated from:
IRIPPLE =
( VCC − VBAT)(VBAT)(0.29 )
(L)(VCC)(f)
(03)
where:
VCC is the maximum voltage at the VCC pin
VBAT is the voltage at BAT pin
f is the switching frequency (200kHz)
L is the minimum inductance of L1
For example, if the input voltage is 12V to 20V, the
inductor, L1, is 30µH ±8% and drops 35% maximum at
the charge current of 1A and the battery is a 3-cell NiCd, the
following values should be plugged into the equation
above:
VBAT = (1.6)(3) = 4.8V
High capacity ceramic capacitors by Tokin or United
Chemi-Con/MARCOM and electrolytic OS-CON capacitors
by Sanyo are recommended for the input. Solid tantalum
capacitors such as AVX TPS and Sprague 593D are
recommended for the output.
VCC = 20V
Boost Capacitor (C1) Selection
Compensation Capacitor (C3) Selection
A 0.22µF ceramic capacitor will work under all conditions.
The voltage across C1 is the battery voltage. An AVX
12065C224MAT2A surface mount capacitor performs
well.
A 0.1µF ceramic surface mount capacitor, such as the AVX
12065C104MAT2A, performs well and gives a soft start
duration of 3ms. For a longer soft start duration, a larger
value is required.
Output Capacitor (C2) Selection
PROG Pin Capacitor (C4) Selection
C2 removes the high frequency components of (smoothes)
the battery charge current.
This capacitor averages the output of CA1 (PROG pin) for
the constant-current loop. A 1µF ceramic capacitor is
recommended. The AVX 12063G105ZAT2A surface mount
capacitor performs well. A 300Ω resistor (see PROG pin
resistor) is required in series with C4.
The highest transient current through C2 occurs when the
battery is plugged in and the power adapter is not live. The
transient current magnitude depends on circuit construction and the components in the power path (traces, leads,
solder joints, etc.). The AVX TPSD226M025R0200 (22µF,
L = (30)(0.92)(0.65) = 17.9µH
After plugging the numbers in, the calculated maximum
ripple current is 0.3A RMS.
AN68-9
Application Note 68
50
Input Capacitor (C5) Selection
45
The trace from C5 to the VCC pin of the LT1510 and to the
ground plane should be as short and wide as possible.
Catch Diode (CR1) Selection
CR1 serves as the catch diode in the schematic of Figure
3. The reverse voltage across it is VIN and the maximum
average forward current is:
I 
ICR1( AVE ) =  OUT  VIN − VOUT
 VIN 
(
)
(04)
where:
IOUT is the maximum output current,
VIN is the maximum input voltage and
VOUT is the lowest output voltage.
A high speed Schottky power rectifier is recommended. A
Motorola 1N5819 (leaded) or MBRS140LT3 (SMT) performs well.
Boost Diode (CR2) Selection
The maximum reverse voltage of CR2 is VIN. The average
current is the BOOST pin current, which can be found in
the “Switch Current vs Boost Current vs Boost Voltage”
graph in Figure 4. The peak current is higher. A switching
diode, such as the Motorola 1N914 (leaded) or
MMBD914LT1 (SMT), performs well here.
AN68-10
VBOOST = 38V
28V
18V
40
BOOST CURRENT (mA)
C5 bypasses the input supply for the LT1510. It is assumed that C5 conducts all input AC switching current.
The maximum RMS ripple current through C5 is approximately half the DC charging current. A high transient
current flows through the input capacitor when a power
adapter is “hot plugged” to the charger. A Tokin
1E106Z5YU-C304F-T (10µF, 25V, ceramic, SMT) performs well. Other alternatives are ceramic capacitors by
United Chemi-Con/MARCON, OS-CON capacitors by Sanyo
and high ripple current electrolytics. Caution must be
exercised when using tantalum capacitor for bypassing.
Only a few tantalum capacitor types, such as AVX TPS and
Sprague 593D series, have the ripple and transient current
ratings required. Consult the manufacturer.
VCC = 16V
35
30
25
20
15
10
5
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
AN68 F04
Figure 4. Switch Current vs Boost Current vs Boost Voltage
Input Diode (CR3) Selection
CR3 provides polarity protection, prevents battery drain
and eliminates current transient spikes. When the power
adapter is removed without the input diode, there could be
battery drain through the following path; positive battery
terminal, BAT pin to SENSE pin through the internal sense
resistor, L1 and the SW pin to the VCC pin through the
internal parasitic diode.
CR3 also eliminates large current transient spikes that can
occur when a power adapter with a large output capacitor
is cold-plugged into the system. The current transient may
compromise the input capacitor (C5) and the connector
contacts unless CR3 is installed.
A low forward voltage rectifier will increase charger efficiency. The reverse voltage is the maximum battery voltage. The average forward current is:
ICR3 ( AVE ) =
(I )(V )
CHRG
BAT
VIN
(05)
CR3 should be able to withstand the current transient that
occurs when the power adapter is “hot-plugged” to the
charger. A Motorola 1N5819 (leaded) or MBRS140LT3
(SMT) performs well.
Switching Inductor (L1) Selection
L1 is an essential part of the positive buck topology as
shown in Figure 3. The average current that flows through
Application Note 68
L1 is the charge current. A 30µH inductor is acceptable for
most applications. A Coiltronics CTX33-2 with windings in
parallel or a Coiltronics CTX8-1 with windings in series
performs well.
L1’s maximum peak current can be calculated from:
(
)
IL1 PEAK = ICHRG +
(0.5)(VIN − VOUT )(VOUT )
(VIN)(L)(f)
(06)
where:
VIN is the maximum input voltage,
VOUT is the lowest output (battery) voltage,
f is the switching frequency (200kHz),
L is the minimum inductance (H) of L1 and
ICHRG is the maximum charge current.
Battery Drain Inhibit MOSFET (Q1) Selection
Q1’s drain current is VBAT/(R1 + R2), when the charger is
active, or 375µA when the charger is in shutdown mode.
If the VIN line has high frequency noise that can penetrate
through the gate-to-source capacitance of Q1, it is recommended that a 220k resistor be added in series with the
gate. A VN2222 or 2N7002 are typical choices for leaded
or SMT parts.
Shutdown MOSFET (Q2) Selection
If the manufacturer does not recommend a float voltage
(indefinite constant voltage across the battery), as in the
case of Li-Ion, the charger must shut down at the end of
the charge regimen. This can be achieved by forcing the VC
pin low with MOSFET Q2. The drain current is 1mA. A
VN2222 or 2N7002 are typical choices for leaded or SMT
parts.
High Charge MOSFET (Q3) Selection
When Q3 is on, the equivalent value of R6 and R7 in parallel
becomes the constant-current programming resistor (see
R6, R7 selection). The off-state drain leakage current of Q3
should be significantly lower than the programming current through R6. A VN2222 or 2N7002 are typical choices
for leaded or SMT parts.
Constant-Voltage Programming Resistors (R1, R2)
Selection
The programming resistors R1 and R2 divide the battery
voltage (constant voltage) down to the threshold level of
2.465V. It is recommended that the voltage divider resistance R1 + R2 have a high value so that the battery drain
is kept to minimum and the need for Q1 is eliminated.
There is, however, a restriction: shutdown output voltage
(see Page 8). If there is a limit to the output voltage when
the battery is removed and the charger is in shutdown
mode, then maximum allowed branch resistance is,
VBAT(MAX)/375µA.
If output voltage with battery removed is not an issue,
divider current can be much lower. As an example, with
VBAT = 8.2V, and drain current of 25µA:
R2 ≅
2.465V
= 98.6 k
25µA
(07)
R2 is selected as 100k.
R1 ≅
(VBAT − VREF) = 232.2k
(2.465 /R2) + 50nA
(08)
R1 is selected as 232k.
Another example for a 2-cell Li-Ion battery that requires
8.2V ±100mV. The maximum output voltage in shutdown
mode (with battery removed) is 8.5V.
The maximum value for R2 is:
R2 ≤
2.465
= 6.57k
375µA
(08.1)
R2 is selected as 4.87k. R1 is calculated as:
R1 ≅
(8.2 − 2.465) = 11.33k
2.465
+ 50nA
4.87k
(09)
R1 is selected as 11.3k.
The battery drain current for an inactive charger is
8.4V/(4.87kΩ + 11.3kΩ) = 0.52mA. Q1 can be added to
eliminate this drain current.
The recommended tolerance for R1 and R2 is 0.25%.
AN68-11
Application Note 68
Compensation Resistor (R4) Selection
R6 is selected as 38.3k.
(2.465)(2000) = 3.79k
This resistor is part of the compensation loop. A 1k, 5%
resistor is recommended for most cases. The stability of
the current loop can be tested by forcing a voltage step
across the output while in a constant-current state.
REQU ≅
PROG Pin Resistor (R5) Selection
1
1
1
≅
−
; R7 ≅ 4.206k
R7 3.79 38.3
This resistor is part of the compensation loop. A 300Ω, 5%
is recommended for most cases. The stability of the
voltage loop can be tested by forcing a current step across
the output while in a constant-voltage state.
Constant-Current Programming Resistors (R6, R7)
Selection
The values of R6 and R7 can be found from the following
formula:
REQU =
(2.465)(2000)
(10)
ICONST
where REQU equals R6 for the low constant-current charge
and R6 in parallel with R7 for high constant-current
charge. As an example, a 1.3AH battery has to be charged
at C and trickle charged at C/10 (0.13A).
(2.465)(2000) = 37.9k
An 8-pin package can be used if only constant-current
operation is required, but a 16-pin package must be used
for constant-current and constant-voltage operation and
for better heat dissipation. The four corner pins of the
SO-16 package are fused (connected to the die internally)
and it is recommended that these pins be soldered to the
ground plane. With these pins soldered to expanded PC
lands, the thermal resistance of the SO-16 package is
50°C/W. The thermal resistance of the DIP package is
75°C/W.
The plots in Figure 5 can be used for selecting an SO-8 or
SO-16 package based on input voltage, output voltage and
output current. These curves can be used to a maximum
ambient temperature of 60°C. Refer to the data sheet for
more accurate thermal calculations.
THERMALLY LIMITED MAXIMUM CHARGING CURRENT
16-PIN SO
1.3
1.5
MAXIMUM CHARGING CURRENT (A)
1.1
4V BATTERY
0.9
8V BATTERY
0.7
12V BATTERY
0.5
16V BATTERY
4V BATTERY
8V BATTERY
1.3
12V BATTERY
1.1
16V BATTERY
0.9
(θJA =50°C/W)
TAMAX =60°C
TJMAX =125°C
0.7
0.5
0.3
0
5
15
10
INPUT VOLTAGE (V)
20
25
0
5
15
10
INPUT VOLTAGE (V)
20
25
AN68 F05
Figure 5. Comparing SO-8 and SO-16 Packages Thermal Limits
AN68-12
(13)
The LT1510 (U1) Package Selection
THERMALLY LIMITED MAXIMUM CHARGING CURRENT
8-PIN SO
(θJA =125°C/W)
TAMAX =60°C
TJMAX =125°C
(12)
R7 is selected as 4.22k.
(11)
0.13
MAXIMUM CHARGING CURRENT (A)
R6 ≅
1.3
Application Note 68
INTEGRATING THE LT1510 INTO A SYSTEM
VIN1 = 8 + VDIN
VIN2 = 2 + VDIN + VBAT
When an LT1510 based charger is integrated into a system
with a power adapter or power supply as a source, and a
battery and a switching regulator as a load, some issues
need to considered. The next paragraphs describe a few
of them.
Minimum Operating Input Voltage
The minimum operating input voltage of an LT1510 based
battery charger has to satisfy three LT1510 requirements:
undervoltage lockout (VIN1), minimum VCC – VBAT voltage
difference (VIN2) and maximum duty cycle (VIN3). See
equations below. Other factors that affect the minimum
operating input voltage are maximum output voltage,
input diode forward voltage, resistance along the power
path (including sense resistor, switch on resistance, trace
resistance, solder joint resistance and connector resistance) coupled with maximum charge current.
The undervoltage lockout is 8V. The input-to-output voltage difference (VCC – VBAT) is 2V and is defined by two
parameters: “maximum VBAT with switch on” and “input
common mode limit (high),” as found in the data sheet.
The maximum duty cycle is 0.93. The following equations
can be used for calculating the minimum input voltage VIN.
VIN3 = VDIN +
(14)
(
)( )
VBAT + ICONST 0.7
(15)
D
where:
VIN is the charger minimum input voltage. Use the highest
of VIN1, VIN2 or VIN3.
VDIN is the forward voltage of the input diode. The input
diode can be replaced by a PMOSFET switch, in which case
this term is removed.
ICONST is the programmed charge current.
VBAT is the maximum battery voltage.
D is the maximum duty cycle. D = 0.93.
To give the designer preliminary data about the typical
lowest input voltage, the circuit in Figure 6 was tested. The
constant current was adjusted with a trim pot connected
to the PROG pin and the output voltage (VOUT) was forced
with the battery simulator board (see Appendix B). The
results are shown in Figure 7.
CR3
MBRS140L
VIN
1
2
C1
0.22µF
CR1
MBRS140L
L1
CTX33-2
CR2†
NC
3
GND
SW
VCC2
BOOST
VCC1
16
15
C5**
14
13
PROG
U1
12
5
OVP LT1510 VC
4
6
7
* AVX TPSD226M025RO200
** TOKIN 1E106ZY5U-C304F-T
† MOTOROLA MMBD914LT1
GND
8
GND
SENSE
BAT
GND
GND
GND
GND
11
10
9
C3
0.1µF
+
C2*
BATTERY
SIMULATOR
R4
1k
C4
1µF
R5
300Ω
RPROG
AN68 F06
Figure 6. Minimum Operating Voltage Test
AN68-13
Application Note 68
mended that a 2200µF capacitor be connected in place of
the battery before making the measurement. (Be sure to
observe capacitor polarity and to connect the capacitor
when the charger is not running.)
25
MINIMUM INPUT VOLTAGE (V)
23
21
19
17
I = 1.3A
I = 0.25A
15
Connecting the System Circuits
to the BAT Pin or SENSE Pin
13
11
9
7
5
5
7
9
11 13 15 17
OUTPUT VOLTAGE (V)
19
21
AN68 F07
Figure 7. Typical Minimum Operating Voltage of an LT1510
Based Charger vs Output Voltage and Output Current
Efficiency
The circuit in Figure 6 was used to test efficiency. The plots
in Figure 8 show the results for output voltage of 8V. Some
power is dissipated in the input diode. See the data sheet
for a circuit with a PMOSFET switch that bypasses the
input diode when VIN is on and saves power.
90
VOUT = 8V
EFFICIENCY (%)
89
88
IOUT = 1A
87
86
IOUT = 1.3A
85
84
8
10
12
14 16 18 20 22
INPUT VOLTAGE (V)
24
26
AN68 F08
Figure 8. Efficiency vs Input Voltage and Output Voltage
Measuring Constant Voltage Without a Battery
At times, it may be necessary to measure the constant
voltage of the charger with high accuracy and with the
battery removed. Since the battery is an integral part of the
regulation loop, some low frequency, low amplitude ripple
may appear on the BAT pin and the DC voltage measurement at the BAT pin will not be accurate. It is recom-
AN68-14
It is possible to connect the portable system circuitry
directly to the battery (BAT pin), but two facts should be
taken into consideration. First, the total current will be
limited and so the system will “steal” battery charge
current. In this case it is not possible to have a termination
such as –∆V that relies on constant charge current.
Second, when the charger is active and the system is
turned off, the full constant current will charge the battery,
so the battery should be able to absorb it.
It is also possible to bypass the sense resistor and connect
the system circuits to the SENSE pin, as shown in Figure
9. In this case the sum of the charging current and system
current should not exceed the LT1510 maximum output
current (limited by thermal considerations or switch peak
current). However, since the system circuitry is capacitive
in nature (input capacitor of a DC-to-DC converter), it
should not be connected directly to the SENSE pin. This is
because the internal sense resistor between the SENSE
and BAT pins will have a large capacitance across it, which
will cause instability. A 2.2µH inductor, such as the
DT1608C-222 by Coilcraft, isolates the input capacitance
of the system circuits well (L2 in Figure 9).
CR4 limits the transient current through the LT1510’s
internal sense resistor when the system is switched on in
battery operation. Q1 is required if the series resistance of
0.2Ω between BAT pin and SENSE pin causes the efficiency to drop. The Si9433’s on-resistance is 0.075Ω. The
charge pump (C3/C4/CR5/CR6/R2) biases the gate of Q1.
Q2 and R1 turn Q1 off when the system operates from the
AC wall adaptor (VIN active). R8 is required if there is no
circuitry connected to VIN.
Switching Between AC and Battery Operation,
2-Diode Configuration
Most systems that employ battery chargers also require
glitch-free switching between AC operation and battery
operation. Figure 10 shows a way to connect the load
(switching regulator) to VIN (when it is present) for AC
Application Note 68
CR3
1N5819
VIN
8V TO 20V
1
2
C1
0.22µF
L1
33µH
CR1
1N5819
CR2
1N914
3
4
7
8
TO
SYSTEM
SW
VCC1
BOOST
VCC2
PROG
LT1510
OVP
6
SYSTEM
SWITCHING
REGULATOR
LT1302
LT1304
LT1307
OR SIMILAR
GND
GND
5
SYSTEM
ON/OFF
SWITCH
GND
VC
SENSE
BAT
GND
GND
GND
GND
16
14
C2
10µF
15
13
12
C6
0.1µF
11
10
BAT1
2-CELL
NiCd
9
Q1
Si9433
Q2
MPS2907
R7
16.2k
+
R4
4.99k
C5
22µF
25V
R2
100k
SW
R1
100k
C3
0.01µF
R6
300Ω
R5
1k
Q3
CR4
1N4001
L2*
2.2µH
R3
1.21k
C7
1µF
CR6
1N914
R8
10k
*COILCRAFT DT1608C-222
C4
0.01µF
CR5
1N914
AN68 F09
Figure 9. Connecting the System to SENSE Pin
CR1
1N5819
VIN
CR3
1N5819
VCC
CR4
1N5819
C1
0.22µF
L1
33µH
CR5
1N914
SWITCHING
REGULATOR
LT1373
LTC1439
OR SIMILAR
C2
10µF
SW
BOOST
VC
C3
0.1µF
CR2
1N5819
R2
1k
LT1510
TO
SYSTEM
SENSE
PROG
R4
4.99k
C4
1µF
R3
300Ω
BAT
+
GND
C5
22µF
BAT
3-CELL
NiCd
Q1
Si9433DY
R1
1k
AN68 F10
Figure 10. LT1510 Charger System, 2-Diode Configuration
AN68-15
Application Note 68
operation or to the battery when VIN is unavailable. When
VIN is active, CR1 conducts the load current, CR2 is
reverse biased and Q1 is off. When VIN is removed, Q1
conducts the load current from the battery. The voltage
drop across Q1 is very low. Note that CR2 is in parallel with
the body diode of Q1. The load has typically high input
capacitance and also demands high current if the battery
voltage is low. CR2 conducts at the transition to battery
operation when Q1 is not fully on (the body diode has
excessive forward voltage drop). R1 is required if there is
no circuitry connected to VIN. Low voltage drop across Q1
is essential for high efficiency when the system is operating from the battery. It was measured at 33mV with a load
of 0.5A and the battery at 3V.
system circuits “steal” charge current from the battery.
This can be circumvented by boosting the output current
by the same amount that is “stolen.”
Figure 11 shows how the charge current can remain
constant regardless of the load. The system circuits’
current is sensed by RS. Q1, U2, R1, R2 and R3 boost the
LT1510’s PROG pin current and thus the output current
increases with the system circuits’ current so that the
current charging the battery remains unchanged. The
LT1510 based charger should be designed for an output
current that is the sum of currents into the battery and
the load.
The Next Generation Battery Charger IC, the LT1511
The next generation constant-voltage/constant-current
battery charger IC, the LT1511, performs like the LT1510,
but has two additional features: input current limiting and
peak switch current of 4A.
Switching Between AC and Battery Operation,
Current Boost Configuration
Placing the system circuits in parallel with the battery
achieves glitch-free switching between AC wall adaptor
and battery operation. In AC operation, however, the
CR2
1N5819
VIN
C3
10µF
VCC
C1
0.22µF
CR1
1N5819
L1
33µH
SW
VC
BOOST
CR3
1N914
C4
0.1µF
R6
1k
U1
LT1510
SENSE
BAT
PROG
R5
10k
C2
1µF
R4
200Ω
VIN
7
Q1
2N3904
+
GND
6
R2
10k
+
U2
LT1006
4
–
3
C5
22µF
25V
BAT
3-CELL
NiCd
R1
10k
2
R3
200Ω
Figure 11. LT1510 Charger System, Current Boost Configuration
AN68-16
SYSTEM
CIRCUITS
RS
0.1Ω
AN68 F11
Application Note 68
A control loop is provided to regulate the current drawn
from the power adapter. This allows simultaneous operation of the portable system and battery charging without
overloading the adapter. When system current increases,
the charging current is reduced to keep the adapter current
within the specified level.
integrator is reset every twenty seconds by the timer U2
and transistor Q1. The output of the integrator is monitored by U1C, a comparator and latch. When U1B’s output
voltage exceeds a threshold, the output of U1C is latched
high and turns Q2 on, pulling the LT1510 VC pin to ground
and shutting the charger off.
The internal switch of the LT1511 is capable of delivering
3A DC current (4A peak current) for charging the battery.
This is a 100% increase over the LT1510.
The bias voltage (VBIAS) for the circuit is generated by
voltage divider (R13/R14) and buffer U1D. The VBIAS level
chosen is close to the thermistor network (RT/R1) output
voltage in order to minimize the turn-on time needed for
charging C1. This also minimizes the effect of C1’s leakage. C1 is a ceramic capacitor.
R2 allows C1 to stabilize rapidly upon turn-on. R2, R3, R6
and R7 supply bias current to U1A and U1B.
The design equations for the dT/dt termination circuit are
presented in the following box.
Sometimes, however, a microcontroller is not available or
is not suitable for fast charge termination. The following
paragraphs describe solutions for both cases.
dT/dt Termination: A safe and reliable way to terminate
fast charging of NiCd and NiMH batteries is based on
detecting the rate at which the battery temperature
increases during constant-current charging. With constant-current charging, the battery temperature increases
rapidly as the battery nears full charge status (see
Figure 12).
The circuit in Figure 13 monitors the battery temperature
with a negative temperature coefficient (NTC) thermistor
RT and detects the rate at which the temperature rises over
a 20-second period.
The thermistor output is amplified by differentiator U1A
and integrator U1B (which, together, form an AC coupled
amplifier). The differentiator is AC coupled and thus eliminates the DC voltage of the RT and R1 network; the
CELL VOLTAGE (V)
Charging NiCd Batteries
1.75
120
90
1.50
100
80
1.25
1.00
0.75
0.50
80
60
40
20
CELL TEMPERATURE (°C)
Any portable equipment that requires fast charge needs to
have proper charge termination. Commonly, the LT1510
is used in conjunction with a microcontroller that has an
internal analog-to-digital converter (ADC) or in conjunction with a microprocessor and an ADC IC.
INTERNAL PRESSURE (PSIG)
CHARGING BATTERIES / TERMINATION METHODS
CELL VOLTAGE
70
PRESSURE
60
50
40
0.25
0
30
0
–20
20
TEMPERATURE
1
50
100
150
CHARGE INPUT (% OF CAPACITY)
Reproduced with permission by Butterworth-Helnemann, Rechargeable
Batteries Applications Handbook, copyright 1992
AN68 F12
Figure 12. Voltage, Pressure and Temperature
During 1C Charge of NiCd Battery
AN68-17
12
13
U1D
VBIAS
R2
100k
4
14
1
VRST
R15
100Ω
11
U1A
LT1014
R3
100k
3
2
R4
10M
C4
0.1µF
VBIAS
100k
VRST
RESET
VDD
16
12V
U1B
8
VSS OUT2
OUT1
CLK
U2
CD4060
Q14
+
–
R7
100k
5
6
9
10
11
12
C2
0.1µF
7
C6
0.1µF
R18
10k
R17
10k
Q1
2N3904
C5
0.1µF
R9
100k
R8
20k
C7
0.22µF
VBIAS
CR4
1N4148
L1
30µH
–
+
R10
100k
9
10
U1C
CR1
1N4148
SENSE
GND
GND
VCC
VC
BAT
+
Q2
2N3904
PROG
U3
LT1510
BOOST
SW
R11
100k
R12
10k
1,7,8,9,10,16††
CR3
1N5819
+
C3
22µF
CR2
1N4148
8
Figure 13. LT1510 NiCd Charger with dT/dt Termination
3
R16
10k
VBIAS
R6
100k
R5
10M
* KETEMA MSC103K. THERMALLY CONNECTED TO B1
** PANASONIC 3-CELL NiCd P130-SCR
† CERAMIC CAPACITOR
†† SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
R14
7.5k
R13
10k
R1
7.5k
C1†
4.7µF
+
–
AN68-18
+
RT*
–
12V
C11
22µF
25V
BAT1**
C10
R21 0.1µF
1k
R20
4.99k
C8
R19 1µF
300Ω
AN68 F13
C9
10µF
CR5
1N5819
Application Note 68
Application Note 68
For the design shown in Figure 13:
RT is a Ketema MSC103k, a 10k thermistor with R25/R125
= 29.25.
Design Equations for dT/dt Termination
Thermistor Design
(1)

T  R 
β =  T O  In T 
 TO − T   RTO 
(2) R1 =
(3)
(
RTO β − 2TO
β + 2TO
)
(16)
(17)
 −β 1 
dVDIV
= VDIVTO 
+ 
dT
 2TO2 TO 
(18)
where:
β is a constant depending on the thermistor material,
T is temperature in °K at which RT is characterized,
TO is temperature in °K at which RTO is characterized,
dVDIV /dT is the rate of divider output voltage change vs
temperature and VDIVTO is the divider DC voltage at TO
Integrator U1A and Differentiator U1B Gain
( )( )( ) (R5)(s1)(C2)
(4) G = R4 s C1
for R4 = R5, G =
(19)
C1
C2
)( )
dVDIV
 dT 
G
(5) VTH =   t
 dt 
dT
( )
(20)
where:
(7) VBIAS =
(V )(R14)
IN
R13 + R14
[
( )] = 7.41k
4004 + 2(298 )
(23)
10k 4004 − 2 298
(24)
R1 is selected as 7.5k.
The gain of the RT, R1 network is:


 10   − 4004
dVDIV
1 
= 12 
+

 2982  298 
+
.
dT
10
7
5
2


(3)
 




= − 0.132V / °C
(25)
For C1 = 4.7µF and C2 = 0.1µF, the gain of the integrator
and differentiator can be written as:
4.7µF
= 47
0.1µF
(26)
The selected slope that will trigger termination is 0.5°C/
min. (a conservative half of the typical 1°C/minute). The
selected timer period is 20 seconds (0.33 minute).
VIN is selected as 12V, R13 = RT = 10k, R14 = R1 = 7.5k
R8 and R9 Selection
( )
(2) R1 ≅
)
(5) VTH = (0.5)(0.33)(0.132)(47) = 1.023V
where:
dT/dt is the selected slope
t is the timer period
VTH VBIAS
=
or:
(6)
R8
R9
 R9 
R8 ≅ VTH 

 VBIAS
(
(4) G =
Threshold of the Latch U1C Stage
()(

298 
In29.25 = 4004
(1) β =  398
398 − 298 

(7) VBIAS =
( ) = 5.14V
12 7.5
10 + 7.5
(27)
R9 is selected as 100k.
(21)
(6) R8 ≅ 1.02
(22)
100
= 19.9 k
5.14
(28)
R8 is selected as 20k.
A secondary termination for the charger is recommended.
Depending on system reliability requirements, the secondary termination circuit may use existing components
such as RT or U1 for absolute temperature or time-out,
AN68-19
Application Note 68
respectively. Also, to avoid premature termination, the
temperature rise rate that results from bringing the system
indoors from the lowest outdoor temperature should be
considered.
–∆V Termination: The internal battery temperature rise
towards the end of charge, coupled with the negative
temperature coefficient of NiCd and NiMH, causes the
battery voltage to drop. The drop can be detected and used
for terminating a fast charge with the LT1510.
In the example shown, Figure 14, the charge current was
selected as 0.8A. To determine the voltage droop rate for
–∆V termination, a fully charged 3-cell (Panasonic P140SCR) NiCd battery was connected to an LT1510 charger
circuit programmed for a 0.8A rate. The negative slope in
voltage, as seen in Figure 14, is calculated to be
– 0.6mV/s. It can be seen that the total voltage drop is
about 300mV (100mV per cell). After the battery voltage
dropped 300mV from the peak of 4.93V (100mV per cell),
the charger was disabled.
4.9
BATTERY VOLTAGE (V)
U3’s output voltage droops at a rate proportional to the
hold capacitor’s internal leakage and the leakage current at
Pin 6 (10pA typical).This droop is very low and does not
affect the operation of the circuit.
The minimum negative battery voltage slope required to
trigger termination can be calculated from:
−
dV
VTRIG
=
dT
t CLK GU2A
( )( )
(29)
where:
VTRIG =
(V )(R12) = 5
REF
R11 + R12
1
= 49.5mV,
101
(30)
tCLK is the clock period, 15 seconds,
5.0
NEGATIVE
SLOPE
GU2A is the gain of the first stage =
4.8
END OF
CHARGE
4.7
R8
= 11
R4 || R5
4.5
− dV 49.5mV
=
= 0.3mV / s
dT
15 11
( )( )
4.4
4.3
0
10
20
TIME (MINUTES)
(31)
hence:
4.6
4.2
capacitor to be charged to the input level. U2B and the
associated parts form a latch that requires a momentary
negative voltage at Pin 6 to change state. R15 supplies the
negative feedback and Q2, R16, R17 and C10 reset the
latch on turn-on.
(32)
30
AN68 F14
Figure 14. – ∆V Test
At the heart of the circuit in Figure 15 is U3, a sample-andhold IC, LF398. The output of U3, Pin 5, samples the input
level, Pin 3, at every clock pulse at Pin 8. When the battery
voltage drops, the input to U3 also drops. If the update step
at the output of U3 is sufficiently negative, U2B latches in
the high state and Q1 turns on and terminates the charge
by pulling VC pin of the LT1510 down and disabling it.
U2A and the associated passive components smooth,
amplify and level shift the battery voltage. The timer U4
updates the hold capacitor C8 every fifteen seconds. The
timer signal stays high for 7ms, sufficient for the hold
AN68-20
Charging Sealed-Lead-Acid (SLA) Batteries
Standard Charge: The LT1510 is ideal for standard charging of SLA batteries because of its constant-current and
constant-voltage features. To extend the battery life, the
float voltage can be temperature matched to the battery
specifications. The circuit in Figure 16 was designed for
the Panasonic SLC-214P, which is a 2-cell, 2.1AH SLA
battery with a maximum charge current of 0.8 amps.
The thermistor RT, selected for temperature matching, is
a Ketema MSC103K. Figure 17 shows minimum and
maximum float voltage vs. temperature, as recommended
by the manufacturer. The output voltage of the charger vs.
temperature fits in this range.
R23
100k
C12
0.01µF
R22
100k
C11
0.1µF
CR1
1N914
L1
30µH
C1
0.22µF
8
7
6
5
4
3
2
1
VDD
VSS
U4
MC14536B
CLK INH
A
B
C
D
DECODE
OSC INH
8-BYPASS
OUT2
OUT1
IN1
BAT
VC
9
10
11
12
13
14
15
16
CLK
*1,7,8,9,
10,16
GND
RESET MONO IN
SET
R21
100k
SENSE
GND
VCC2
VCC1
PROG
U1
LT1510
BOOST
SW
C4
0.1µF
R20
100k
C5
22µF
25V
Q1
2N3904
+
R3
1k
R2
6.19k
C3
R1
1µF
300Ω
C6
0.1µF
3
2
R4
10k
U2A
4
8
CLK
R8
100k
C7
0.1µF
1
+
–
R10
30.1k
U3
LF398
30k
Figure 15. LT1510 NiCd Charger with –∆V Termination
4
OFFSET
2
NC
LOGIC
7 REFERENCE
8 LOGIC
3 INPUT
R9
30.1k
* SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
** B1 IS A NiCd 3-CELL PANASONIC PI40-SCR
† PANASONIC ECQV1HIOSJL
B1**
R6
100k
R5
100k
C2
10µF
5VREF
+
CR2
1N5819
R7
10Ω
+
–
6
C8†
1µF
HOLD
CAPACITOR
150Ω
1
OUTPUT
5
R12
1k
C9
0.1µF
5
6
R13
10k
R11
100k
5VREF
U2B
LT1013
+
–
CR3
1N5819
–
12V
5VREF
Q2
2N3904
R15
100k
7
LT1029CZ
R14
10k
+
AN68 F15
R17
100k
R16
100k
C10
22µF
Application Note 68
AN68-21
Application Note 68
VIN
1
2
C1
0.22µF
L1
30µH
CR2
1N5819
CR3
1N914
3
GND
GND
SW
VCC2
BOOST
VCC1
CR1
1N5819
16
15
C2
10µF
14
13
PROG
LT1510
12
5
OVP
VC
4
6
7
8
GND
SENSE
BAT
GND
GND
GND
GND
10
C4
1µF
C3
0.1µF
11
+
9
* PANASONIC LSC-214P
**KETEMA THERMISTOR MSC103K
THERMISTOR IS THERMALLY CONNECTED TO BATTERY BAT1
C5
22µF
25V
BAT1*
R1
162k
0.25%
R4
1k
R5
300Ω
R6
6.19k
R2**
10k
R3
210k
0.25%
AN68 F16
Figure 16. Temperature Compensated Standard Sealed-Lead-Acid Battery Charger
4.70
tor is higher than the voltage across R7 or equivalent to the
charge current being above 0.4A. As long as U1’s output
is low, the charging voltage is boosted to 5V by changing
the OVP voltage divider ratio by switching R4 in parallel
with R3.
MAX
BATTERY VOLTAGE (V)
4.65
LT1510
CHARGER
VOLTAGE
4.60
4.55
4.50
Charging Li-Ion Batteries
MIN
4.45
4.40
0
5 10 15 20 25 30 35 40 45 50
BATTERY TEMPERATURE (°C)
AN68 F17
Figure 17. Output Voltage vs Temperature of SLA
Charger and MIN/MAX Float Voltage vs Temperature
(as Recommended by Panasonic)
Li-Ion batteries are charged with a constant-voltage/constant-current charger. Constant current is supplied until
the output voltage reaches 4.1V or 4.2V per cell (depending on the manufacturer) followed by constant-voltage
charging with required accuracy of ±50mV per cell. The
charging current then tapers down naturally.
Fast Charge: The circuit in Figure 18 is a fast SLA battery
charger. It is based on the standard SLA battery charger
circuit in Figure 16. When the charge current is high, the
constant-voltage level increases from 4.5V to 5V. At a
constant voltage of 5V, the battery reaches a full charge
state faster than at a constant voltage of 4.5V.
IMIN + Timer Termination: To maximize battery cycle life,
several lithium-ion battery manufacturers recommend
termination of constant-voltage float mode 30 to 90 minutes after charge current has dropped below a specified
threshold level, IMIN. The float voltage is 4.1V or 4.2V per
cell and the charge current threshold level is typically
50mA to 100mA. Check with the battery manufacturer
for details.
R9’s value programs the constant current of the LT1510 to
0.8A. R1, R2 and R3 program the constant voltage to 4.5V.
U2, an open-collector voltage comparator, is at low state
when the voltage across the internal LT1510 sense resis-
Figure 19 shows a constant-current, constant-voltage
charger with IMIN + 30-minute termination. When the
LT1510 is charging, U2 compares the voltage across the
LT1510 internal 0.2Ω sense resistor to the voltage across
AN68-22
Application Note 68
8V TO
15V
CR3
1N5819
C1
10µF
R9
6.19k
VCC2
VCC1
SW
PROG
C2
0.22µF
CR1
1N5819
R11
1k
BOOST
L1
30µH
CR2
1N914
U1
LT1510
C3
1µF
R10
300Ω
CR4
1N914
C4
0.1µF
VC
C7
0.1µF
R6
15k
GND
2
BAT
+
OVP
C5
22µF
25V
BAT1*
SENSE
R1
162k
0.25%
R7
15k
†
R2
10k
GND
C6
0.1µF
R8
1M
3
8
+
7
U2
LT1011
4
–
1
1,7,8,9, 10,16**
R4
825k
R3
210k
0.25%
AN68 F18
* PANASONIC LSC-214P
** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
† KETEMA MSC103K IS THERMALLY CONNECTED TO BAT1
Figure 18. Fast, Temperature Compensated SLA Charger
R7. When the voltage across the sense resistor is lower
than the voltage across R7 (or, alternatively, when the
charge current drops below 75mA), U2’s output voltage
drops. When U2’s output voltage drops, U2 latches at low
state via CR5. U2’s output is connected to the RESET pin
of U3; when switched, this signal releases the reset (active
high) of U3. U3, a timer, starts clocking and 30 minutes
later its DECODE output (Pin 13) changes to a high state.
The DECODE output is connected to the SET input of U3
(active high). This signal latches U3’s DECODE pin high.
The high at the DECODE pin also terminates the charge by
pulling the LT1510’s VC pin down via Q2.
Q3, R14, C8, R15 and CR6 reset U2 and U3 on turn-on.
A secondary termination can be based on total charge
time.
Terminating with a Microprocessor
The LT1510 gives the designer an easy solution for the
power section of a battery charger and also a smooth
transition from constant-current to constant-voltage operation. When a sophisticated charging regimen is required, connecting a dedicated or system microprocessor
to the charger is the solution of choice.
NiCd or NiMH Charger: The charger in Figure 20 has two
charge rates that depend on the HI_CHARGE signal and
are programmed by R1 and R7 (see the Component
Selection section). The microprocessor reads the battery
voltage by clocking U2, a serial data ADC. C7 smoothes the
ADC input, but averaging a number of ADC readings is
recommended. The voltage divider R4/R5/R6 divides the
voltage at the BAT pin for both the ADC and the OVP pin.
The LT1510 is programmed to 5V in constant-voltage
mode. The microprocessor can terminate charge based
on – ∆V, zero ∆V or dV/dt. After termination, the low
charge can serve as trickle for NiCd type batteries; the
charger may have to shut down for NiMH cells. Check the
battery manufacturer’s specifications.
AN68-23
AN68-24
L1
30µH
VCC2
VCC1
GND
Q1
VN2222
BAT
VC
PROG
U1
LT1510
SENSE
OVP
GND
BOOST
SW
1,7,8,9, 10,16**
CR1
1N5819
VC
R5
4.87k
0.25%
R4
11.3k
0.25%
R3
1k
+
C5
22µF
25V
C4, 0.1µF
R2 C3, 1µF
300Ω
R1
4.12k
* MOLI ENERGY ICR-18650
** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
CR2
1N4148
C1
0.22µF
R7
2.2k
R6
2.2k
R9
430k
R8
560k
C9
0.1µF
2
3
Q3
2N3904
1
U2
LT1011
8
7
R10
10k
+
C6
0.1µF
CR5
1N4148
4
CR4
1N4148
R15
100k
C8
22µF
R14
100k
Figure 19. Li-Ion IMIN + Timer Charger
BAT1*
2-CELL
Li-Ion
C2
10µF
+
CR3
1N5819
–
11V TO
15V
CR6
1N4148
C7
R12
4.64k 0.01µF
R11, 10k
7
6
5
4
3
2
1
8
VSS
CLK INH
A
B
C
D
DECODE
OSC INH
MONO IN
VDD
8-BYPASS
OUT2
OUT1
IN1
RESET
SET
U3
MC14536B 16
9
10
11
12
13
14
15
R13
10k
AN68 F19
Q2
2N3904
VC
Application Note 68
Application Note 68
Where (N)10 is the decimal value of the data entered into
the PWM register of the microprocessor and 28 is the
maximum decimal value of an 8-bit register representing
D = 1.
Controlling the LT1510 Charger with a Microprocessor
PWM Charge Current Control: Figure 21 shows how to
control the charge current with the PWM output of the
microcontroller. The constant-current charge can be calculated from:
ICONST
(2.465)(2000)(D)
=
LT1510
PROG
(33)
RPROG + 300Ω
R1
300Ω
IPROG
C1
1µF
AN68 F21
Figure 21. PWM Charge Current Control
(N)
2
10
8
(34)
12V TO
24V
VIN
CR1
1N5819
+
C1
10µF
25V
CR2
1N5819
L1
30µH
FROM PWM
OUTPUT OF
MCU
Q1
VN2222
Where RPROG is the value of the programming resistor and
D is the duty cycle of the PWM signal. The maximum value
of D is 1. D can be calculated from:
D=
RPROG
C2
0.22µF
R8
100k
U3
LT1086-5CT
VOUT
+
GND
R1
71.5k
VCC2
VCC1
SW
PROG
C4
0.1µF
U1
LT1510
GND
C3
1µF
R2
300Ω
R3
1k
R9
100Ω
VC
BOOST
C6
0.1µF
BAT
CR3
1N4148
R4
1k
SENSE
OVP
R5
4.99k
1,7,8,9,
10,16**
R7
7.87k
VREF
+IN
GND
* 3-CELL NiCd OR NiMH
** CONNECT TO GROUND PLANE FOR
THERMAL DISSIPATION
C8
1µF
10V
BAT1*
C5
22µF
25V
VCC
CS/
SHDN
U2
LTC1096
C7
Q1
0.1µF
VN2222
+
VCC
U4
MICROPROCESSOR
CLK
R6
4.99k
I/O PORTS
–IN
DOUT
Q2
VN2222
NO_CHARGE
HI_CHARGE
Q3
VN2222
AN68 F20
Figure 20. NiCd or NiMH Microprocessor Controlled Charger
AN68-25
Application Note 68
Parallel and Serial Control: There are many ways to
control the constant current and constant voltage of the
charger. Some of them are described here.
PROG pin of the LT1510 and thus the charge current is
controlled by the microprocessor.
In the circuit in Figure 23, U2, U3 and Q1 form a microprocessor-controlled current sink. The data on the microprocessor serial bus controls the voltage at the OUT1 pin of
U2. U3 regulates the voltage across R2 to equal that at the
OUT1 pin of U2. The current through R2 flows out of the
PROG pin of the LT1510 and thus the charge current is
controlled by the microprocessor.
In the circuit in Figure 22, U2, U3 and Q1 form a microprocessor-controlled current sink. The data on the microprocessor parallel bus controls the voltage at the OUT1 pin of
U2. U3 regulates the voltage across R2 to equal that at the
OUT1 pin of U2. The current through R2 flows out of the
U1
LT1510
PROG
R1
300Ω
–1.25VREF
C1
1µF
VREF
C2
33pF
RFB
OUT1
–
U3
LT1097
U2
LTC7541A
OUT2
Q1
2N3904
+
GND
R2
1.65k
AN68 F22
MICROPROCESSOR
Figure 22. 12-Bit, Parallel Loading Microprocessor Charge Current Control
U1
LT1510
PROG
R1
300Ω
–1.25VREF
C1
1µF
CLOCK
DATA
LOAD
VREF
STB1
SRI
C2
33pF
RFB
OUT1
U2
LTC7543
U3
OUT2
LD1
–
+ LT1097
Q1
2N3904
GND
R2
1.65k
MICROPROCESSOR
AN68 F23
Figure 23. 12-Bit Serial Interface Microprocessor Charge Current Control
AN68-26
Application Note 68
Figure 24 shows a circuit to control the constant-voltage
output of an LT1510-based battery charger. U2, R1, and
R2 invert the polarity of the battery voltage. U3 and U4 act
as a voltage divider and also change the polarity of the
voltage back to positive. The divided voltage is fed to the
OVP pin of the LT1510.
U3 can be LTC7541A for parallel data interface with the
microprocessor or LTC7543 for serial data interface with
the microprocessor. The programmed voltage VCONST can
be calculated from the following:
VCONST = 2.465
212
N
( )10
(35)
where (N)10 is the decimal value of the microprocessor
bus data and 212 is the maximum data value based on
12-bit data.
CONCLUSION
The LT1510 is a high efficiency charger building block that
relieves the designer of the burdens of switcher design,
heat sinking, and even power-transistor and sense resistor selection. In some cases, the LT1510 and a few passive
parts are all that is necessary to build a high efficiency
battery charger. Its high accuracy constant-voltage and
constant-current features make the LT1510 an excellent
choice for Li-Ion, NiCd, NiMH and SLA charging. Its
control over all charging parameters makes the LT1510 an
easy device with which to design.
R2
200k
R1
200k
BAT
U1
LT1510
–
U2
+LT1097
B1
OVP
VREF
C1
33pF
RFB
OUT1
U3
LTC7541A/LTC7543*
OUT2
GND
–
+
U4
LT1097
AN68 F24
* LTC7541A FOR PARALLEL INTERFACE
OR LTC7543 FOR SERIAL INTERFACE
SERIAL OR
PARALLEL BUS
MICROPROCESSOR
Figure 24. Microprocessor Voltage Control
AN68-27
Application Note 68
APPENDIX A: TEST RESULTS
Testing a statistically significant number of batteries and
charge/discharge cycles is essential for validating a charger
design. The circuits in the body of this document were
tested and the results are presented in this appendix.
tion times; otherwise it is regulated at 0.5A. The LT1510
control loop corrects the charge current at a rate of 0.5A
per 1ms. If the load changes at a slower rate, it will not
affect the charge current.
Output Current Boost Configuration Test
The purpose of this test is to see how stable the charge
current is with a dynamic load. The 0.5A charger circuit in
Figure 11 was connected to the constant-current load
(Figure B2), which was adjusted to 0.5A and connected to
a function generator as shown in Figure A1. The function
generator switched the load between 0A and 0.5A at 100Hz
with a 50% duty cycle. The battery charge current was
monitored with an oscilloscope across a 0.1Ω sense
resistor connected in series with the battery.
0mV
AN68 FA2
Figure A2. Battery Charge Current of Current Boost System
with Dynamic Load
The test results are shown in Figure A2. The battery charge
current is affected by the dynamic load only at the transi-
1N5819
VIN
10µF
VCC
VC
SW
0.1µF
0.22µF
BOOST
33µH
1N5819
1k
U1
LT1510
SENSE
1N914
GND
(+)
BAT
+
C1
22µF
3-CELL
NiCd
PROG
RPROG
10k
R4
300Ω
C2
1µF
7
+
U2
LT1006
–
R2
10k
FUNCTION
GENERATOR
RSENSE
0.1Ω
VIN
6
(+)
TO
O’SCOPE
GND
Q1
2N3904
CONTROL
4
3
2
R1
10k
(–)
(–)
RS
0.1Ω
AN68 FA1
CONSTANTCURRENT LOAD
ADJUSTED TO 0.5A
(SEE FIGURE B2)
R3
200Ω
Figure A1. Battery Charging Current System Test Circuit
AN68-28
AMPLITUDE: 0V TO 10V
FREQUENCY: 100Hz
Application Note 68
The purpose of this test is to establish that the charger
circuit in Figure A3 is reliable and terminates consistently.
The circuit was connected to a test system that discharged
the battery to the same level after termination and reactivated the charger circuit. Data from nineteen charge/
discharge cycles was collected. The test conditions are
given in Table A1.
The test results are presented below. Figure A4 shows a
typical battery voltage during one charge/discharge cycle.
The data collected was analyzed and presented in Table
A2.
4.8
4.6
4.4
BATTERY VOLTAGE (V)
dT/dt Termination Test
Table A1
4.2
4.0
Battery
3 NiCd Cells of Panasonic
P130-SCR in Series (1.3AH)
Constant Charge Current
1A
Discharge Current
1A
3.2
Minimum dT/dt Slope That Will
Trigger Termination
0.5°C/minute
3.0
Required Duration of the Above
Slope to Trigger Termination
20 seconds
3.8
3.6
3.4
0
0:30 1:00
1:30 2:00 2:30
TIME (HR)
3:00 3:30
AN68 FA4
Figure A4. Typical Battery Voltage at Charge/Discharge
Cycle of NiCd Charger with dT/dt Termination
12V
VRST
R4
10M
RT*
C1†
4.7µF
CR1
1N4148
Q1
2N3904
C2
0.1µF
2
3
–
4
1
U1A
LT1014
R5
10M
6
+
11
5
–
7
R8
20k
U1B
10
+
U1C
9
R2
100k
R1
7.5k
R3
100k
R6
100k
R16
10k
+
R10
100k
12
–
+
R11
100k
CR2
1N4148
14
VBIAS
12V
VCC
SW
C5
0.1µF
16
3
U1D
C3
22µF
Q2
2N3904
CR5
1N5819
VRST
13
8
VBIAS
100k
R13
10k
–
R7
100k
R9
100k
VBIAS
R12
10k
+
R15
100Ω
VBIAS
C4
0.1µF
VDD
Q14
RESET
CLK
U2
CD4060
OUT1
R14
7.5k
12
C7
0.22µF
C8
R19 1µF
300Ω
CR3
1N5819
C9
10µF
PROG
11
R17
10k
10
R18
10k
BOOST
L1
30µH
CR4
1N4148
R20
4.99k
U3
LT1510
C10
R21 0.1µF
1k
GND
VC
VSS OUT2
* KETEMA MSC103K. THERMALLY CONNECTED TO B1
** PANASONIC 3-CELL NiCd P130-SCR
† CERAMIC CAPACITOR
†† SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
8
9
SENSE
C6
0.1µF
BAT
GND
1,7,8,9,10,16††
+
C11
22µF
25V
BAT1**
AN68 F13
Figure A3. LT1510 NiCd Charger with dT/dt Termination
AN68-29
Application Note 68
Table A2
Average Charge Time
1:20:22 hours
Standard Deviation of Charge Time
0:01:00 hours (1 minute)
Average Discharge Time
1:19:25 hours
Standard Deviation of Discharge Time
0:00:58 hours (58 seconds)
The consistent discharge time (58 seconds standard deviation with constant-current load) proves that the charge
termination is repeatable and that the LT1510 charger
performs well. Also, because there is no “top-off” or trickle
charge, the charge efficiency (discharge ampere-hours
over charge ampere-hours) is close to 100%.
NiCd –∆V Termination Test
SLA Fast Charge Test
The purpose of this test is to establish that the SLA fast
charge circuit in Figure A7 is reliable and terminates
consistently. The circuit was connected to a test system
that, after about four hours of charging, discharged the
battery to the same level every cycle and then reactivated
the charger circuit. Data from seventeen charge/discharge
cycles was collected. Test conditions are presented in
Table A5.
Table A5
The purpose of this test is to establish that the charger
circuit in Figure A5 is reliable and terminates consistently.
The circuit was connected to a test system that discharged
the battery to the same level after termination and reactivated the charger circuit. Data from 68 charge/discharge
cycles was collected. Test conditions are given in Table
A3.
Table A3
Battery
performs well. Also, because there is no “top-off” or trickle
charge, the charge efficiency (discharge ampere-hours
over charge ampere-hours) is close to 100%.
3 NiCd Cells of Panasonic
P140-SCR in Series (1.4AH)
Constant Charge Current
0.8A
Discharge Current
0.8A
– ∆V That Will Trigger Termination
4.5mV
End of Discharge Voltage
2.7V
The test results are presented below. Figure A6 shows
typical battery voltage during charge/discharge cycle. The
data collected was analyzed and presented in Table A4.
Battery
2-Cell SLA Panasonic
LSC-214P
Constant Charging Voltage
4.5V
Boosted Constant Charging Voltage (Until
Charge Current Drops Below 0.4A)
5V
Constant Charge Current
0.8A
Average Charge Time
3:53 hours
Standard Deviation of Charge Time
0:04:15 hours
Discharge Current
1A
End of Discharge Voltage
3.6V
The test results are presented below. Figure A8 shows
typical current during the charge and discharge (positive
and negative, respectively) of one cycle. The data collected
was analyzed and is presented in Table A6.
Table A6
Average Discharge Time
1:31:39 hours
Standard Deviation of Discharge Time
0:02:06 hours
Table A4
Average Charge Time
2:00:55 hours
Standard Deviation of Charge Time
0:05:37 hours
Average Discharge Time
1:59:14 hours
Standard Deviation of Discharge Time
0:00:48 hours (48 seconds)
The consistent discharge time (48 seconds standard deviation with constant-current load) proves that the charge
termination is repeatable and that the LT1510 charger
AN68-30
The consistent discharge time (standard deviation is 2
minutes) proves that the charger and regimen are reliable.
Charge current of 0.8A and charge level of 1.5AH (discharge time of 1.5 hours at 1A) are conservative for a
2.1AH battery. To reach full charge at the required four
hours, a higher charge current battery can be used, or a
current threshold lower than 0.4A can be programmed.
R23
100k
C12
0.01µF
R22
100k
C11
0.1µF
CR1
1N914
L1
30µH
C1
0.22µF
8
7
6
5
4
3
2
1
VDD
VSS
U4
MC14536B
CLK INH
A
B
C
D
DECODE
OSC INH
8-BYPASS
OUT2
OUT1
IN1
BAT
VC
9
10
11
12
13
14
15
16
CLK
*1,7,8,9,
10,16
GND
RESET MONO IN
SET
R21
100k
SENSE
GND
VCC2
VCC1
PROG
U1
LT1510
BOOST
SW
C4
0.1µF
R20
100k
C5
22µF
25V
Q1
2N3904
+
R3
1k
R2
6.19k
C3
R1
1µF
300Ω
C6
0.1µF
3
2
R4
10k
U2A
4
8
CLK
R8
100k
C7
0.1µF
1
+
–
R10
30.1k
U3
LF398
30k
Figure A5. LT1510 NiCd Charger with – ∆V Termination
4
OFFSET
2
NC
LOGIC
7 REFERENCE
8 LOGIC
3 INPUT
R9
30.1k
* SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
** B1 IS A NiCd 3-CELL PANASONIC PI40-SCR
† PANASONIC ECQV1HIOSJL
B1**
R6
100k
R5
100k
C2
10µF
5VREF
+
CR2
1N5819
R7
10Ω
+
–
6
C8†
1µF
HOLD
CAPACITOR
150Ω
1
OUTPUT
5
R12
1k
C9
0.1µF
5
6
R13
10k
R11
100k
5VREF
U2B
LT1013
+
–
CR3
1N5819
–
12V
5VREF
Q2
2N3904
R15
100k
7
LT1029CZ
R14
10k
+
AN68 F15
R17
100k
R16
100k
C10
22µF
Application Note 68
AN68-31
BATTERY CHARGE
CURRENT (A)
Application Note 68
5.0
4.8
4.4
4.2
0.5
0
4.0
BATTERY DISCHARGE
CURRENT (A)
BATTERY VOLTAGE (V)
4.6
1.0
3.8
3.6
3.4
3.2
3.0
0
1
2
3
4
TIME (HR)
5
–0.5
–1.0
–1.5
0
6
1
2
3
4
TIME (HR)
5
6
AN68 FA8
AN68 FA6
Figure A8. Typical Battery Current at Charge/Discharge
Cycle of SLA Fast Charger
Figure A6. Typical Battery Voltage During Charge/Discharge
Cycle of NiCd Charger with – ∆T Termination
8V TO
15V
CR3
1N5819
C1
10µF
R9
6.19k
VCC2
VCC1
SW
PROG
C2
0.22µF
CR1
1N5819
R11
1k
BOOST
L1
30µH
CR2
1N914
U1
LT1510
C3
1µF
R10
300Ω
CR4
1N914
C4
0.1µF
VC
C7
0.1µF
R6
15k
GND
2
BAT
+
OVP
C5
22µF
25V
SENSE
BAT1*
R1
162k
0.25%
R7
15k
†
GND
R2
10k
C6
0.1µF
R8
1M
3
8
+
7
U2
LT1011
4
–
1
1,7,8,9, 10,16**
R4
825k
R3
210k
0.25%
* PANASONIC LSC-214P
** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
† KETEMA MSC103K IS THERMALLY CONNECTED TO BAT1
Figure A7. Fast, Temperature Compensated SLA Charger
AN68-32
AN68 F18
Application Note 68
Li-Ion with Time-Out Termination Test
The purpose of this test is to establish that the charger
circuit in Figure A9 is reliable and terminates consistently.
The circuit was connected to a test system that, after
termination, discharged the battery to the same level and
reactivated the charger circuit. Data from thirty-two charge/
discharge cycles was collected. Other test conditions are
given in Table A7.
The test results are presented below. Figure A10 shows a
typical charge/discharge battery voltage. The test data is
analyzed and given in Table A8.
Table A8
Average Charge Time
2:14:12 hours
Standard Deviation of Charge Time
0:01:00 hours
Average Discharge Time
1:11:33 hours
Standard Deviation of Discharge Time
0:00:40 hours
Table A7
Battery
2 Li-Ion Cells of Moli Energy
ICR18650 in Series
Constant Voltage
8.2V
Constant Charge Current
1.3A
Discharge Current
1A
Charge Current That Will Trigger Timer
<100mA
Timer Duration
30 minutes
The consistent discharge time (40 seconds standard deviation with 1A constant-current load) proves that the
charge termination is reliable and that the LT1510 is a
good solution for Li-Ion charging.
AN68-33
AN68-34
L1
30µH
GND
VCC2
VCC1
Q1
VN2222
BAT
VC
PROG
U1
LT1510
SENSE
OVP
GND
BOOST
SW
1,7,8,9, 10,16**
CR1
1N5819
VC
R5
4.87k
0.25%
R4
11.3k
0.25%
R3
1k
+
C5
22µF
25V
C4, 0.1µF
R2 C3, 1µF
300Ω
R1
4.12k
* MOLI ENERGY ICR-18650
** SOLDER TO GROUND PLANE FOR HEAT DISSIPATION
CR2
1N4148
C1
0.22µF
R7
2.2k
R6
2.2k
R9
430k
R8
560k
C9
0.1µF
2
3
Q3
2N3904
1
U2
LT1011
8
7
R10
10k
+
C6
0.1µF
CR5
1N4148
4
CR4
1N4148
R15
100k
C8
22µF
R14
100k
CR6
1N4148
C7
R12
4.64k 0.01µF
R11, 10k
Figure A9. Li-Ion Charger with Time-Out Termination
BAT1*
2-CELL
Li-Ion
C2
10µF
+
CR3
1N5819
–
11V TO
15V
7
6
5
4
3
2
1
8
VSS
CLK INH
A
B
C
D
DECODE
OSC INH
MONO IN
VDD
8-BYPASS
OUT2
OUT1
IN1
RESET
SET
U3
MC14536B 16
9
10
11
12
13
14
15
R13
10k
AN68 F19
Q2
2N3904
VC
Application Note 68
Application Note 68
8.5
BATTERY VOLTAGE (V)
8.0
7.5
7.0
6.5
6.0
5.5
5.0
0
1.0 1.5
0.5
2.0 2.5 3.0 3.5 4.0 4.5
TIME (HR)
AN68 FA10
Figure A10. Typical Li-Ion Battery Voltage at Charge/Discharge
with Time-Out Termination
APPENDIX B: AUXILIARY CIRCUITS FOR TESTING
BATTERIES AND CHARGERS
While working on battery charger design and testing, two
“home-brewed” circuits were frequently used: a battery
simulator and a controlled constant-current load. The
former was used for checking the operation of the charger
and the latter for charge/discharge tests. A variety of
timers, charge/discharge controllers and the like were
constructed as the need arose.
The Battery Simulator
There are two advantages to using the battery simulator
(Figure B1) over a battery in board tests. In case of an
accidental short, the power supply has current limiting,
whereas a shorted battery can conduct more than 20
amperes and vaporize everything, including traces. Also,
the simulator voltage is static and controllable, which
makes it easy for testing (efficiency, for instance).
The user has to adjust the power supply to the desired
voltage and connect the positive and negative “battery
input” terminals of the simulator in place of the battery.
In discharge mode, the battery simulator uses the current
limited lab power supply PS1 as the source and the
simulator circuit is inactive. In charge mode, current is
forced through the battery input terminals. The low voltage that develops across R8 is amplified by U1 and causes
Q1 to shunt the charge current while maintaining the
+
R10
0.1Ω
2W
WW
BATTERY
INPUT
Q1
BUZ11
+
R7
10Ω
R1
1Ω
2W
WW
C5
4700µF
ELECT
C1
0.1µF
R2
1k
R3
100k
–
1
8
+
U1
LT1078
R4 C2
10k 0.1µF
–
4
3
CR1
1N5818
2
R5
1k
NC
L1
120µH
R8
0.1Ω
2W
WW
R6
1k
+
3
6
NC
7
VIN 2
SW1
U2
LT1073-12
1
8
ILIM
SENSE
C3
47µF
ELECT
SW2
GND
4
5
R9
200Ω
2W
WW
+
PS1
ADJUSTABLE
LAB POWER
SUPPLY WITH
CURRENT LIMIT
C4
47µF
ELECT
AN58 B1
NOTE: UNLESS OTHERWISE SPECIFIED:
CAPACITORS ARE 25V, CERAMIC
Figure B1. Battery Simulator
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN68-35
Application Note 68
power supply PS1 voltage. U2, L1, CR1, C3 and C4
produce housekeeping 12V that is required to operate U1
and drive Q1. The power supply range is 1.5V to 15V. Q1
requires a heat sink. R1 is used for measuring the charge
current. R10 and C5 simulate the AC characteristics of the
battery.
Controlled Constant-Current Load
Although a constant-power load is a more realistic load,
the constant-current load (Figure B2) works better for
battery discharge testing because the discharge time
gives immediate data on the battery charge level. All that
needs to be done is to multiply the current by the discharge
time.
switch to BOOST. The minimum control voltage is then
3.3V. The user has to adjust the potentiometer to the
desired current. This can be done by connecting a power
supply or battery at above 2V in series with a current meter
to the (+) and (–) terminals and adjusting R4.
When the load is connected to a battery, Q1 and Q2 operate
in a negative feedback mode and maintain the VBE of Q1 at
0.5V. The voltage across R6 must be between 0.5V and
1.5V, depending on the wiper position of R4, a trim pot.
Since the value of R6 is 1Ω, this translates to a constant
current of 0.5A to 1.5A. Q1 requires a heat sink.
A voltage doubler (U1, C1, C2, C3, C4, CR1, CR2, R1 and
R2) boosts the voltage for the gate of Q2.
For operation in DIRECT mode, the control input has to be
above 7V. If the control voltage available is less than 7V,
DIRECT
SW1 B
BOOST
SW1 A
CONTROL
R2
10k
R1
100k
4
5
C2
0.1µF
2
6
C1
1000pF
U1
LMC555
CR1
1N914
C3
1µF
CR2
1N914
R3
100k
(+)
Q2
MTP50N06E
3
1
C4
0.1µF
Q1
MPS2222
R4
500Ω
R5
249Ω
R6
1Ω
2W
WW
0.5A TO 1.5A
LOAD
(–)
AN68 FB2
Figure B2. Controlled Constant Current Load
Note: Linear Technology would like to thank Arie Ravid for
contributing the majority of the material in this application note.
AN68-36
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417● (408)432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
an68f LT/GP 1296 5K • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1996