Datasheet

LTC3780
14
Rev G
For more information www.analog.com
OPERATION
on, switches B and D are then turned on. Finally, switches
A and D are turned on for the remainder of the time.
Boost Region (V
IN
< V
OUT
)
Switch A is always on and synchronous switch B is always
off in boost mode. Every cycle, switch C is turned on first.
Inductor current is sensed when switch C is turned on.
After the sensed inductor current exceeds the reference
voltage which is proportional to V
ITH
, switch C is turned off
and synchronous switch D is turned on for the remainder
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
0V
HIGH
3780 F03
Figure 3. Buck Mode (V
IN
> V
OUT
)
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
3780 F04a
Figure 4a. Buck-Boost Mode (V
IN
≥ V
OUT
)
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
3780 F04b
Figure 4b. Buck-Boost Mode (V
IN
≤ V
OUT
)
Figure 4. Buck-Boost Mode
of the cycle. switches C and D will alternate, behaving like
a typical synchronous boost regulator.
The duty cycle of switch C decreases until the minimum duty
cycle of the converter in boost mode reaches D
MIN_BOOST
,
given by:
D
MIN_BOOST
= D
BUCK-BOOST
where D
BUCK-BOOST
is the duty cycle of the buck-boost
switch range:
D
BUCK-BOOST
= (200ns • f) • 100%
and f is the operating frequency in Hz.
Figure 5 shows typical boost mode waveforms. If V
IN
ap-
proaches V
OUT
, the buck-boost region is reached.
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
0V
HIGH
3780 F05
Figure 5. Boost Mode (V
IN
< V
OUT
)
LOW CURRENT OPERATION
The FCB pin is used to select among three modes for both
buck and boost operations by accepting a logic input.
Figure 6 shows the different modes.
FCB PIN BUCK MODE BOOST MODE
0V to 0.75V Force Continuous Mode Force Continuous Mode
0.85V to 5V Skip-Cycle Mode Burst Mode Operation
>5.3V DCM with Constant Freq DCM with Constant Freq
Figure 6. Different Operating Modes
When the FCB pin voltage is lower than 0.8V, the controller
behaves as a continuous, PWM current mode synchronous
switching regulator. In boost mode, switch A is always on.
switch C and synchronous switch D are alternately turned
on to maintain the output voltage independent of direction
of inductor current. Every ten cycles, switch A is forced off
for about 300ns to allow boost capacitor C
A
(Figure 13) to
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.