LTC3444_15 Datasheet, PDF (11/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

U
OPERATIO
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.
LTC3444
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_IN
=
1–
V_OUT
(125ns
•
f)
V
The point at which the four switch region ends is given by:
V_IN = V_OUT(1–D) = V_OUT(1–125ns • f) V
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