LTC3780_12 Datasheet, PDF (13/28 Pages) Linear Technology – High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller

LTC3780
OPERATION
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I_L
Figure 3. Buck Mode (V_IN > V_OUT)
0V
HIGH
3780 F03
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I_L
(4a) Buck-Boost Mode (V_IN ≥ V_OUT)
3780 F04a
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I_L
3780 F04b
(4b) Buck-Boost Mode (V_IN ≤ V_OUT)
Figure 4. Buck-Boost Mode
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
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
SWITCH C
HIGH
0V
SWITCH D
I_L
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
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
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
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