Datasheet

LT8705
24
8705fb
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 per-
centage 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,BOOS T)
≅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).