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LT1513/LT1513-2

sn1513 1513fas

PIN FUNCTIONS

VC (Pin 1): The compensation pin is primarily used for

frequency 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 3.6A as the VC voltage

varies from 1V to 1.9V. Current out of the VC pin is about

200

µA 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 directly to the ground pin (avoid ground loops).

FB (Pin 2): The feedback pin is used for positive output

voltage sensing. 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. FB is the

inverting input to the voltage error amplifier. 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 LT1513 ground pin (avoid ground loops).

IFB (Pin 3): 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 a programmed limit. During constant-current

operation, the LT1513 IFB pin regulates at – 100mV. Input

resistance of this pin is 5k

Ω, so filter resistance (R4,

Figure 1) should be less than 50

Ω. The 39Ω, 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. The LT1513-2 IFB pin regulates at 0mV to provide

programmable current limit. The current through R5,

Figure 5, is balanced by the current through R4, program-

ming the maximum voltage across R3.

GND (Pin 4): The ground pin is common to both control

circuitry and switch current. VC, FB and S/S signals must

be Kelvin and connected as close as possible to this pin.

The TAB of the R package should also be connected to the

power ground.

VSW (Pin 5): The switch pin is the collector of the power

switch, carrying up to 3A of current with fast rise and fall

times. Keep the traces on this pin as short as possible to

minimize radiation and voltage spikes. In particular, the

path in Figure 1 which includes SW to C2, D1, C1 and

around to the LT1513 ground pin should be as short as

possible to minimize voltage spikes at switch turn-off.

S/S (Pin 6): 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 switch-

ing frequency to the external signal. Shutdown is avoided

in this mode with an internal timer.

VIN (Pin 7): 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 TAB is connected.

TAB: The TAB on the surface mount R package is electri-

cally connected to the ground pin, but a low inductance

connection must be made to both the TAB and the pin for

proper circuit operation. See suggested PC layout in

Figure 4.

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