Datasheet

LT3791
14
3791fa
operaTion
Boost Region (V
IN
< V
OUT
)
Switch M1 is always on and synchronous switch M2 is
always off in boost operation. Every cycle switch M3 is
turned on first. Inductor current is sensed when synchro-
nous switch M3 is turned on. After the sensed inductor
current exceeds the reference voltage which is proportional
to V
C
, switch M3 turns off and synchronous switch M4
is turned on for the remainder of the cycle. Switches M3
and M4 alternate, behaving like a typical synchronous
boost regulator.
The duty cycle of switch M3 decreases until the minimum
duty cycle of the converter in boost operation reaches
D
MIN(BOOST,BG2)
, given by:
D
MIN(BOOST,BG2)
= D
(BUCK-BOOST)
where D
(BUCK-BOOST)
is the duty cycle of the buck-boost
switch range:
D
(BUCK-BOOST)
= 8%
Figure 6 shows typical boost operation waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
M1 + M3
M1 + M3
M1 + M4 M1 + M4
M1 + M3
M1 + M4
3791 F06
Figure 6. Boost Operation (V
IN
< V
OUT
)
Low Current Operation
The LT3791 runs in forced continuous mode. In this mode
the controller behaves as a continuous, PWM current
mode synchronous switching regulator. In boost opera-
tion, switch M1 is always on, switchM3 and synchronous
switch M4 are alternately turned on to maintain the output
voltage independent of the direction of inductor current.
In buck operation, synchronous switch M4 is always on,
switch M1 and synchronous switch M2 are alternately
turned on to maintain the output voltage independent of
the direction of inductor current. In the forced continuous
mode, the output can source or sink current.