Linear Technology LT1512 Datasheet

FEATURES

LT1512

SEPIC Constant-Current/

Constant-Voltage

Battery Charger

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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

■

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

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APPLICATIONS

■

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

The LT®1512 is a 500kHz current mode switching regula­tor 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 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.

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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

TYPICAL APPLICATION

LT1512

V

L1 A*

•

V

SW

I

FB

C

C5

0.1µF

R5
1k

WALL

ADAPTER

INPUT

CHARGE

SHUTDOWN

C3

+

22µF
25V

SYNC

AND/OR

SHUTDOWN

V

IN

S/S

GND GND S

*

L1 A, L1 B ARE TWO 33µH WINDINGS ON A
SINGLE INDUCTOR: COILTRONICS CTX33-3



**

TOKIN CERAMIC 1E225ZY5U-C203-F

Figure 1. SEPIC Charger with 0.5A Output Current

FB

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C4

0.22µF

C2**

2.2µF

L1 B*

R4

24Ω

D1

MBRS130LT3

R1

•

R2

R3

0.2Ω

0.5A

+

1512 F01

Maximum Charging Current

1.0
SINGLE

LITHIUM 

0.8

CELL (4.1V)

0.6

0.4

C1
22µF
25V

CURRENT (A)

0.2

0

0

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.

6V BATTERY

12V BATTERY

5

10

INPUT VOLTAGE (V)

DOUBLE
LITHIUM 
CELL (8.2V)

INDUCTOR = 33µH

15

20

1512 TA02

25

1

LT1512

WU

U

PACKAGE

/

O

RDER I FOR ATIO

1
2
3
4

8
7
6
5



TOP VIEW

V

C



FB

I

FB



S/S

V

SW


GND
GND S
V

IN

N8 PACKAGE
8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

A

W

O

LUTEXI T

S

A

WUW

ARB

U
G

I

S

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 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

LECTRICAL C CHARA TERIST

E

VIN = 5V, VC = 0.6V, VFB = V

, IFB = 0V, VSW and S/S pins open, unless otherwise noted.

REF

ICS

T

= 125°C, θ

JMAX

T

= 125°C, θ

JMAX

NOTE: CONTACT FACTORY CONCERNING 16-LEAD
FUSED-LEAD GN PACKAGE WITH LOWER THERMAL
RESISTANCE

Consult factory for Military grade parts.

= 100°C/ W (N)

JA

= 130°C/ W (S)

JA

ORDER PART

NUMBER

LT1512CN8
LT1512CS8
LT1512IN8
LT1512IS8

S8 PART

MARKING

1512
1512I

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

REF

V

IREF

g

m

A

V

f Switching Frequency 2.7V ≤ VIN ≤ 25V 450 500 550 kHz

BV Output Switch Breakdown Voltage 0°C ≤ TJ ≤ 125°C ● 40 47 V

2

VFB Reference Voltage Measured at FB Pin 1.233 1.245 1.257 V

= 0.8V ● 1.228 1.245 1.262 V

V

C

FB Input Current VFB = V

FB Reference Voltage Line Regulation 2.7V ≤ VIN ≤ 25V, VC = 0.8V ● 0.01 0.03 %/V
IFB Reference Voltage Measured at IFB Pin –107 –100 – 93 mV

IFB Input Current V
IFB Reference Voltage Line Regulation 2.7V ≤ VIN ≤ 25V, VC = 0.8V ● 0.01 0.05 %/V
Error Amplifier Transconductance ∆IC = ±25µA 1100 1500 1900 µmho

Error Amplifier Source Current VFB = V
Error Amplifier Sink Current VFB = V
Error Amplifier Clamp Voltage High Clamp, VFB = 1V 1.70 1.95 2.30 V

Error Amplifier Voltage Gain 500 V/V
VC Pin Threshold Duty Cycle = 0% 0.8 1 1.25 V

Maximum Switch Duty Cycle ● 88 95 %
Switch Current Limit Blanking Time 130 260 ns

REF

= 0V, VC = 0.8V ● –110 –100 – 90 mV

V

FB

= V

IFB

Low Clamp, V

–40°C ≤ TJ < 20°C (LT1512I) 35 V

(Note 2) ● 10 25 35 µA

IREF

– 150mV, VC = 1.5V ● 120 200 350 µA

REF

+ 150mV, VC = 1.5V ● 1400 2400 µA

REF

= 1.5V 0.25 0.40 0.52 V

FB

≤ 125°C ● 430 500 580 kHz

0°C ≤ T

J

–40°C ≤ T

< 0°C (LT1512I) 400 580 kHz

J

● 600 nA

● 700 2300 µmho

300 550 nA

LT1512

TEMPERATURE (°C)

–50

1.8

INPUT VOLTAGE (V)

2.0

2.2

2.4

2.6

050

100

150

1512 G03

2.8

3.0

–25 25

75

125

LECTRICAL C CHARA TERIST

E

VIN = 5V, VC = 0.6V, VFB = V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V
I

∆I
∆I

I

SAT

LIM

IN

SW

Q

Output Switch ON Resistance ISW = 2A ● 0.5 0.8 Ω
Switch Current Limit Duty Cycle = 50% ● 1.5 1.9 2.7 A

Supply Current Increase During Switch ON Time 15 25 mA/A

Control Voltage to Switch Current 2A/V
Transconductance

Minimum Input Voltage ● 2.4 2.7 V
Supply Current 2.7V ≤ VIN ≤ 25V ● 4 5.5 mA
Shutdown Supply Current 2.7V ≤ VIN ≤ 25V, V

Shutdown Threshold 2.7V ≤ VIN ≤ 25V ● 0.6 1.3 2 V
Shutdown Delay ● 51225µs
S/S Pin Input Current 0V ≤ V
Synchronization Frequency Range ● 600 800 kHz

, IFB = 0V, VSW and S/S pins open, unless otherwise noted.

REF

ICS

Duty Cycle = 80% (Note 1)

≤ 125°C ● 12 30 µA

0°C ≤ T

J

–40°C ≤ T

J

≤ 5V ● –10 15 µA

S/S

● 1.3 1.7 2.5 A

≤ 0.6V

S/S

≤ 0°C (LT1512I) 50 µA

The ● 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

Note 2: The I
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

TYPICAL PERFORMANCE CHARACTERISTICS

SWITCH SATURATION VOLTAGE (V)

pin is servoed to its regulating state with VC = 0.8V.

FB

Switch Saturation Voltage
vs Switch Current

1.0

0.9

0.8

0.7

0.4

0.3

0.2

0.1

0.6

0.5

0

0.4

0.2

0

0.6

SWITCH CURRENT (A)

0.8

100°C

1.0

1.2

150°C

–55°C

1.4

= 0.667 (2.75 – DC).

LIM

W

U

25°C

1.6

2.0

1.8

1512 G01

Switch Current Limit
vs Duty Cycle

3.0

2.5

2.0

1.5

1.0

SWITCH CURRENT LIMIT (A)

0.5

0

20 40 60 80

DUTY CYCLE (%)

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.

Minimum Input Voltage
vs Temperature

25°C AND
125°C

–55°C

10010030 50 70 90

1512 G02

3

LT1512

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Minimum Peak-to-Peak
Synchronization Voltage vs Temp

)

3.0
f

0

–50

= 700kHz

SYNC

050

–25 25

TEMPERATURE (°C)

75

100

P-P

2.5

2.0

1.5

1.0

0.5

MINIMUM SYNCHRONIZATION VOLTAGE (V

125

150

1512 G04

Feedback Input Current
vs Temperature

800

VFB = V

–25

REF

0

25

TEMPERATURE (°C)

700

600

500

400

300

200

FEEDBACK INPUT CURRENT (nA)

100

0

–50

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PIN FUNCTIONS

V

: The compensation pin is primarily used for frequency

C

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 200µ A 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

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 100µ A 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

directly to

Negative Feedback Input Current
vs Temperature

0

–10

–20

–30

–40

NEGATIVE FEEDBACK INPUT CURRENT (µA)

75

50

100

125

150

1512 G05

–50

–50

–25 25

0

50

TEMPERATURE (°C)

75

100

125

150

1512 G06

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, 0.22µ F 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

4