LTC3780
OPERATION
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I
L
3780 F03
the remainder 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.
CLOCK
SWITCH A
SWITCH B
3780 F04a
0V
HIGH
Figure 3. Buck Mode (V
IN
> V
OUT
)
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I
L
HIGH
0V
SWITCH C
SWITCH D
I
L
3780 F05
(4a) Buck-Boost Mode (V
IN
≥ V
OUT
)
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I
L
3780 F04b
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.
(4b) Buck-Boost Mode (V
IN
≤ V
OUT
)
Figure 4. Buck-Boost Mode
FCB PIN
0V to 0.75V
0.85V to 5V
>5.3V
BUCK MODE
Force Continuous Mode
Skip-Cycle Mode
DCM with Constant Freq
BOOST MODE
Force Continuous Mode
Burst Mode Operation
DCM with Constant Freq
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 synchronous 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
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
recharge. In buck mode, synchronous switch D is always
3780fe
13
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
