Datasheet

LT8705
16
8705fb
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operaTionoperaTion
charging up to 2.5V and be held there in the case of a fault
event that persists. After the fault condition had ended and
SS is greater than 1.6V, SS will then slowly discharge to
50mV (post fault delay state). This timeout period relieves
the part and other downstream power components from
electrical and thermal stress for a minimum amount of
time as set by the voltage ramp rate on the SS pin. After
SS has discharged to < 50mV, the LT8705 will enter the
soft-start state and restart switching activity.
Power Switch Control
Figure 3 shows a simplified diagram of how the four
power switches are connected to the inductor, V
IN
, V
OUT
and ground. Figure 4 shows the regions of operation for
the LT8705 as a function of V
OUT
-V
IN
or switch duty cycle
DC. The power switches are properly controlled so the
transfer between modes is continuous.
is turned on first. Inductor current is sensed by amplifier
A5 while switch M2 is on. A slope compensation ramp is
added to the sensed voltage which is then compared by A8
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.
TG1
BG1
TG2
BG2
R
SENSE
8705 F03
M1
M2
M4
M3
L
SW1 SW2
V
IN
V
OUT
M1 ON, M2 OFF
PWM M3, M4 SWITCHES
M4 ON, M3 OFF
PWM M1, M2 SWITCHES
4-SWITCH PWM
V
OUT
-V
IN
SWITCH
M3 DC
MAX
SWITCH
M2 DC
MAX
SWITCH
M3 DC
MIN
SWITCH
M2 DC
MIN
BOOST REGION
BUCK REGION
0
BUCK/BOOST REGION
8705 F04
Figure 3. Simplified Diagram of the Output Switches
Figure 4. Operating Regions vs V
OUT
-V
IN
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
OFF
ON
8705 F05
Figure 5. Buck Region (V
IN
>> V
OUT
)
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 buck mode 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)
• f • 100%
where:
t
ON(M2,MIN)
is the minimum on-time for the synchronous
switch in buck operation (260ns typical, see Electrical
Characteristics).
f is the switching frequency
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
245ns (typical, see Electrical Characteristics) to maintain
steady-state operation, avoid duty cycle jitter, increased
output ripple and reduction in maximum output current.
Power 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 switch M3 is always
off. Also, switch M4 is always on unless reverse current is
detected while in Burst Mode operation or discontinuous
mode. At the start of every cycle, synchronous switch M2