LTC3789_15 Datasheet, PDF (13/30 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
Operation
Power switch Control
Figure 1 shows a simplified diagram of how the four
power switches are connected to the inductor, V_IN, V_OUT
and GND. Figure 2 shows the regions of operation for
the LTC3789 as a function of duty cycle, D. The power
switches are properly controlled so the transfer between
regions is continuous.
Buck Region (V_IN >> V_OUT)
Switch D is always on and switch C is always off in this
region. At the start of every cycle, synchronous switch
B is turned on first. Inductor current is sensed when
synchronous switch B is turned on. After the sensed
inductor valley current falls below a reference voltage,
which is proportional to V_ITH, synchronous switch B is
turned off and switch A is turned on for the remainder of
the cycle. Switches A and B will alternate, behaving like
a typical synchronous buck regulator. The duty cycle of
switch A increases until the maximum duty cycle of the
converter reaches D_{MAX_BUCK}, given by:
D_{MAX _ BUCK}
=
_1−
1^
12 
• 100%
=
91.67%
V_IN
V_OUT
TG1
A
D
TG2
SW1 ^L SW2
BG1
B
C
BG2
R_SENSE
3789 F01
Figure 1. Simplified Diagram of the Output Switches
90%
D_MAX
OOST
D_MIN
BOOST
D_MAX
BUCK
D_MIN
BUCK
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
FOUR SWITCH PWM BUCK/BOOST REGION
D ON, C OFF
PWM A, B SWITCHES
BUCK REGION
3789 F02
Figure 3 shows typical buck region waveforms. If V_IN
approaches V_OUT, the buck-boost region is reached.
CLOCK
SWITCH A
SWITCH B
SWITCH C
SWITCH D
I_L
Figure 3. Buck Region (V_IN >> V_OUT)
LOW
HIGH
3780 F03
Buck-Boost Region (V_IN ≈ V_OUT)
When V_IN is close to V_OUT, the controller enters buck-
boost region. Figure 4 shows the typical waveforms in this
region. At the beginning of a clock cycle, if the controller
starts with B and D on, the controller first operates as a
buck region. When I_CMP trips, switch B is turned off, and
switch A is turned on. At 120° clock phase, switch C is
turned on. The LTC3789 starts to operate as a boost until
I_CMP trips. Then, switch D is turned on for the remainder
of the clock period. If the controller starts with switches
A and C on, the controller first operates as a boost, until
I_CMP trips and switch D is turned on. At 120°, switch B is
turned on, making it operate as a buck. Then, I_CMP trips,
turning switch B off and switch A on for the remainder of
the clock period.
Boost Region (V_IN << V_OUT)
Switch A is always on and synchronous switch B is always
off in the boost region. In every cycle, switch C is turned
on first. Inductor current is sensed when synchronous
switch C is turned on. After the sensed inductor peak
current exceeds what the reference voltage demands,
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.
Figure 2. Operating Region vs Duty Cycle
For more information www.linear.com/LTC3789
3789fc
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