LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 13

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

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
Switch A is always on and synchronous switch B is always
off in boost mode. Every cycle, switch C is turned on fi rst.
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
is turned off and synchronous switch D is turned on for
SWITCH A
SWITCH B
SWITCH C
SWITCH D
SWITCH D
SWITCH A
SWITCH B
SWITCH C
SWITCH D
SWITCH A
SWITCH B
SWITCH C
CLOCK
CLOCK
CLOCK
I
L
I
I
L
L
(4a) Buck-Boost Mode (V
(4b) Buck-Boost Mode (V
Figure 3. Buck Mode (V
IN
Figure 4. Buck-Boost Mode
< V
OUT
)
IN
IN
IN
> V
≥ V
≤ V
OUT
OUT
OUT
)
)
)
ITH
, switch C
3780 F04b
3780 F03
3780 F04a
HIGH
0V
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
given by:
where D
switch range:
and f is the operating frequency in Hz.
Figure 5 shows typical boost mode waveforms. If V
proaches V
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.
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
recharge. In buck mode, synchronous switch D is always
SWITCH D
SWITCH A
SWITCH B
SWITCH C
0V to 0.75V
0.85V to 5V
CLOCK
D
D
FCB PIN
MIN_BOOST
BUCK-BOOST
>5.3V
I
L
BUCK-BOOST
OUT
Figure 6. Different Operating Modes
Figure 5. Boost Mode (V
, the buck-boost region is reached.
DCM with Constant Freq
= D
Force Continuous Mode
= (200ns • f) • 100%
Skip-Cycle Mode
BUCK-BOOST
BUCK MODE
is the duty cycle of the buck-boost
IN
DCM with Constant Freq
Force Continuous Mode
< V
Burst Mode Operation
LTC3780
OUT
BOOST MODE
A
)
(Figure 13) to
MIN_BOOST
13
IN
3780fe
3780 F05
HIGH
ap-
0V
,