LT1512
SEPIC Constant-Current/
Constant-Voltage
Battery Charger
FEATURES
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DESCRIPTION
The LT
®
1512 is a 500kHz current mode switching regula-
tor specially configured to create a constant-current/
constant-voltage 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 sepa-
rated from the battery itself, simplifying battery switching
and system grounding problems. In addition, these to-
pologies allow charging even when the input voltage is
lower than the battery voltage.
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 cur-
rent can be easily programmed for all battery types.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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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
100mV Current Sense Voltage for High Efficiency
Battery Can Be Directly Grounded
500kHz Switching Frequency Minimizes
Inductor Size
Charging Current Easily Programmable or Shut Down
APPLICATIONS
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s
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Battery Charging of NiCd, NiMH, Lead-Acid
or Lithium Rechargeable Cells
Precision Current Limited Power Supply
Constant-Voltage/Constant-Current Supply
Transducer Excitation
*Maximum Input Voltage = 40V – V
BAT
TYPICAL APPLICATION
1.0
Maximum Charging Current
SINGLE
LITHIUM
CELL (4.1V)
WALL
ADAPTER
INPUT
CHARGE
•
L1 A*
C2**
D1
2.2µF MBRS130LT3
CURRENT (A)
0.8
+
C3
22µF
25V
V
IN
LT1512
S/S
GND GND S V
C
V
SW
L1 B*
FB
I
FB
C5
0.1µF
R5
1k
C4
0.22µF
R3
0.2Ω
•
R4
24Ω
R2
R1
0.5A
0.6
SYNC
AND/OR
SHUTDOWN
SHUTDOWN
0.4
+
C1
22µF
25V
0.2
0
0
*L1 A, L1 B ARE TWO 33µH WINDINGS ON A
SINGLE INDUCTOR: COILTRONICS CTX33-3
**TOKIN CERAMIC 1E225ZY5U-C203-F
1512 F01
Figure 1. SEPIC Charger with 0.5A Output Current
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.
U
U
U
DOUBLE
LITHIUM
CELL (8.2V)
6V BATTERY
12V BATTERY
INDUCTOR = 33µH
5
15
10
INPUT VOLTAGE (V)
20
25
1512 TA02
1
LT1512
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
C
1
FB 2
I
FB
3
S/S 4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
V
SW
GND
GND S
V
IN
Input Voltage .......................................................... 30V
Switch Voltage ........................................................ 40V
S/S Pin Voltage ....................................................... 30V
FB Pin Voltage (Transient, 10ms) .........................
±10V
V
FB
Pin Current .................................................... 10mA
I
FB
Pin Voltage (Transient, 10ms) .........................
±10V
Storage Temperature Range ................ – 65°C to 150°C
Ambient Temperature Range
LT1512C (Note 3) .................................... 0°C to 70°C
LT1512I .............................................. – 40°C to 85°C
Operating Junction Temperature Range
LT1512C (Note 3) ............................ – 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
ORDER PART
NUMBER
LT1512CN8
LT1512CS8
LT1512IN8
LT1512IS8
S8 PART
MARKING
1512
1512I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 100°C/ W (N)
T
JMAX
= 125°C,
θ
JA
= 130°C/ W (S)
NOTE: CONTACT FACTORY CONCERNING 16-LEAD
FUSED-LEAD GN PACKAGE WITH LOWER THERMAL
RESISTANCE
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, I
FB
= 0V, V
SW
and S/S pins open, unless otherwise noted.
SYMBOL
V
REF
PARAMETER
V
FB
Reference Voltage
FB Input Current
FB Reference Voltage Line Regulation
V
IREF
I
FB
Reference Voltage
I
FB
Input Current
I
FB
Reference Voltage Line Regulation
g
m
Error Amplifier Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
Error Amplifier Clamp Voltage
A
V
f
Error Amplifier Voltage Gain
V
C
Pin Threshold
Switching Frequency
Duty Cycle = 0%
2.7V
≤
V
IN
≤
25V
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 0°C (LT1512I)
q
q
CONDITIONS
Measured at FB Pin
V
C
= 0.8V
V
FB
= V
REF
q
q
MIN
1.233
1.228
TYP
1.245
1.245
300
MAX
1.257
1.262
550
600
0.03
– 93
– 90
35
0.05
1900
2300
350
2400
2.30
0.52
1.25
550
580
580
260
UNITS
V
V
nA
nA
%/V
mV
mV
µA
%/V
µmho
µmho
µA
µA
V
V
V/ V
V
kHz
kHz
kHz
%
ns
V
V
2.7V
≤
V
IN
≤
25V, V
C
= 0.8V
Measured at I
FB
Pin
V
FB
= 0V, V
C
= 0.8V
V
IFB
= V
IREF
(Note 2)
2.7V
≤
V
IN
≤
25V, V
C
= 0.8V
∆I
C
=
±25µA
q
q
q
q
q
0.01
– 107
–110
10
1100
700
120
1.70
0.25
0.8
450
430
400
88
40
35
– 100
– 100
25
0.01
1500
200
1400
1.95
0.40
500
1
500
500
95
130
47
V
FB
= V
REF
– 150mV, V
C
= 1.5V
V
FB
= V
REF
+ 150mV, V
C
= 1.5V
High Clamp, V
FB
= 1V
Low Clamp, V
FB
= 1.5V
q
q
Maximum Switch Duty Cycle
Switch Current Limit Blanking Time
BV
Output Switch Breakdown Voltage
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
< 20°C (LT1512I)
q
2
U
W
U
U
W W
W
LT1512
ELECTRICAL CHARACTERISTICS
V
IN
= 5V, V
C
= 0.6V, V
FB
= V
REF
, I
FB
= 0V, V
SW
and S/S pins open, unless otherwise noted.
SYMBOL
V
SAT
I
LIM
∆I
IN
∆I
SW
PARAMETER
Output Switch ON Resistance
Switch Current Limit
Supply Current Increase During Switch ON Time
Control Voltage to Switch Current
Transconductance
Minimum Input Voltage
I
Q
Supply Current
Shutdown Supply Current
2.7V
≤
V
IN
≤
25V
2.7V
≤
V
IN
≤
25V, V
S/S
≤
0.6V
0°C
≤
T
J
≤
125°C
– 40°C
≤
T
J
≤
0°C (LT1512I)
2.7V
≤
V
IN
≤
25V
0V
≤
V
S/S
≤
5V
q
q
q
q
q
q
q
CONDITIONS
I
SW
= 2A
Duty Cycle = 50%
Duty Cycle = 80% (Note 1)
q
q
q
MIN
1.5
1.3
TYP
0.5
1.9
1.7
15
2
2.4
4
12
MAX
0.8
2.7
2.5
25
UNITS
Ω
A
A
mA/A
A/V
2.7
5.5
30
50
2
25
15
800
V
mA
µA
µA
V
µs
µA
kHz
Shutdown Threshold
Shutdown Delay
S/S Pin Input Current
Synchronization Frequency Range
0.6
5
– 10
600
1.3
12
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
For duty cycles (DC) between 50% and 85%, minimum
guaranteed switch current is given by I
LIM
= 0.667 (2.75 – DC).
Note 2:
The I
FB
pin is servoed to its regulating state with V
C
= 0.8V.
Note 3:
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.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
vs Switch Current
1.0
SWITCH SATURATION VOLTAGE (V)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
150°C
100°C
SWITCH CURRENT LIMIT (A)
2.0
–55°C
1.5
1.0
0.5
0
INPUT VOLTAGE (V)
–55°C
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
SWITCH CURRENT (A)
1512 G01
U W
25°C
Switch Current Limit
vs Duty Cycle
3.0
2.5
25°C AND
125°C
3.0
2.8
2.6
2.4
2.2
2.0
Minimum Input Voltage
vs Temperature
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1512 G02
1.8
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G03
3
LT1512
TYPICAL PERFORMANCE CHARACTERISTICS
Minimum Peak-to-Peak
Synchronization Voltage vs Temp
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
f
SYNC
= 700kHz
FEEDBACK INPUT CURRENT (nA)
2.5
2.0
1.5
1.0
0.5
0
–50 –25
700
600
500
400
300
200
100
V
FB
= V
REF
NEGATIVE FEEDBACK INPUT CURRENT (µA)
3.0
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G04
PIN FUNCTIONS
V
C
:
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 V
C
voltage varies from 1V
to 1.9V. Current out of the V
C
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 V
C
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).
I
FB
:
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 I
FB
pin regulates at – 100mV. Input resis-
tance 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 synchroni-
zation. It is logic level compatible, but can be tied to V
IN
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.
V
IN
:
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 I
FB
bypass capacitor should
4
U W
Feedback Input Current
vs Temperature
800
0
Negative Feedback Input Current
vs Temperature
–10
–20
–30
–40
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G05
–50
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1512 G06
U
U
U
LT1512
PIN FUNCTIONS
be connected directly to the GND S pin or to the ground
plane close to the point where the GND S pin is connected.
V
SW
:
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.
BLOCK DIAGRAM
S/S
5k
I
FB
62k
FB
1.245V
REF
GND S
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
+
–
–
W
U
U
U
U
V
IN
SHUTDOWN
DELAY AND RESET
500kHz
OSC
LOW DROPOUT
2.3V REG
SW
ANTI-SAT
SYNC
LOGIC
DRIVER
SWITCH
+
I
FBA
–
COMP
+
EA
V
C
A
V
≈
6
IA
0.08Ω
–
GND
1512 F02
Figure 2
5
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