Datasheet

LT3686A
16
3686afa
ApplicAtions inForMAtion
BAT54) in parallel with the internal diode to reduce V
D
. The
following equations can be used to calculate and minimize
boost capacitance in μF:
C
BOOST
=
0.065
(V
BD
+ V
CATCH
V
D
2.2) f
V
D
is the forward drop of the boost diode, V
CATCH
is the
forward drop of the catch diode (D1), and frequency is in
MHz. A typical value of 0.22µF can be used for C
BOOST
.
For lower output voltages the BD pin can be tied to an
external voltage source with adequate local bypassing
(Figure 8b). The above equations still apply for calculating
the optimal boost capacitor for the chosen BD voltage.
The absence of BD voltage during startup will increase
minimum voltage to start and reduce efciency. You must
also be sure that the maximum voltage rating of BOOST
pin is not exceeded. The BD pin can also be tied to V
IN
(Figure 8c) but V
IN
will be limited to 25V and the active
load circuit is automatically disabled.
The minimum operating voltage of an LT3686A applica-
tion is limited by the undervoltage lockout (3.6V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
Figure 8.
SW
DA
V
IN
V
IN
BD
V
OUT
GND
V
BOOST
– V
SW
V
OUT
MAX V
BOOST
V
IN
+ V
OUT
8a
8b
LT3686A
BOOST
SW
DA
V
IN
V
IN
BD
V
OUT
GND
V
BOOST
– V
SW
V
DD
MAX V
BOOST
V
IN
+ V
DD
LT3686A
BOOST
V
DD
8c
SW
DA
V
IN
V
IN
BD
V
OUT
GND
V
BOOST
– V
SW
V
IN
MAX V
BOOST
2V
IN
LT3686A
3686A F08
BOOST