LINER LT1512 Sepic constant-current/ constant-voltage battery charger Datasheet

LT1512
SEPIC Constant-Current/
Constant-Voltage Battery Charger
Features
n
n
n
n
n
n
n
Description
Charger Input Voltage May Be Higher, Equal to or
Lower Than Battery Voltage
Charges Any Number of Cells Up to 30V*
1% Voltage Accuracy for Rechargeable Lithium
Batteries
500kHz Switching Frequency Minimizes
Inductor Size
100mV Current Sense Voltage for High Efficiency
Battery Can Be Directly Grounded
Charging Current Easily Programmable or Shut Down
The LT®1512 is a 500kHz current mode switching regulator
specially configured to create a constant-current/constantvoltage battery charger. In addition to the usual voltage
feedback node, it has a current sense feedback circuit for
accurately controlling output current of a flyback or SEPIC
(Single-Ended Primary Inductance Converter) topology
charger. These topologies allow the current sense circuit
to be ground referred and completely separated from the
battery itself, simplifying battery switching and system
grounding problems. In addition, these topologies allow
charging even when the input voltage is lower than the
battery voltage.
Applications
n
n
n
n
Maximum switch current on the LT1512 is 1.5A. This allows
battery charging currents up to 1A for a single lithium-ion
cell. Accuracy of 1% in constant-voltage mode is perfect
for lithium battery applications. Charging current can be
easily programmed for all battery types.
Battery Charging of NiCd, NiMH, Lead-Acid or
Lithium Rechargeable Cells
Precision Current Limited Power Supply
Constant-Voltage/Constant-Current Supply
Transducer Excitation
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Maximum Input Voltage = 40V – VBAT
Typical Application
Maximum Charging Current
CHARGE
SHUTDOWN
+
•
C3
22µF
25V
SYNC
AND/OR
SHUTDOWN
VIN
S/S
1.0
C2**
D1
2.2µF MBRS130LT3
VSW
L1 B*
LT1512
GND GND S VC
IFB
C5
0.1µF
R5
1k
R1
•
FB
R4
24Ω
C4
0.22µF
R2
+
C1
22µF
25V
Figure 1. SEPIC Charger with 0.5A Output Current
0.6
DOUBLE
LITHIUM
CELL (8.2V)
0.4
6V BATTERY
12V BATTERY
0.2
R3
0.2Ω
*L1 A, L1 B ARE TWO 33µH WINDINGS ON A
SINGLE INDUCTOR: COILTRONICS CTX33-3
**TOKIN CERAMIC 1E225ZY5U-C203-F
SINGLE
LITHIUM
CELL (4.1V)
0.8
0.5A
CURRENT (A)
WALL
ADAPTER
INPUT
L1 A*
INDUCTOR = 33µH
0
1512 F01
0
5
15
10
INPUT VOLTAGE (V)
20
25
1512 TA02
ACTUAL PROGRAMMED CHARGING current will be independent of input
voltage and battery voltage if it does not exceed the values shown.
THESE ARE ELECTRICAL LIMITATIONS BASED ON MAXIMUM SWITCH CURRENT.
PACKAGE THERMAL LIMITATIONS MAY REDUCE MAXIMUM CHARGING CURRENT.
SEE APPLICATIONs INFORMATION.
1512fc
For more information www.linear.com/LT1512
1
LT1512
Absolute Maximum Ratings
(Note 1)
Pin Configuration
Input Voltage ............................................................ 30V
Switch Voltage.......................................................... 40V
S/S Pin Voltage......................................................... 30V
FB Pin Voltage (Transient, 10ms)............................ ±10V
VFB Pin Current...................................................... 10mA
IFB Pin Voltage (Transient, 10ms) ........................... ±10V
Storage Temperature Range................... –65°C to 150°C
Ambient Temperature Range
LT1512C (Note 2)...................................... 0°C to 70°C
LT1512I................................................. –40°C to 85°C
Operating Junction Temperature Range
LT1512C (Note 2)............................... –20°C to 125°C
LT1512I............................................... –40°C to 125°C
Short Circuit.......................................... 0°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
TOP VIEW
VC 1
8
VSW
FB 2
7
GND
IFB 3
6
GND S
S/S 4
5
VIN
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 125°C, qJA = 100°C/W (N)
TJMAX = 125°C, qJA = 130°C/W (S)
NOTE: CONTACT FACTORY CONCERNING 16-LEAD
FUSED-LEAD GN PACKAGE WITH LOWER THERMAL RESISTANCE
order information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1512CN8#PBF
LT1512CN8#TRPBF
1512
8-Lead PDIP
0°C to 70°C
LT1512CS8#PBF
LT1512CS8#TRPBF
1512
8-Lead Plastic SO
0°C to 70°C
LT1512IN8#PBF
LT1512IN8#TRPBF
1512I
8-Lead PDIP
–40°C to 85°C
LT1512IS8#PBF
LT1512IS8#TRPBF
1512I
8-Lead Plastic SO
–40°C to 85°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1512CN8
LT1512CN8#TR
1512
8-Lead PDIP
0°C to 70°C
LT1512CS8
LT1512CS8#TR
1512
8-Lead Plastic SO
0°C to 70°C
LT1512IN8
LT1512IN8#TR
1512I
8-Lead PDIP
–40°C to 85°C
LT1512IS8
LT1512IS8#TR
1512I
8-Lead Plastic SO
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VC = 0.6V, VFB = VREF, IFB = 0V, VSW and S/S pins open,
unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VREF
VFB Reference Voltage
Measured at FB Pin
VC = 0.8V
FB Input Current
l
VFB = VREF
MIN
TYP
MAX
UNITS
1.233
1.228
1.245
1.245
1.257
1.262
V
V
300
550
600
nA
nA
0.01
0.03
%/V
l
FB Reference Voltage Line Regulation
2
2.7V ≤ VIN ≤ 25V, VC = 0.8V
l
1512fc
For more information www.linear.com/LT1512
LT1512
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VC = 0.6V, VFB = VREF, IFB = 0V, VSW and S/S pins open,
unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
VIREF
IFB Reference Voltage
Measured at IFB Pin
VFB = 0V, VC = 0.8V
IFB Input Current
gm
AV
f
MIN
TYP
MAX
UNITS
l
–107
–110
–100
–100
–93
–90
mV
mV
VIFB = VIREF (Note 3)
l
10
25
35
µA
IFB Reference Voltage Line Regulation
2.7V ≤ VIN ≤ 25V, VC = 0.8V
l
0.01
0.05
%/V
Error Amplifier Transconductance
∆IC = ±25µA
1100
700
1500
l
1900
2300
µmho
µmho
120
200
350
µA
1400
2400
µA
1.95
0.40
2.30
0.52
V
V
Error Amplifier Source Current
VFB = VREF – 150mV, VC = 1.5V
l
Error Amplifier Sink Current
VFB = VREF + 150mV, VC = 1.5V
l
Error Amplifier Clamp Voltage
High Clamp, VFB = 1V
Low Clamp, VFB = 1.5V
1.70
0.25
VC Pin Threshold
Duty Cycle = 0%
0.8
1
1.25
V
Switching Frequency
2.7V ≤ VIN ≤ 25V
0°C ≤ TJ ≤ 125°C
–40°C ≤ TJ < 0°C (LT1512I)
l
450
430
400
500
500
550
580
580
kHz
kHz
kHz
l
85
95
40
35
Error Amplifier Voltage Gain
500
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
130
BV
Output Switch Breakdown Voltage
0°C ≤ TJ ≤ 125°C
–40°C ≤ TJ < 20°C (LT1512I)
l
VSAT
Output Switch ON Resistance
ISW = 2A
l
ILIM
Switch Current Limit
Duty Cycle = 50%
Duty Cycle = 80% (Note 4)
l
l
∆IIN
∆ISW
Supply Current Increase During Switch ON Time
1.5
1.3
Control Voltage to Switch Current Transconductance
%
260
47
ns
V
V
0.5
0.8
Ω
1.9
1.7
2.7
2.5
A
A
15
25
mA/A
2
Minimum Input Voltage
IQ
V/V
A/V
l
2.4
2.7
V
Supply Current
2.7V ≤ VIN ≤ 25V
l
4
5.5
mA
Shutdown Supply Current
2.7V ≤ VIN ≤ 25V, VS/S ≤ 0.6V
0°C ≤ TJ ≤ 125°C
–40°C ≤ TJ ≤ 0°C (LT1512I)
l
12
30
50
µA
µA
2.7V ≤ VIN ≤ 25V
l
0.6
1.3
2
V
l
5
12
25
µs
l
–10
15
µA
l
600
800
kHz
Shutdown Threshold
Shutdown Delay
S/S Pin Input Current
0V ≤ VS/S ≤ 5V
Synchronization Frequency Range
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: Commercial devices are guaranteed over 0°C to 125°C junction
temperature range and 0°C to 70°C ambient temperature range. These
parts are also designed, characterized and expected to operate over the
–20°C to 85°C extended ambient temperature range, but are not tested at
–20°C or 85°C. Devices with full guaranteed electrical specifications over
the ambient temperature range –40°C to 85°C are available as industrial
parts with an “I” suffix.
Maximum allowable ambient temperature may be limited by power
dissipation. Parts may not necessarily be operated simultaneously
at maximum power dissipation and maximum ambient temperature.
Temperature rise calculations must be done as shown in the Applications
Information section to ensure that maximum junction temperature does
not exceed 125°C limit. With high power dissipation, maximum ambient
temperature may be less than 70°C.
Note 3: The IFB pin is servoed to its regulating state with VC = 0.8V.
Note 4: For duty cycles (DC) between 50% and 85%, minimum guaranteed
switch current is given by ILIM = 0.667 (2.75 – DC).
1512fc
For more information www.linear.com/LT1512
3
LT1512
Typical Performance Characteristics
Switch Saturation Voltage
vs Switch Current
3.0
0.7
0.6
–55°C
0.4
0.3
0.2
0.1
0
0
2.5
2.8
25°C AND
125°C
2.0
–55°C
1.5
1.0
0
FEEDBACK INPUT CURRENT (nA)
MINIMUM SYNCHRONIZATION VOLTAGE (VP-P)
1.0
0.5
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G04
4
700
VFB = VREF
600
500
400
300
200
100
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
Negative Feedback Input Current
vs Temperature
800
1.5
0
1512 G03
Feedback Input Current
vs Temperature
fSYNC = 700kHz
2.0
2.2
1512 G02
Minimum Peak-to-Peak
Synchronization Voltage vs Temp
2.5
2.4
1.8
–50 –25
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1512 G01
3.0
2.6
2.0
0.5
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
3.0
INPUT VOLTAGE (V)
25°C
0.8
0.5
Minimum Input Voltage
vs Temperature
NEGATIVE FEEDBACK INPUT CURRENT (µA)
150°C
100°C
0.9
SWITCH CURRENT LIMIT (A)
SWITCH SATURATION VOLTAGE (V)
1.0
Switch Current Limit
vs Duty Cycle
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G05
0
–10
–20
–30
–40
–50
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G06
1512fc
For more information www.linear.com/LT1512
LT1512
Pin Functions
VC: The compensation pin is primarily used for frequency
compensation, but it can also be used for soft starting and
current limiting. It is the output of the error amplifier and
the input of the current comparator. Peak switch current
increases from 0A to 1.8A as the VC voltage varies from
1V to 1.9V. Current out of the VC pin is about 200µA when
the pin is externally clamped below the internal 1.9V clamp
level. Loop frequency compensation is performed with a
capacitor or series RC network from the VC pin directly to
the ground pin (avoid ground loops).
FB: The feedback pin is used for positive output voltage
sensing. This pin is the inverting input to the voltage
error amplifier. The R1/R2 voltage divider connected to
FB defines Li-Ion float voltage at full charge, or acts as a
voltage limiter for NiCd or NiMH applications. Input bias
current is typically 300nA, so divider current is normally
set to 100µA to swamp out any output voltage errors due
to bias current. The noninverting input of this amplifier is
tied internally to a 1.245V reference. The grounded end of
the output voltage divider should be connected directly to
the LT1512 ground pin (avoid ground loops).
IFB: The current feedback pin is used to sense charging
current. It is the input to a current sense amplifier that
controls charging current when the battery voltage is below
the programmed voltage. During constant-current operation, the IFB pin regulates at –100mV. Input resistance of
this pin is 5kΩ, so filter resistance (R4, Figure 1) should be
less than 50Ω. The 24Ω, 0.22µF filter shown in Figure 1 is
used to convert the pulsating current in the sense resistor
to a smooth DC current feedback signal.
S/S: This pin can be used for shutdown and/or synchronization. It is logic level compatible, but can be tied to VIN
if desired. It defaults to a high ON state when floated. A
logic low state will shut down the charger to a micropower
state. Driving the S/S pin with a continuous logic signal of
600kHz to 800kHz will synchronize switching frequency
to the external signal. Shutdown is avoided in this mode
with an internal timer.
VIN: The input supply pin should be bypassed with a
low ESR capacitor located right next to the IC chip. The
grounded end of the capacitor must be connected directly
to the ground plane to which the GND pin is connected.
GND S, GND: The LT1512 uses separate ground pins for
switch current (GND) and the control circuitry (GND S).
This isolates the control ground from any induced voltage created by fast switch currents. Both pins should be
tied directly to the ground plane, but the external control
circuit components such as the voltage divider, frequency
compensation network and IFB bypass capacitor should be
connected directly to the GND S pin or to the ground plane
close to the point where the GND S pin is connected.
VSW: The switch pin is the collector of the power switch,
carrying up to 1.5A of current with fast rise and fall times.
Keep the traces on this pin as short as possible to minimize radiation and voltage spikes. In particular, the path
in Figure 1 which includes SW to C2, D1, C1 and around
to the LT1512 ground pin should be as short as possible
to minimize voltage spikes at switch turn-off.
1512fc
For more information www.linear.com/LT1512
5
LT1512
Block Diagram
VIN
SHUTDOWN
DELAY AND RESET
S/S
SYNC
SW
LOW DROPOUT
2.3V REG
500kHz
OSC
ANTI-SAT
LOGIC
DRIVER
SWITCH
+
IFB
IFBA
5k
–
COMP
62k
–
–
+
FB
1.245V
REF
+
EA
IA
VC
AV ≈ 6
0.08Ω
–
GND
1512 F02
GND S
Figure 2
6
1512fc
For more information www.linear.com/LT1512
LT1512
Operation
The LT1512 is a current mode switcher. This means that
switch duty cycle is directly controlled by switch current
rather than by output voltage or current. Referring to the
Block Diagram, the switch is turned “on” at the start of
each oscillator cycle. It is turned “off” when switch current
reaches a predetermined level. Control of output voltage and
current is obtained by using the output of a dual feedback
voltage sensing error amplifier to set switch current trip
level. This technique has the advantage of simplified loop
frequency compensation. A low dropout internal regulator provides a 2.3V supply for all internal circuitry on the
LT1512. This low dropout design allows input voltage to
vary from 2.7V to 25V. A 500kHz oscillator is the basic
clock for all internal timing. It turns “on” the output switch
via the logic and driver circuitry. Special adaptive antisat
circuitry detects onset of saturation in the power switch
and adjusts driver current instantaneously to limit switch
saturation. This minimizes driver dissipation and provides
very rapid turn-off of the switch.
A unique error amplifier design has two inverting inputs
which allow for sensing both output voltage and current.
A 1.245V bandgap reference biases the noninverting input.
The first inverting input of the error amplifier is brought out
for positive output voltage sensing. The second inverting
input is driven by a “current” amplifier which is sensing
output current via an external current sense resistor. The
current amplifier is set to a fixed gain of –12.5 which
provides a –100mV current limit sense voltage.
The error signal developed at the amplifier output is brought
out externally and is used for frequency compensation.
During normal regulator operation this pin sits at a voltage
between 1V (low output current) and 1.9V (high output
current). Switch duty cycle goes to zero if the VC pin is
pulled below the VC pin threshold, placing the LT1512 in
an idle mode.
Applications Information
The LT1512 is an IC battery charger chip specifically optimized to use the SEPIC converter topology. The SEPIC
topology has unique advantages for battery charging. It
will operate with input voltages above, equal to or below
the battery voltage, has no path for battery discharge when
turned off and eliminates the snubber losses of flyback
designs. It also has a current sense point that is ground
referred and need not be connected directly to the battery.
The two inductors shown are actually just two identical
windings on one inductor core, although two separate
inductors can be used.
A current sense voltage is generated with respect to ground
across R3 in Figure 1. The average current through R3 is
always identical to the current delivered to the battery. The
LT1512 current limit loop will servo the voltage across R3
to –100mV when the battery voltage is below the voltage
limit set by the output divider R1/R2. Constant current
charging is therefore set at 100mV/R3. R4 and C4 filter
the current signal to deliver a smooth feedback voltage to
the IFB pin. R1 and R2 form a divider for battery voltage
sensing and set the battery float voltage. The suggested
value for R2 is 12.4k. R1 is calculated from:
R1=
R2( VBAT – 1.245)
1.245 + R2(0.3µA)
VBAT = battery float voltage
0.3µA = typical FB pin bias current
A value of 12.4k for R2 sets divider current at 100µA.
This is a constant drain on the battery when power to the
charger is off. If this drain is too high, R2 can be increased
to 41.2k, reducing divider current to 30µA. This introduces
an additional uncorrectable error to the constant voltage
float mode of about ±0.5% as calculated by:
VBAT Error=
±0.15µA(R1)(R2)
1.245(R1+R2)
±0.15µA = expected variation in FB bias current around
the nominal 0.3µA typical value.
With R2 = 41.2k and R1 = 228k, (VBAT = 8.2V), the error
due to variations in bias current would be ±0.42%.
A second option is to disconnect the voltage divider with
a small NMOS transistor as shown in Figure 3. To ensure
1512fc
For more information www.linear.com/LT1512
7
LT1512
Applications Information
CONNECT D2 ANODE HERE FOR FULLY
CHARGED BATTERY VOLTAGE LESS
THAN 3.5V. Q1 WILL NOT BE TURNED OFF
IN SHUTDOWN IF VIN IS PRESENT
•
CONNECT D2 ANODE HERE IF FULLY CHARGED
BATTERY VOLTAGE IS GREATER THAN 3.5V AND
Q1 MUST BE TURNED OFF IN SHUTDOWN WITH
VIN STILL ACTIVE
L1 A
C2
VIN
D1
VSW
R1
SHUTDOWN
S/S
LT1512
GND
R5
470k
C6
470pF
D2
1N4148
L1 B
+
Q1
2N7002
FB
R3
R2
1512 F03
Figure 3. Eliminating Divider Current
adequate drive to the transistor (even when the VIN voltage
is at its lowest operating point of 2.4V), the FET gate is
driven wth a peak detected voltage via D2. Note that there
are two connections for D2. The L1 A connection must be
used if the voltage divider is set for less than 3.5V (fully
charged battery). Gate drive is equal to battery voltage
plus input voltage. The disadvantage of this connection is
that Q1 will still be “on” if the VIN voltage is active and the
charger is shut down via the S/S pin. The L1 B connection
allows Q1 to turn off when VIN is off or when shutdown is
initiated, but the reduced gate drive (= VBAT) is not adequate
to ensure a Q1 on-state for fully charged battery voltages
less than 3.5V. Do not substitute for Q1 unless the new
device has adequate VGS maximum rating, especially if
D2 is connected to L1A. C6 filters the gate drive and R5
pulls the gate low when switching stops.
Disconnecting the divider leaves only D1 diode leakage
as a battery drain. See Diode Selection for a discussion
of diode leakage.
Maximum Input Voltage
Maximum input voltage for the circuit in Figure 1 is partly
determined by battery voltage. A SEPIC converter has a
maximum switch voltage equal to input voltage plus output voltage. The LT1512 has a maximum input voltage of
30V and a maximum switch voltage of 40V, so this limits
maximum input voltage to 30V, or 40V – VBAT, whichever
is less. Maximum VBAT = 40V – VIN.
8
Shutdown and Synchronization
The dual function S/S pin provides easy shutdown and
synchronization. It is logic level compatible and can be
pulled high or left floating for normal operation. A logic
low on the S/S pin activates shutdown, reducing input
supply current to 12µA. To synchronize switching, drive
the S/S pin between 600kHz and 800kHz.
Inductor Selection
L1A and L1B are normally just two identical windings on
one core, although two separate inductors can be used.
A typical value is 33µH, which gives about 0.25A peak-topeak inductor current. Lower values will give higher ripple
current, which reduces maximum charging current. 15µH
can be used if charging currents are at least 20% lower than
the values shown in the maximum charging current graph.
Higher inductance values give slightly higher maximum
charging current, but are larger and more expensive. A
low loss toroid core such as KoolMµ, Molypermalloy or
Metglas is recommended. Series resistance should be
less than 0.1Ω for each winding. “Open core” inductors,
such as rods or barrels are not recommended because
they generate large magnetic fields which may interfere
with other electronics close to the charger.
Input Capacitor
The SEPIC topology has relatively low input ripple current
compared to other topologies and higher harmonics are
1512fc
For more information www.linear.com/LT1512
LT1512
Applications Information
especially low. RMS ripple current in the input capacitor is
less than 0.1A with L = 33µH and less than 0.2A with L =
15µH. A low ESR 22µF, 25V solid tantalum capacitor (AVX
type TPS or Sprague type 593D) is adequate for most applications with the following caveat. Solid tantalum capacitors
can be destroyed with a very high turn-on surge current
such as would be generated if a low impedance input source
were “hot switched” to the charger input. If this condition
can occur, the input capacitor should have the highest possible voltage rating, at least twice the surge input voltage if
possible. Consult with the capacitor manufacturer before
a final choice is made. A 2.2µF ceramic capacitor such as
the one used for the coupling capacitor can also be used.
These capacitors do not have a turn-on surge limitation.
The input capacitor must be connected directly to the VIN
pin and the ground plane close to the LT1512.
Output Capacitor
It is assumed as a worst case that all the switching output ripple current from the battery charger could flow in
the output capacitor. This is a desirable situation if it is
necessary to have very low switching ripple current in
the battery itself. Ferrite beads or line chokes are often
inserted in series with the battery leads to eliminate high
frequency currents that could create EMI problems. This
forces all the ripple current into the output capacitor. Total
RMS current into the capacitor has a maximum value of
about 0.5A, and this is handled with a 22µF, 25V capacitor
shown in Figure 1. This is an AVX type TPS or Sprague
type 593D surface mount solid tantalum unit intended
for switching applications. Do not substitute other types
without ensuring that they have adequate ripple current
ratings. See Input Capacitor section for details of surge
limitation on solid tantalum capacitors if the battery may
be “hot switched” to the output of the charger.
Coupling Capacitor
C2 in Figure 1 is the coupling capacitor that allows a SEPIC
converter topology to work with input voltages either
higher or lower than the battery voltage. DC bias on the
capacitor is equal to input voltage. RMS ripple current
in the coupling capacitor has a maximum value of about
0.5A at full charging current. A conservative formula to
calculate this is:
ICOUP(RMS) =
ICHRG( VIN + VBAT )(1.1)
2( VIN )
(1.1 is a fudge factor to account for inductor ripple current
and other losses)
GND
VIN
1
4
+VIN
R4
R1
L1A
2 WINDING
INDUCTOR
L1B
4
L1B
3
1
L1A
2
R3
C4 R2
3
D1
D1
GND
C1
VBATT
C5
GND
R1 R2
1512 F04a
a. Double-Sided (Vias Connect to the Backside of Ground Plane. Dash
Lines Indicate Interconnects on Backside. Demo Board Uses This
Layout, Except that R5 Has Been Added to Increase Phase Margin)
R5
+
C3
R3
VIN
U1
C1
S/S
C5
+
GND
R5
VBATT
IFB GND S
C2B C2A
VC VSW
C3
S/S
C2
FB
2
R4
C4
S/S
1512 F04b
b. Single-Sided Alternative Layout
Figure 4. LT1512 Suggested Layouts for Critical Thermal and Electrical Paths
For more information www.linear.com/LT1512
1512fc
9
LT1512
Applications Information
With ICHRG = 0.5A, VIN = 15V and VBAT = 8.2V, ICOUP = 0.43A
The recommended capacitor is a 2.2µF ceramic type from
Marcon or Tokin. These capacitors have extremely low ESR
and high ripple current ratings in a small package. Solid
tantalum units can be substituted if their ripple current
rating is adequate, but typical values will increase to 22µF
or more to meet the ripple current requirements.
Diode Selection
The switching diode should be a Schottky type to minimize
both forward and reverse recovery losses. Average diode
current is the same as output charging current , so this
will be under 1A. A 1A diode is recommended for most
applications, although smaller devices could be used at
reduced charging current. Maximum diode reverse voltage
will be equal to input voltage plus battery voltage.
Diode reverse leakage current will be of some concern
during charger shutdown. This leakage current is a direct
drain on the battery when the charger is not powered. High
current Schottky diodes have relatively high leakage currents
(2µA to 200µA) even at room temperature. The latest verylow-forward devices have especially high leakage currents.
It has been noted that surface mount versions of some
Schottky diodes have as much as ten times the leakage of
their through-hole counterparts. This may be because a low
forward voltage process is used to reduce power dissipation
in the surface mount package. In any case, check leakage
specifications carefully before making a final choice for the
switching diode. Be aware that diode manufacturers want to
specify a maximum leakage current that is ten times higher
than the typical leakage. It is very difficult to get them to
specify a low leakage current in high volume production.
This is an on going problem for all battery charger circuits
and most customers have to settle for a diode whose typical leakage is adequate, but theoretically has a worst-case
condition of higher than desired battery drain.
Average supply current (including driver current) is:
IIN = 4mA +
( VBAT )(ICHRG )(0.024)
VIN
Switch power dissipation is given by:
PSW =
(ICHRG )2(RSW )( VBAT + VIN )( VBAT)
( VIN )2
RSW = output switch ON resistance
Total power dissipation of the die is equal to supply current
times supply voltage, plus switch power:
PD(TOTAL) = (IIN)(VIN) + PSW
For VIN = 10V, VBAT = 8.2V, ICHRG = 0.5A, RSW = 0.65Ω
IIN = 4mA + 10mA = 14mA
PSW = 0.24W
PD = (0.014)(10) + 0.24 = 0.38W
The S8 package has a thermal resistance of 130°C/W.
(Contact factory concerning 16-lead fused-lead package with footprint approximately same as S8 package
and with lower thermal resistance.) Die temperature rise
will be (0.38W)(130°C/W) = 49°C. A maximum ambient
temperature of 60°C will give a die temperature of 60°C +
49°C = 109°C. This is only slightly less than the maximum
junction temperature of 125°C, illustrating the importance
of doing these calculations!
Programmed Charging Current
LT1512 charging current can be programmed with a PWM
signal from a processor as shown in Figure 5. C6 and D2
form a peak detector that converts a positive logic signal
to a negative signal. The average negative signal at the
LT1512
Thermal Considerations
Care should be taken to ensure that worst-case conditions
do not cause excessive die temperatures. Typical thermal
resistance is 130°C/W for the S8 package but this number
will vary depending on the mounting technique (copper
area, air flow, etc).
10
PWM
INPUT
≥1kHz
+
C6
1µF
IFB
R6
4.02k
R5
4.02k
D2
+
C7
10µF
R4
200Ω
C4
0.22µF
L1B
R3
1512 F05
Figure 5. Programming Charge Current
1512fc
For more information www.linear.com/LT1512
LT1512
Applications Information
input to R5 is equal to the processor VCC level multiplied
by the inverse PWM ratio. This assumes that the PWM
signal is a CMOS output that swings rail-to-rail with a
source resistance less than a few hundred ohms. The
negative voltage is converted to a current by R5 and R6
and filtered by C7. This current multiplied by R4 generates
a voltage that subtracts from the 100mV sense voltage
of the LT1512. This is not a high precision technique
because of the errors in VCC and the diode voltage, but
it can typically be used to adjust charging current over a
20% to 100% range with good repeatability (full charging current accuracy is not affected). To reduce the load
on the logic signal, R4 has been increased from 24Ω to
200Ω. This causes a known increase in full-scale charging
Package Description
current (PWM = 0) of 3% due to the 5k input resistance of
the IFB pin. Note that 100% duty cycle gives full charging
current and that very low duty cycles (especially zero!)
will not operate correctly. Very low duty cycle (<10%)
is a problem because the peak detector requires a finite
up-time to reset C6.
More Help
Linear Technology Field Application Engineers have a CAD
spreadsheet program for detailed calculations of circuit
operating conditions, and our Applications Department is
always ready to lend a helping hand. For additional information refer to the LT1372 data sheet. This part is identical to
the LT1512 except for the current amplifier circuitry.
Dimensions in inches (millimeters) unless otherwise noted.
N Package
Package.300 Inch)
8-Lead PDIPN8(Narrow
8-Lead PDIP (Narrow 0.300)
(Reference LTC DWG # 05-08-1510 Rev I)
(LTC DWG # 05-08-1510 Rev I)
.400*
(10.160)
MAX
8
7
6
5
1
2
3
4
.255 ±.015*
(6.477 ±0.381)
.300 – .325
(7.620 – 8.255)
.008 – .015
(0.203 – 0.381)
(
+.035
.325 –.015
8.255
+0.889
–0.381
)
.045 – .065
(1.143 – 1.651)
.065
(1.651)
TYP
.100
(2.54)
BSC
.130 ±.005
(3.302 ±0.127)
.120
(3.048) .020
MIN
(0.508)
MIN
.018 ±.003
(0.457 ±0.076)
N8 REV I 0711
NOTE:
1. DIMENSIONS ARE
INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
For more information www.linear.com/LT1512
1512fc
11
LT1512
Package Description
Dimensions in inches (millimeters) unless otherwise noted.
Package
S88 Package
8-Lead
Plastic
Small
Outline
(Narrow
0.150)
8-Lead
Plastic
Small
Outline
(Narrow
.150
Inch)
(LTCLTC
DWGDWG
# 05-08-1610
Rev G)
(Reference
# 05-08-1610
Rev G)
.189 – .197
(4.801 – 5.004)
NOTE 3
.045 ±.005
.050 BSC
8
.245
MIN
.160 ±.005
.010 – .020
× 45°
(0.254 – 0.508)
2
.053 – .069
(1.346 – 1.752)
0°– 8° TYP
.016 – .050
(0.406 – 1.270)
.014 – .019
(0.355 – 0.483)
TYP
INCHES
(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE
12
5
.150 – .157
(3.810 – 3.988)
NOTE 3
1
RECOMMENDED SOLDER PAD LAYOUT
.008 – .010
(0.203 – 0.254)
6
.228 – .244
(5.791 – 6.197)
.030 ±.005
TYP
NOTE:
1. DIMENSIONS IN
7
3
4
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
SO8 REV G 0212
1512fc
For more information www.linear.com/LT1512
LT1512
Revision History
(Revision history begins at Rev B)
REV
DATE
DESCRIPTION
B
6/14
Reconfigured inputs to LM301
PAGE NUMBER
C
3/15
Changed inductor value units from “mH” to “µH”
14
1, 14
1512fc
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.
13
LT1512
Typical Application
The circuit in Figure 6 will provide adapter current limiting to ensure that the battery charger never overloads
the adapter. In addition, it adjusts charging current to a
lower value if other system power increases to the point
where the adapter would be overloaded. This allows the
WALL
ADAPTER
INPUT
SYSTEM
POWER
R6
0.2Ω
+
+
3
2
–
4
D2
1N4148
VIN
VSW
7
6
8
0.5A
L1 B*
LT1512
SYNC
4
AND/OR
S/S
SHUTDOWN
GND GND S VC
FB
2
IFB
1
C5
0.1µF
R1
•
R4
24Ω
3
TO FB PIN
R7
12k
C2**
D1
2.2µF MBRS130LT3
5
30pF
8
6
C3
22µF
25V
1
LM301
Q1
2N3904
L1 A*
•
R5
1k
7
LT1512 to charge the battery at the maximum possible
rate without concern about varying system power levels.
The LM301 op amp used here is unusual in that it can
operate with its inputs at a voltage equal to the positive
supply voltage.
C4
0.22µF
R2
+
C1
22µF
25V
R3
0.2Ω
*L1 A, L1 B ARE TWO 33µH WINDINGS ON A
COMMON CORE: COILTRONICS CTX33-3
**TOKIN CERAMIC 1E225ZY5U-C203-F
1512 F06
Figure 6. Adding Adapter Current Limiting
Related Parts
PART NUMBER DESCRIPTION
COMMENTS
LT1239
Backup Battery Management System
Charges Backup Battery and Regulates Backup Battery Output when
Main Battery Removed
LTC®1325
Microprocessor Controlled Battery Management System
Can Charge, Discharge and Gas Gauge NiCd, NiMH and Pb-Acid Batteries
with Software Charging Profiles
LT1510
1.5A Constant-Current/Constant-Voltage Battery Charger
Step-Down Charger for Li-Ion, NiCd and NiMH
LT1511
3.0A Constant-Current/Constant-Voltage Battery Charger
with Input Current Limiting
Step-Down Charger that Allows Charging During Computer Operation and
Prevents Wall-Adapter Overload
LT1513
SEPIC Constant-Current/Constant-Voltage Battery Charger
Step-Up/Step-Down Charger for Up to 2A Current
LTC4020
55V Buck-Boost Multi-Chemistry Battery Charger
4.5V to 55V input, up to 20+A charge current, up to 55V output/charge
voltage, Li-Ion and SLA battery termination algorithms on board
14 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT1512
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT1512
1512fc
LT 0315 REV C • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2008