LTC3441
8
3441fc
For more information www.linear.com/LTC3441
operaTion
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, output A begins to switch. 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 volt
-
age 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
=
OUT
1– (150ns • f)
V
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.
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 25µA. 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
≈
IN
V
+ V
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 main
-
tain 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 ≈
η
LOA
25µA +I
where (ηbm) is typically 75% during Burst Mode
operation
.
