3681 datasheet(7/24 Pages) LINER

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LT3681

3681f

PI FU CTIO S

PG (Pin 1): The PG pin is the open collector output of an

internal comparator. PG remains low until the FB pin is

within 10% of the ﬁnal regulation voltage. PG output is

valid when VIN is above 3.5V and RUN/SS is high.

BIAS (Pin 2): The BIAS pin supplies the current to the

LT3681’s internal regulator. Tie this pin to the lowest

available voltage source above 3V (typically VOUT). This

architecture increases efﬁciency especially when the input

voltage is much higher than the output.

FB (Pin 3): The LT3681 regulates the FB pin to 1.265V.

Connect the feedback resistor divider tap to this pin.

VC (Pin 5): The VC pin is the output of the internal error

ampliﬁer. The voltage on this pin controls the peak switch

current. Tie an RC network from this pin to ground to

compensate the control loop.

RT (Pin 6): Oscillator Resistor Input. Connecting a resistor

to ground from this pin sets the switching frequency.

DA (Pin 8): This is the anode of the integrated power

Schottky diode. High frequency, large amplitude currents

ﬂow through this pin, so tie it to ground through a low

impedance connection.

DC (Pin 9, Exposed Pad 16): These pins connect to the

cathode of the integrated power Schottky diode. High fre-

quency, large amplitude currents ﬂow through these pins,

so tie them to SW through a low impedance connection.

BD (Pin 10): This pin connects to the anode of the internal

boost Schottky diode.

BOOST (Pin 11): This pin is used to provide a drive

voltage, higher than the input voltage, to the internal

bipolar NPN power switch. Connect a capacitor between

this pin and SW.

SW (Pin 12): The SW pin is the output of the internal

power switch. Connect this pin to the inductor, DC and

boost capacitor.

VIN (Pin 13): The VIN pin supplies current to the LT3681’s

internal regulator and to the internal power switch. This

pin must be locally bypassed.

RUN/SS (Pin 14): The RUN/SS pin is used to put the

LT3681 in shutdown mode. Tie to ground to shut down

the LT3681. Tie to 2.3V or more for normal operation. If

the shutdown feature is not used, tie this pin to the VIN

pin. RUN/SS also provides a soft-start function; see the

Applications Information section.

GND (Pins 4, 7, Exposed Pad 15): All three of these

terminals internally connect to the LT3681 control IC’s

signal return, while exposed pad 15 performs the added

function of providing a low thermal resistance heat ﬂow

path between the IC and the system heatsink. Tie all of

these terminals to a copper pour on the top layer of the

printed circuit board. Please refer to the Applications

Information section for more details.

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