Datasheet

LT3686A
7
3686afa
V
IN
(Pin 1/2): The V
IN
pin supplies current to the LT3686A’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
BD (Pin 2/3): When the SYNC/MODE Pin is driven with
clock pulses or tied greater than 0.8V, the LT3686A will
prevent pulse-skipping at light loads by regulating an
active load on the BD pin; see Applications Information
section Fixed Frequency at Light Load.
FB (Pin 3/4): The LT3686A regulates its feedback pin to
0.8V. Connect the feedback resistor divider tap to this pin.
Set the output voltage according to:
R1= R2
V
OUT
0.8V
– 1






A good value for R2 is 10k.
SS (Pin 4/5): 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 UVLO, OVLO and thermal
shutdown conditions. Float the pin if soft-start feature is
not being used.
RT (Pin 5/6): 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.
pin Functions
(DFN/MSE)
EN/UVLO (Pin 6/7): The EN/UVLO pin is used to start
up the LT3686A. Pull the pin below 0.4V to shutdown the
LT3686A. The 1.28V 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
.
SYNC/MODE (Pin 7/8): The SYNC/MODE pin is used
to synchronize the internal oscillator of the LT3686A to
an external signal. The SYNC signal can be driven by a
signal with pulse width of at least 200ns on and off time.
The SYNC/MODE Pin also acts as mode select for the BD
active load; when it is driven with pulses or tied above
0.8V, the LT3686A will prevent pulse skipping at light loads
by regulating an active load on the BD pin. To disable the
active load, tie SYNC/MODE to below 0.4V.
BOOST (Pin 8/9): The BOOST pin is used to provide a
drive voltage, higher than the input voltage, to the internal
bipolar NPN power switch.
DA (Pin 9/10): Connect catch diode (D1) anode to this
pin.
SW (Pin 10/11, 12): 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/Pin 1, Exposed Pad Pin 13):
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.