LTC3443 Datasheet, PDF (9/16 Pages) Linear Technology – High Current Micropower 600kHz Synchronous Buck-Boost DC/DC Converter

U
OPERATIO
similar to a typical synchronous buck regulator. As the
control voltage increases, the duty cycle of switch A
increases until the maximum duty cycle of the converter in
Buck mode reaches D_MAX__BUCK, given by:
D_MAX__BUCK = 100 – D4_SW %
where D4_SW = duty cycle % of the four switch range.
D4_SW = (150ns • 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,
switch pair AD remain on for duty cycle D_{MAX_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
(150ns •
V
f)
The point at which the four switch region ends is given by:
V_IN = V_OUT(1 – D) = V_OUT(1 – 150ns • f) V
Boost Region (V_IN < V_OUT)
Switch A is always on and switch B is always off during
this mode. When the internal control voltage, V_CI, is above
voltage V3, switch pair CD will alternately switch to
provide a boosted output voltage. This operation is typical
to a synchronous boost regulator. The maximum duty
cycle of the converter is limited to 88% typical and is
reached when V_CI is above V4.
LTC3443
Burst Mode OPERATION
Burst Mode operation is when the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
28µA. In this mode the output ripple has a variable
frequency component that depends upon load current.
During the period where the device is delivering energy to
the output, the peak current will be equal to 800mA typical
and the inductor current will terminate at zero current for
each cycle. In this mode the typical maximum average
output current is given by:
I_{OUT(MAX)BURST}
≈
0.2 • V_IN
V_OUT + V_IN
A
Burst Mode operation is user controlled, by driving the
MODE/SYNC pin high to enable and low to disable.
The peak efficiency during Burst Mode operation is less
than the peak efficiency during fixed frequency because
the part enters full-time 4-switch mode (when servicing
the output) with discontinuous inductor current as illus-
trated in Figures 3 and 4. During Burst Mode operation, the
control loop is nonlinear and cannot utilize the control
voltage from the error amp to determine the control mode,
therefore full-time 4-switch mode is required to maintain
the Buck/Boost function. The efficiency below 1mA
becomes dominated primarily by the quiescent current
and not the peak efficiency. The equation is given by:
Efficiency Burst ≈ ^{(ηbm) • ILOAD}
25µA + I_LOAD
where (ηbm) is typically 80% during Burst Mode
operation.
sn3443 3443fs
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