4

LT1512

Minimum Peak-to-Peak

Synchronization Voltage vs Temp

Feedback Input Current

vs Temperature

Negative Feedback Input Current

vs Temperature

TEMPERATURE (

°C)

–50

0

0.5

1.0

1.5

2.0

050

100

150

1512 G04

2.5

3.0

–25

25

75

125

TEMPERATURE (

°C)

–50

400

500

600

150

1512 G05

300

200

0

0

50

100

100

800

700

–25

25

75

125

TEMPERATURE (

°C)

–50

–50

–30

0

0

50

75

1512 G06

–40

–10

–20

–25

25

100 125

150

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

pin is externally clamped below the internal 1.9V clamp

level. Loop frequency compensation is performed with a

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 100

µA 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).

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

tance of this pin is 5k

Ω, so filter resistance (R4, Figure 1)

should be less than 50

Ω. 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-

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.

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