Datasheet

LT3791-1
13
37911f
operaTion
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.
Boost Region (V
IN
< V
OUT
)
Switch M1 is always on and synchronous switch M2 is al-
ways off in boost operation. Every cycle switch M3 is turned
on first. Inductor current is sensed when synchronous 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.
Low Current Operation
The LT3791-1 is recommended to run in forced continuous
conduction mode at heavy load by pulling the CCM pin
higher than 1.5V. In this mode the controller behaves as
a continuous, PWM current mode synchronous switching
regulator. In boost operation, 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 syn-
chronous 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
sour
ce or sink current.
However, reverse inductor current from the output to the
input is not desired for certain applications. For these ap-
plications, the CCM pin must be connected to C/10 (pin 4)
with a pull-up resistor to INTV
CC
(see front page Typical
Application). Therefore, the CCM pin will be pulled lower
than 0.3V for discontinuous conduction mode by the C/10
pin when the output current is low. In this mode, switch
M4 turns off when the inductor current flows negative.
Figure 3. Buck Operation (V
IN
> V
OUT
)
M2 + M4 M2 + M4 M2 + M4
M1 + M4
37911 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
37911 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
37911 F05
M1 + M3
M1 + M3
M1 + M4 M1 + M4
M1 + M3
M1 + M4
37911 F06
Figure 6. Boost Operation (V
IN
< V
OUT
)