Datasheet

LTC3530
8
3530fb
Depending on the control voltage, the IC will operate in
either buck, buck/boost or boost mode. The V
CI
voltage
is a level shifted voltage from the output of the error amp
(V
C
). The four power switches are properly phased so the
transfer between operating modes is continuous, smooth
and transparent to the user. When V
IN
approaches V
OUT
the buck/boost region is reached where the conduction
time of the four switch region is typically 150ns. Referring
to Figures 1 and 2, the various regions of operation will
now be described.
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
=
V
OUT
1– (150ns • 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 of a
synchronous boost regulator. The maximum duty cycle
of the converter is limited to 90% typical and is reached
when V
CI
is above V4.
BURST MODE OPERATION
Burst mode reduces the LTC3530’s quiescent current
consumption at light loads and improves overall conver-
sion effi ciency, increasing battery life. During Burst Mode
operation the LTC3530 delivers energy to the output until
it is regulated and then goes into sleep mode where the
outputs are off and quiescent current drops to 40μA (typ).
In this mode the output ripple has a variable frequency
component that depends upon load current, and will
typically be about 2% peak-to-peak. Burst Mode opera-
tion ripple can be reduced slightly by using more output
capacitance (47μF or greater). Another method of reducing
Burst Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the V
OUT
feedback
divider network (as in Type III compensation).
During the period where the device is delivering energy to
the output, the peak switch current will be equal to 450mA
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
MAX(BURST)BUCK

450mA
2
;V
OUT
< V
IN
I
MAX(BURST)BOOST

450mA
2
•
V
IN
V
OUT






;V
OUT
> V
IN
I
MAX(BURST)
Buck-Boost ≈ 350mA; V
OUT
≈ V
IN
.
Since the input and output are connected together for
most of the cycle.
OPERATION