8

LT1769

1769fa

TEST CIRCUITS

Test Circuit 1

≈ 0.65V

CA2

–

+

–

+

CA1

+

300

Ω

20k

1k

1k

100

Ω

BAT

SENSE

SPIN

1769 TC01

PROG

0.047

µF

LT1769

1

µF

60k

LT1006

+

200

Ω

200

Ω

Test Circuit 2

2.465V

+

10k

10k

OVP

1769 TC02

LT1769

PROG

LT1013

0.47

µF

–

+

VA

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

switcher. The battery DC charge current is programmed

pin (see Block Diagram). Amplifier CA1 converts the

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

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

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

battery voltage, forcing the charger into a low battery drain

(3

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