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LT1511

Constant-Current/Constant-Voltage 3A Battery Charger with Input Current Limiting

Linear Technology 

LT1511

TEST CIRCUITS

VC

0.047µF

1k

+

≈ 0.65V

Test Circuit 1

LT1511

CA2

CA1

60k

1k

VREF

PROG

SPIN

RS3

SENSE 200Ω

RS2

BAT 200Ω

+

LT1006

–

1µF

300Ω

RPROG

20k

Test Circuit 2

LT1511

OVP

VA

IPROG

PROG

10k

VREF

10k

–

0.47µF RPROG

LT1013

+

+

2.465V

1511 • TC02

RS1

10Ω

VBAT

1511 • TC01

U

OPERATION

The LT1511 is a current mode PWM step-down (buck)

switcher. The battery DC charging current is programmed

by a resistor RPROG (or a DAC output current) at the PROG

pin (see Block Diagram). Amplifier CA1 converts the

charging current through RS1 to a much lower current

IPROG fed into the PROG pin. Amplifier CA2 compares the

output of CA1 with the programmed current and drives the

PWM loop to force them to be equal. High DC accuracy is

achieved with averaging capacitor CPROG. Note that IPROG

has both AC and DC components. IPROG goes through R1

and generates a ramp signal that is fed to the PWM control

comparator C1 through buffer B1 and level shift resistors

R2 and R3, forming the current mode inner loop. The

Boost pin drives the switch NPN QSW into saturation and

reduces power loss. For batteries like lithium-ion that

require both constant-current and constant-voltage charg-

ing, the 0.5%, 2.465V reference and the amplifier VA

reduce the charging current when battery voltage reaches

the preset level. For NiMH and NiCd, VA can be used for

overvoltage protection. When input voltage is not present,

the charger goes into low current (3µA typically) sleep

mode as input drops down to 0.7V below battery voltage.

To shut down the charger, simply pull the VC pin low with

a transistor.

8

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