LTC3534
10
3534fb
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shifted voltage from the output of the error amplifier (V
C
pin), see Figure 3. The four power switches are properly
phased so the transfer between operating modes is con
-
tinuous, smooth and transparent to the user. When V
IN
approaches V
OUT
the buck-boost region is entered, where
the conduction time of the four switch region is typically
125ns. 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 period. Switches A and B will alternate
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
= (125ns • f) • 100%
where f = operating frequency in Hz, typically 1MHz.
Hence, D4
SW
= 12.5% for the LTC3534.
D
MAX_BUCK
= 87.5%
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
V
CI
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− 125ns• f
( )
V
The V
IN
potential at which the four switch region ends is
given by:
V
IN
= V
OUT
• (1 – D) = V
OUT
• (1 – 125ns • ƒ) V
where f = operating frequency in Hz, typically 1MHz.
Hence, for the LTC3534,
V
IN(ENTER4SW)
≅
OUT
V
Approximate V
IN
potential at which the four switch
region is entered.
V
IN(4SWEXIT)
@ 0.875 • V
OUT
V
Approximate V
IN
potential at which the four switch
region is exited.
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 85% typical and is reached
when V
CI
is above V4.
Burst Mode OPERATION
Burst Mode operation reduces the LTC3534’s quiescent
current consumption at light loads and improves overall
conversion efficiency, increasing battery life. During Burst
Mode operation the LTC3534 delivers energy to the output
until it is regulated and then enters a sleep state where
the switches are off and the quiescent current drops to
25µA typical. 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
operation 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
feedforward capacitor across the upper resistor in the
V
OUT
feedback divider network (as in Type III compensa-
tion), see Figure 6.
operaTion
