Datasheet

LT3686
7
3686fc
V
IN
(Pin 1): The V
IN
pin supplies current to the LT3686’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
BD (Pin 2): The BD pin is used to provide current to
the internal Boost Schottky diode. Tie this pin to output
whenever possible. When the MODE pin is greater than
0.8V, the LT3686 will prevent pulse-skipping at light
loads by regulating an active load on the BD pin; see the
Applications Information section Fixed Frequency at Light
Load.
FB (Pin 3): The LT3686 regulates its feedback pin to 0.8V.
Connect the feedback resistor divider tap to this pin. Set
the output voltage according to V
OUT
= 0.8(1 + R1/R2). A
good value for R2 is 10k.
SS (Pin 4): Provides Soft-Start and Tracking. An internal
2µA current source tied to a 2.5V reference supplies cur-
rent to this pin to charge an external capacitor to create a
voltage ramp at the pin. Feedback voltage and switching
frequency both track SS voltage. Feedback voltage stops
tracking at 0.8V. SS is reset under all fault conditions. Float
the pin if soft-start feature is not being used.
RT (Pin 5): The RT pin is used to program the oscillator
frequency. Select the value of R
T
resistor according to
Table 1 in the applications section of the data sheet.
EN/UVLO (Pin 6): The EN/UVLO pin is used to start up the
LT3686. Pull the pin below 0.4V to shutdown the LT3686. The
1.27V threshold can function as an accurate undervoltage
lockout (UVLO), preventing the regulator from operating
until the input voltage has reached the programmed level.
Do not drive the EN/UVLO pin above V
IN
.
MODE (Pin 7): The MODE pin acts as mode select for the
BD active load; when it is tied high, the LT3686 will prevent
pulse skipping at light loads by regulating an active load on
the BD pin. To disable the active load, tie MODE to GND.
BOOST (Pin 8): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
DA (Pin 9): Connect catch diode (D1) anode to this pin.
SW (Pin 10): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
GND (Exposed Pad Pin 11): The exposed pad GND pin is
the only ground connection for the device. The exposed
pad should be soldered to a large copper area to reduce
thermal resistance.
pin Functions