11
LTC3444
3444fb
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during this
mode. When the internal control voltage, V
CI
, is above
voltage V1, Switch A is on. During the off time of switch A,
synchronous switch B turns on for the remainder of the
time. Switches A and B will alternate similar to a typical
synchronous buck regulator. As the control voltage in-
creases, the duty cycle of switch A increases until the
maximum duty cycle of the converter in buck mode
reaches DMAX_BUCK, given by:
DMAX_BUCK = 100% – D4
SW
where D4
SW
= duty cycle % of the four switch range.
D4
SW
= (125ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or Buck-Boost region
is reached.
Buck-Boost or Four Switch (V
IN
~ V
OUT
)
When the internal control voltage, V
CI
, is above voltage V2,
but below V3, switch pair AD remain on for duty cycle
DMAX_BUCK, and the switch pair AC begins to phase in.
As switch pair AC phases in, switch pair BD phases out
accordingly. When the V
CI
voltage reaches the edge of the
buck-boost range, at voltage V3, the AC switch pair
completely phase out the BD pair, and the boost phase
begins at duty cycle D4
SW
. The input voltage, V
IN
, where
the four switch region begins is given by:
V
V
ns f
V
IN
OUT
=
•1 125–( )
The point at which the four switch region ends is given by:
V
IN
= V
OUT
(1–D) = V
OUT
(1–125ns • f) V
OPERATIO
U
