LT8708
20
Rev 0
For more information
turns off and M2 turns on. The LT8708 then operates as
if in buck mode until A5 trips. Finally, switch M2 turns off
and M1 turns on until the end of the cycle.
Switch Control: Boost Region (V
IN
<< V
OUT
)
When V
OUT
is significantly higher than V
IN
, the part oper-
ates in the boost region. In this region switch M1 is always
on and switch M2 is always off. At the start of every cycle,
switch M3 is turned on first. Inductor current is sensed by
amplifier A4 while switch M3 is on. A slope compensation
ramp is added to the sensed voltage which is then compared
(A2) to a reference that is proportional to V
C
. After the
sensed inductor current rises above the reference voltage,
switch M3 is turned off and switch M4 is turned on for the
remainder of the cycle. Switches M3 and M4 will alternate,
behaving like a typical synchronous boost regulator.
The part will continue operating in the boost region over a
range of switch M3 duty cycles. The duty cycle of switch
M3 in the boost region is given by:
DC
(M3,BOOST)
=
1–
V
IN
V
OUT
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
•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 boost region, reaches DC
(ABSMIN,M3,BOOST)
. If the
duty cycle becomes lower than DC
(ABSMIN,M3,BOOST)
, the
part will move to the buck-boost region.
DC
(ABSMIN,M3,BOOST)
≅
t
ON(M3,MIN)
• ƒ • 100%
where:
t
ON(M3,MIN)
is the minimum on-time for the main
switch in boost operation (200ns typical, see Electrical
Characteristics).
ƒ is the switching frequency.
When V
OUT
is much higher than V
IN
, the duty cycle of
switch M3 will increase, causing the M3 switch off-time
to decrease. The M3 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.
Figure 7. Boost Region (V
IN
<< V
OUT
)
Switch Control: Soft-Start
During soft-start, the LT8708 operates in the same three
regions discussed above (buck, buck-boost and boost).
However, a few differences in switch control happen dur-
ing soft-start.
First, M1 and M4 are not turned on simultaneously while
SS ramps up to 0.8V (typical). When M1 and M4 would
normally both be on, they are instead turned off, leaving
all four switches off. After SS rises above 0.8V, during the
time when M1 and M4 would normally both be on, they
are turned on briefly instead. This brief amount of time
increases as SS rises until M1 & M4 are allowed to remain
on as long as the normal switching sequence requires.
Second, M2 and M3 will occasionally turn on together for
one cycle to refresh both boost capacitors. This refresh
cycle happens because M1 and M4 switch more frequently
during soft-start than in normal operation. As such, the
Boost Capacitor Charge Control block (see Figure 1) can-
not always keep the boost capacitors charged. M2 and M3
are turned on when either BOOSTx-SWx voltage drops
below 5V (typical). Note that during the refresh cycle,
the inductor current slope is nearly zero, thus the boost
capacitors can be refreshed without much disturbance to
the ongoing switching operations.
UNI AND BIDIRECTIONAL CONDUCTION
The LT8708 has one bidirectional and three unidirectional
current conduction modes, primarily selected by the MODE
pin. The bidirectional mode (CCM: continuous conduction
mode) allows current and power to flow from V
IN
to V
OUT
,
or vice versa, under control of the V
C
pin. The unidirectional
OPERATION
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
OFF
ON
8708 F07
