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LT8705MPFE#PBF

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LT8705
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8705ff
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Otherwise, if the inductor value is already known then
∆I
L(MAX,BOOST)
can be more accurately calculated as
follows:
∆I
L(MAX,BOOST)
=
DC
(MAX,M3,BOOST)
100%
• V
IN(MIN)
f•L
A
where:
DC
(MAX,M3,BOOST)
is the maximum duty cycle percentage
in the boost region as calculated previously.
f is the switching frequency
L is the inductance of the main inductor
After the maximum ripple current is known, the maximum
allowed R
SENSE
in the boost region can be calculated as
follows:
R
SENSE(MAX,BOOST)
=
2• V
RSENSE(MAX,BOOST,MAX)
• V
IN(MIN)
2•I
OUT(MAX,BOOST)
• V
OUT(MIN)
( )
+ ∆I
L(MAX,BOOST)
• V
IN(MIN)
( )
Ω
where V
RSENSE(MAX,BOOST,MAX)
is the maximum inductor
current sense voltage as discussed in the previous section.
Using values from the previous examples:
R
SENSE(MAX,BOOST)
=
2•93mV •12
2•2A •36V
( )
+ 3A•12V
( )
=12.4mΩ
Buck Region: In the buck region, the maximum output cur-
rent capability
is the least when operating at the minimum
duty cycle. This is because the slope compensation ramp
increases the maximum R
SENSE
voltage with increasing
duty cycle. The minimum duty cycle for buck operation
can be calculated using:
DC
(MIN,M2,BUCK)
≅ t
ON(M2,MIN)
• f • 100%
where t
ON(M2,MIN)
is 260ns (typical value, see Electrical
Characteristics)
Before calculating the maximum R
SENSE
resistance,
however, the inductor ripple current must be determined.
If the main inductor L is not known, the ripple current
∆I
L(MIN,BUCK)
can be estimated by choosing ∆I
L(MIN,BUCK)
to be 10% of the maximum inductor current in the buck
region as follows:
∆I
L(MIN,BUCK)
≅
I
OUT(MAX,BUCK)
100%
10%
–0.5
A
where:
I
OUT(MAX,BUCK)
is the maximum output load current
required in the buck region.
If the inductor value is already known then ∆I
L(MIN,BUCK)
can be calculated as follows:
∆I
L MIN,BUCK
( )
=
DC
(MIN,M2,BUCK)
100%
• V
OUT(MIN)
f•L
A
where:
DC
(MIN,M2,BUCK)
is the minimum duty cycle percentage
in the buck region as calculated previously.
f is the switching frequency
L is the inductance of the main inductor
After the inductor ripple current is known, the maximum
allowed R
SENSE
in the buck region can be calculated as
follows:
R
SENSE(MAX,BUCK)
=
2•86mV
2•I
OUT(MAX,BUCK)
( )
–∆I
L(MIN,BUCK)
applicaTions inForMaTion
LT8705
23
8705ff
For more information www.linear.com/LT8705
Final R
SENSE
Value: The final R
SENSE
value should be
lower than both R
SENSE(MAX,BOOST)
and R
SENSE(MAX,BUCK)
.
A margin of 30% or more is recommended.
Figure 8 shows approximately how the maximum output
current and maximum inductor current would vary with
V
IN
/V
OUT
while all other operating parameters remain
constant (frequency = 350
kHz, inductance = 10μH, R
SENSE
=
10mΩ). This graph is normalized and accounts for changes
in maximum current due to the slope compensation ramps
and the effects of changing ripple current. The curve is
theoretical, but can be used as a guide to predict relative
changes in maximum output and inductor current over a
range of V
IN
/V
OUT
voltages.
Reverse Current Limit
When the forced continuous mode is selected (MODE pin
low), inductor current is allowed to reverse directions and
flow from the V
OUT
side to the V
IN
side. This can lead to
current sinking from the output and being forced into the
input. The reverse current is at a maximum magnitude
when V
C
is lowest. The graph of Minimum Inductor
Current Sense
Voltage in FCM in the Typical Performance
Characteristics section can help to determine the maximum
reverse current capability.
Inductor Selection
For
high efficiency, choose an inductor with low core
loss, such as ferrite. Also, the inductor should have low
DC resistance to reduce the I
2
R losses, and must be able
to handle the peak inductor current without saturating. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor.
The operating frequency and inductor selection are
interrelated in that higher operating frequencies allow
the use of smaller inductor and capacitor values. The
following sections discuss several criteria to consider
when choosing an inductor value. For optimal perfor
-
mance, choose an inductor that meets all of the following
criteria.
Inductor Selection: Adequate Load Current in the
Boost Region
Sm
all value inductors result in increased ripple cur
-
rents and thus, due to the limited peak inductor current,
decrease
the maximum average current that can be
provided to the load (I
OUT
) while operating in the boost
region.
applicaTions inForMaTion
Figure 8. Currents vs V
IN
/V
OUT
Ratio
V
IN
/V
OUT
(V/V)
NORMALIZED CURRENT
1.0
0.8
0.6
8705 F08
0
0.4
0.2
10
0.1 1
MAXIMUM
INDUCTOR
CURRENT
MAXIMUM
OUTPUT
CURRENT
LT8705
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8705ff
For more information www.linear.com/LT8705
In order to provide adequate load current at low V
IN
volt-
ages in the boost region, L should be at least:
L
(MIN1,BOOST)
≅
V
IN(MIN)
•
DC
(MAX,M3,BOOST)
100%
2• f•
V
RSENSE(MAX,BOOST,MAX)
R
SENSE
–
I
OUT(MAX)
• V
OUT(MAX)
V
IN(MIN)
where:
DC
(MAX,M3,BOOST)
is the maximum duty cycle percentage
of the M3 switch (see R
SENSE
Selection and Maximum
Current section).
f is the switching frequency
V
RSENSE(MAX,BOOST,MAX)
is the maximum current sense
voltage in the boost region at maximum duty cycle (see
R
SENSE
Selection and Maximum Current section)
Negative values of L
(MIN1,BOOST)
indicate that the output
load current I
OUT
can’t be delivered in the boost region
because the inductor current limit is too low. If L
(MIN1,BOOST)
is too large or is negative, consider reducing the R
SENSE
resistor value to increase the inductor current limit.
Inductor Selection: Subharmonic Oscillations
The LT8705’s internal slope compensation circuits will
prevent subharmonic oscillations that can otherwise oc
-
cur when
V
IN
/V
OUT
is less than 0.5 or greater than 2. The
slope compensation circuits will prevent these oscillations
provided that the inductance exceeds a minimum value
(see the earlier section Inductor Current Sensing and Slope
Compensation for more information). Choose an induc
-
tance greater
than
all of the relevant L
(MIN)
limits discussed
below. Negative results can be interpreted as zero.
applicaTions inForMaTion
In the boost region, if V
OUT
can be greater than twice V
IN
,
calculate L
(MIN2,BOOST)
as follows:
L
(MIN2,BOOST)
=
V
OUT(MAX)
–
V
IN(MIN)
• V
OUT(MAX)
V
OUT(MAX)
– V
IN(MIN)
•R
SENSE
0.08• f
H
In the buck region, if V
IN
can be greater than twice V
OUT
,
calculate L
(MIN1,BUCK)
as follows:
L
(MIN1,BUCK)
=
V
IN(MAX)
• 1–
V
OUT(MAX)
V
IN(MAX)
– V
OUT(MIN)
•R
SENSE
0.08• f
H
Inductor Selection: Maximum Current Rating
The inductor must have a rating greater than its peak
operating current to prevent inductor saturation resulting
in efficiency loss. The peak inductor current in the boost
region is:
I
L(MAX,BOOST)
≅I
OUT(MAX)
•
V
OUT(MAX)
V
IN(MIN)
+
V
IN(MIN)
•
DC
(MAX,M3,BOOST
100%
2•L • f
A
where DC
(MAX,M3,BOOST)
is the maximum duty cycle
percentage of the M3 switch (see R
SENSE
Selection and
Maximum Current section).
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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
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