LT1769IGN datasheet(8/16 Pages) LINER

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LT1769

TEST CIRCUITS

Test Circuit 1

VREF

≈ 0.65V

VBAT

CA2

–

CA1

300

Ω

20k

RS1

100

Ω

BAT

SENSE

SPIN

1769 TC01

PROG

RPROG

0.047

µF

LT1769

µF

60k

LT1006

RS2

200

Ω

RS3

200

Ω

Test Circuit 2

VREF

2.465V

10k

OVP

1769 TC02

IPROG

RPROG

LT1769

PROG

LT1013

0.47

µF

–

OPERATION

The LT1769 is a current mode PWM step-down (buck)

switcher. The battery DC charge current is programmed

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

pin (see Block Diagram). Amplifier CA1 converts the

charge 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

control 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 con-

stant-voltage charging, the 0.5%, 2.465V reference and

the amplifier VA reduce the charge current when battery

voltage reaches the preset level. For NiMH and NiCd, VA

can be used for overvoltage protection. When the input

voltage is removed, the VCC pin drops to 0.7V below the

battery voltage, forcing the charger into a low battery drain

µA typical) sleep mode. To shut down the charger,

simply pull the VC pin low with a transistor.

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