Datasheet

LTC3789
13
3789fa
Figure 3. Buck Region (V
IN
>> V
OUT
)
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%
Figure 3 shows typical buck region waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
operaTion
Figure 1. Simplified Diagram of the Output Switches
Figure 2. Operating Region vs Duty Cycle
TG1
BG1
TG2
BG2
R
SENSE
3789 F01
A
B
D
C
L
SW1 SW2
V
IN
V
OUT
A ON, B OFF
PWM C, D SWITCHES
D ON, C OFF
PWM A, B SWITCHES
FOUR SWITCH PWM
90%
D
MAX
OOST
D
MIN
BUCK
D
MIN
BOOST
D
MAX
BUCK
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
3789 F02
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
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.