LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 12

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

LTC3789
operaTion
POWER SWITCH CONTROL
Figure 1 shows a simplified diagram of how the four
power switches are connected to the inductor, V
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
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 syn-
chronous switch B is turned on. After the sensed voltage
falls below a reference voltage, which is proportional to
V
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
given by:
12
ITH
D
, synchronous switch B is turned off and switch A is
MAX _ BUCK
Figure 1. Simplified Diagram of the Output Switches
BOOST
OOST
BUCK
BUCK
D
D
D
D
Figure 2. Operating Region vs Duty Cycle
90%
MAX
MAX
MIN
MIN
TG1
BG1
IN
A ON, B OFF
PWM C, D SWITCHES
FOUR SWITCH PWM
D ON, C OFF
PWM A, B SWITCHES
= 1−
>> V
V



IN
A
B
SW1
OUT
12
1
)

 • 100% = 91.67%

L
R
3789 F01
SENSE
SW2
BUCK/BOOST REGION
BOOST REGION
V
D
C
BUCK REGION
OUT
TG2
BG2
3789 F02
MAX_BUCK
IN
, V
OUT
,
Figure 3 shows typical buck region waveforms. If V
approaches V
SWITCH A
SWITCH B
SWITCH C
SWITCH D
Buck-Boost Region (V
When V
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
switch A is turned on. At 120° clock phase, switch C is
turned on. The LTC3789 starts to operate as a boost until
I
of the clock period. If the controller starts with switches
A and C on, the controller first operates as a boost, until
I
turned on, making it operate as a buck. Then, I
turning switch B off and switch A on for the remainder of
the clock period.
Boost Region (V
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 current exceeds
what the reference voltage demands, which is proportional
to V
D is turned on for the remainder of the cycle. Switches C
and D will alternate, behaving like a typical synchronous
boost regulator.
CMP
CMP
CLOCK
ITH
trips. Then, switch D is turned on for the remainder
trips and switch D is turned on. At 120°, switch B is
I
L
, switch C is turned off and synchronous switch
IN
is close to V
Figure 3. Buck Region (V
OUT
IN
, the buck-boost region is reached.
<< V
CMP
IN
OUT
OUT
trips, switch B is turned off, and
≈ V
)
, the controller enters buck-
OUT
)
IN
>> V
OUT
)
CMP
trips,
3780 F03
HIGH
3789f
LOW
IN