Datasheet

LT3791
13
3791fa
operaTion
Buck Region (V
IN
> V
OUT
)
Switch M4 is always on and switch M3 is always off during
this mode. At the start of every cycle, synchronous switch
M2 is turned on first. Inductor current is sensed when
synchronous switch M2 is turned on. After the sensed
inductor current falls below the reference voltage, which
is proportional to V
C
, synchronous switch M2 is turned off
and switch M1 is turned on for the remainder of the cycle.
Switches M1 and M2 will alternate, behaving like a typical
synchronous buck regulator. The duty cycle of switch M1
increases until the maximum duty cycle of the converter
in buck operation reaches D
MAX(BUCK, TG1)
, given by:
D
MAX(BUCK,TG1)
= 100% – D
(BUCK-BOOST)
where D
(BUCK-BOOST)
is the duty cycle of the buck-boost
switch range:
D
(BUCK-BOOST)
= 8%
Figure 3 shows typical buck operation waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
Buck-Boost Region (V
IN
~ V
OUT
)
When V
IN
is close to V
OUT
, the controller is in buck-boost
operation. Figure 4 and Figure 5 show typical waveforms in
this operation. Every cycle the controller turns on switches
M2 and M4, then M1
and M4 are turned on until 180° later
when switches M1 and M3 turn on, and then switches
M1 and M4 are turned on for the remainder of the cycle.
Figure 3. Buck Operation (V
IN
> V
OUT
)
M2 + M4 M2 + M4 M2 + M4
M1 + M4
3791 F03
M1 + M4M1 + M4
Figure 4. Buck-Boost Operation (V
IN
≤ V
OUT
)
Figure 5. Buck-Boost Operation (V
IN
≥ V
OUT
)
M2 + M4 M2 + M4 M2 + M4
M1 + M4M1 + M4M1 + M4
M1 + M4 M1 + M4 M1 + M4
M1+ M3 M1+ M3 M1+ M3
3791 F04
M2 + M4 M2 + M4 M2 + M4
M1 + M4 M1 + M4 M1 + M4
M1 + M4 M1 + M4 M1 + M4
M1 + M3 M1 + M3 M1 + M3
3791 F05