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LT8705MPFE#PBF

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LT8705
16
8705ff
For more information www.linear.com/LT8705
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 switchM2
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
LT8705
17
8705ff
For more information www.linear.com/LT8705
operaTionoperaTion
Power Switch Control: Buck-Boost (V
IN
≅ V
OUT
)
When V
IN
is close to V
OUT
, the controller enters the buck-
boost region. Figure 6 shows typical waveforms in this
region. Every cycle, if the controller starts with switchesM2
and M4 turned on, the controller first operates as if in the
buck region. When A8 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 LT8705 then
operates as if in boost mode until A9 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 A9 trips, switch M3 is turned off and M4 is turned
on until the middle of the clock cycle. Next, switch M1
turns off and M2 turns on. The LT8705 then operates as
if in buck mode until A8 trips. Finally switch M2 turns off
and M1 turns on until the end of the cycle.
Power Switch Control: Boost Region (V
IN
<< V
OUT
)
When V
OUT
is significantly higher than V
IN
, the part will
run 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 A5 while switch M3 is on. A slope compensa
-
tion ramp
is added to the sensed voltage which is then
compared
(A9) 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 switchM3
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 boost mode 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)
• f • 100%
where:
t
ON(M3,MIN)
is the minimum on-time for the main
switch in boost operation (265ns typical, see Electrical
Characteristics)
f is the switching frequency
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
8705 F06a
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
8705 F06b
(6a) Buck-Boost Region (V
IN
≥ V
OUT
)
(6b) Buck-Boost Region (V
IN
≤ V
OUT
)
Figure 6. Buck-Boost Region
Figure 7. Boost Region (V
IN
<< V
OUT
)
SWITCH M1
CLOCK
SWITCH M2
SWITCH M3
SWITCH M4
I
L
OFF
ON
8705 F07
LT8705
18
8705ff
For more information www.linear.com/LT8705
operaTion
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
245ns (typical, see Electrical Characteristics) to maintain
steady-state operation, avoid duty cycle jitter, increased
output ripple and reduction in maximum output current.
Light Load Current Operation (MODE Pin)
Under light current load conditions, the LT8705 can be set
to operate in discontinuous mode, forced continuous mode,
or Burst Mode operation. To select forced continuous mode,
tie the MODE pin to a voltage below 0.4V (i.e., ground). To
select discontinuous mode, tie MODE to a voltage above
2.3V (i.e., LDO33). To select Burst Mode operation, float
the MODE pin or tie it between 1.0V and 1.7V.
Discontinuous Mode: When the LT8705 is in discontinu
-
ous mode, synchronous switch M4 is held off whenever
reverse current in the inductor is detected. This is to prevent
current draw from the output and/or feeding current to the
input supply. Under very light loads, the current compara
-
tor may also remain tripped for several cycles and force
switches
M1 and M3 to stay off for the same number of
cycles
(i.e., skipping pulses). Synchronous switch M2 will
remain on during the skipped cycles, but since switch M4
is off, the inductor current will not reverse.
Burst Mode Operation: Burst Mode operation sets a V
C
level, with about 25mV of hysteresis, below which switch-
ing activity
is inhibited and above which switching activity
is
re-enabled. A typical example is when, at light output
currents, V
OUT
rises and forces the V
C
pin below the thresh-
old that temporarily inhibits switching. After V
OUT
drops
slightly and V
C
rises ~25mV the switching is resumed,
initially in the buck-boost region. Burst Mode operation
can increase efficiency at light load currents by eliminating
unnecessary switching activity and related power losses.
Burst Mode operation handles reverse-current detection
similar to discontinuous mode. The M4 switch is turned
off when reverse current is detected.
Forced Continuous Mode: The forced continuous mode
allows the inductor current to reverse directions without
any switches being forced “off” to prevent this from hap
-
pening. At very light load currents the inductor current will
swing
positive and negative as the appropriate average
current is
delivered
to the output. During soft-start,
when the SS pin is below 1.6V, the part will be forced
into discontinuous mode to prevent pulling current from
the output to the input. After SS rises above 1.6V, forced
continuous mode will be enabled.
Voltage Regulation Loops
The LT8705 provides two constant-voltage regulation
loops, one for output voltage and one for input voltage.
A resistor divider between V
OUT
, FBOUT and GND senses
the output voltage. As with traditional voltage regulators,
when FBOUT rises near or above the reference voltage of
EA4 (1.207V typical, see Block Diagram), the V
C
voltage
is reduced to command the amount of current that keeps
V
OUT
regulated to the desired voltage.
The input voltage can also be sensed by connecting a
resistor divider between V
IN
, FBIN and GND. When the
FBIN voltage falls near or below the reference voltage of
EA3 (1.205V typical, see Block Diagram), the V
C
voltage is
reduced to also reduce the input current. For applications
with a high input source impedance (i.e., a solar panel), the
input voltage regulation loop can prevent the input voltage
from becoming too low under high output load conditions.
For applications with a lower input source impedance (i.e.,
batteries
and voltage supplies), the FBIN pin can be used
to stop switching activity when the input power supply
voltage gets too low for proper system operation. See
the Applications Information section for more information
about setting up the voltage regulation loops.
Current Monitoring and Regulation
The LT8705 provides two constant-current regulation
loops, one for input current and one for output current.
A sensing resistor close to the input capacitor, sensed by
CSPIN and CSNIN, monitors the input current. A current,
linearly proportional to the sense voltage (V
CSPIN
-V
CSNIN
),
is forced out of the IMON_IN pin and into an external
resistor. The resulting voltage V
IMON_IN
is therefore linearly
proportional to the input current. Similarly, a sensing
resistor close to the output capacitor, and sensed by
CSPOUT and CSNOUT will monitor the output current and
generate a voltage V
IMON_OUT
that is linearly proportional
to the output current.
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LT8705MPFE#PBF

Mfr. #:
Buy LT8705MPFE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 80V Vin and Vout Synchronous 4-Switch Buck- Boost DC/DC Controller
Lifecycle:
New from this manufacturer.
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