LT8708
33
Rev 0
To provide adequate reverse I
IN
current at low V
OUT
volt-
ages in the buck region, L should be at least:
L
(MIN1,BUCK)
V
OUT(MIN,BUCK)
•
DC
(MAX,M2,BUCK)
100%
2• ƒ •
| V
RSENSE(MIN,BUCK,MAXDC)
|
R
SENSE
–
I
IN(MAX,RVS)
• V
IN(MAX,BUCK)
V
OUT(MIN,BUCK)
Ω
≅
where:
DC
(MAX,M2,BUCK)
is the maximum duty cycle percentage
of the M2 switch (see R
in the Buck Region section)
ƒ is the switching frequency
V
RSENSE(MIN,BUCK,MAXDC)
is the minimum current
sense voltage in the buck region at maximum duty
I
IN(MAX,RVS)
is the maximum reverse V
IN
current in
buck region
Negative values of L
(MIN1,BOOST)
or L
(MIN1,BUCK)
indicate
that the load current can’t be delivered because the induc-
tor current limit is too low. If L
(MIN1,BOOST)
or L
(MIN1,BUCK)
is too large or is negative, consider reducing the R
SENSE
resistor value to increase the inductor current limit.
L Selection: Subharmonic Oscillations
The LT8708’s internal slope compensation circuits will
prevent subharmonic oscillations that can otherwise
occur when V
IN
/V
OUT
is less than 0.5 or greater than 2.
The slope compensation circuits will prevent these oscil-
lations provided that the inductance exceeds a minimum
value (see the earlier section Inductor Current Sensing
and Slope Compensation for more information). Choose
an inductance greater than all of the relevant L
(MIN)
lim-
its discussed below. Negative calculation results can be
interpreted as zero.
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,BOOST)
–
V
IN(MIN,BOOST)
• V
OUT(MAX,BOOST)
V
OUT(MAX,BOOST)
– V
IN(MIN,BOOST)
•R
SENSE
0.08 • f
H
In the buck region, if V
IN
can be greater than twice V
OUT
,
calculate L
(MIN2,BUCK)
as follows:
L
(MIN2,BUCK)
=
V
IN(MAX,BUCK)
• 1–
V
OUT(MIN,BUCK)
V
IN(MAX,BUCK)
– V
OUT(MIN,BUCK)
•R
SENSE
0.08 • ƒ
H
L Selection: Maximum Current Rating
The inductor must have a rating greater than its maximum
operating current to prevent inductor saturation resulting
in efficiency loss. The maximum forward inductor current
in the boost region is:
I
L(MAX,BOOST,FWD)
≅
I
OUT(MAX,FWD)
•
V
OUT(MAX,BOOST)
V
IN(MIN,BOOST)
+
V
IN(MIN,BOOST)
•
DC
(MAX,M3,BOOST)
100%
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
2 •L • ƒ
⎛
⎝
⎜
⎜
⎜
⎜
⎜
⎜
⎜⎜
⎞
⎠
⎟
⎟
⎟
⎟
⎟
⎟
⎟⎟
A
where:
DC
(MAX,M3,BOOST)
is the maximum duty cycle per-
centage of the M3 switch (see R
APPLICATIONS INFORMATION
