LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 12

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
LTC3780
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 in
Buck mode reaches D
where D
range:
and f is the operating frequency in Hz.
Figure 3 shows typical Buck mode waveforms. If V
approaches V
SWITCH D
Buck-Boost (V
When V
mode. Figure 4 shows typical waveforms in this mode.
Every cycle, if the controller starts with Switches B and D
turned on, Switches A and C are then turned on. Finally,
Switches A and D are turned on for the remainder of the
time. If the controller starts with Switches A and C turned
on, Switches B and D are then turned on. Finally, Switches
A and D are turned on for the remainder of the time.
SWITCH A
SWITCH B
SWITCH C
12
CLOCK
D
D
MAX_BUCK
BUCK-BOOST
I
IN
BUCK-BOOST
is close to V
OUT
Figure 3. Buck Mode (V
= (1 – D
IN
= (200ns • f) • 100%
≅ V
, the Buck-Boost region is reached.
U
= duty cycle of the Buck-Boost switch
OUT
MAX_BUCK
OUT
BUCK-BOOST
)
, the controller is in Buck-Boost
, given by:
) • 100%
IN
> V
OUT
)
3780 F03
2.4V
IN
0V
Boost Region (V
Switch A is always on and Synchronous Switch B is always
off in Boost mode. 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 the
reference voltage which is proportional to V
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.
SWITCH D
SWITCH A
SWITCH B
SWITCH C
SWITCH A
SWITCH B
SWITCH C
SWITCH D
CLOCK
CLOCK
I
I
L
L
(4a) Buck-Boost Mode (V
(4b) Buck-Boost Mode (V
Figure 4. Buck-Boost Mode
IN
< V
OUT
)
IN
IN
≥ V
≤ V
OUT
OUT
)
)
ITH
, Switch C
3780 F04b
3780 F04a
3780f