LT3681
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 V
IN
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 V
OUT
). 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.
V
C
(Pin 5):
The V
C
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.
V
IN
(Pin 13):
The V
IN
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 V
IN
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.
U
U
U
3681f
7
LINER [ LINEAR TECHNOLOGY ]
