LT8708
19
Rev 0
Switch Control: Buck Region (V
IN
>> V
OUT
)
When V
IN
is significantly higher than V
OUT
, the part will run
in the buck region. In this region M3 is always off and switch
M4 is always on. At the start of every cycle, synchronous
switch M2 is turned on first. Inductor current is sensed by
amplifier A4 while switch M2 is on. A slope compensation
ramp is added to the sensed voltage which is then compared
by A5 to a reference that is proportional to V
C
. After the
sensed inductor current falls below the reference, 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. Figure 5 shows
the switching waveforms in the buck region.
When V
IN
is much higher than V
OUT
, the duty cycle of
switch M2 will increase, causing the M2 switch off-time
to decrease. The M2 switch off-time should be kept above
230ns (typical, see Electrical Characteristics) to maintain
steady-state operation and avoid duty cycle jitter, increased
output ripple and reduction in maximum output current.
Switch Control: Buck-Boost (V
IN
≅
V
OUT
)
When V
IN
is close to V
OUT
, the controller operates in the
buck-boost region. Figure 6 shows typical waveforms
in this region. Every cycle, if the controller starts with
switches M2 and M4 turned on, the controller first oper-
ates as if in the buck region. When A5 trips, switch M2
is turned off and M1 is turned on until the middle of the
clock cycle. Next, switch M4 turns off and M3 turns on.
The LT8708 then operates as if in boost mode until A2
trips. Finally, switch M3 turns off and M4 turns on until
the end of the cycle.
If the controller starts with switches M1 and M3 turned
on, the controller first operates as if in the boost region.
When A2 trips, switch M3 is turned off and M4 is turned
on until the middle of the clock cycle. Next, switch M1
Figure 5. Buck Region (V
IN
>> V
OUT
)
OPERATION
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
OFF
ON
8708 F05
The part will continue operating in the buck region over a
range of switch M2 duty cycles. The duty cycle of switch
M2 in the buck region is given by:
DC
(M2,BUCK)
=
1–
V
OUT
V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
•100%
As V
IN
and V
OUT
get closer to each other, the duty cycle
decreases until the minimum duty cycle of the converter,
in the buck region, reaches DC
(ABSMIN,M2,BUCK)
. If the duty
cycle becomes lower than DC
(ABSMIN,M2,BUCK)
the part will
move to the buck-boost region.
DC
(ABSMIN,M2,BUCK)
≅
t
ON(M2,MIN)
• ƒ •100%
where:
t
ON(M2,MIN)
is the minimum on-time for the synchro-
nous switch in buck operation (200ns typical, see
Electrical Characteristics).
ƒ is the switching frequency.
Figure 6. Buck-Boost Region
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
8708 F06a
6(a) Buck-Boost Region (V
IN
≥ V
OUT
)
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
8708 F06b
6(a) Buck-Boost Region (V
IN
≤ V
OUT
)
