LT8705
APPLICATIONS INFORMATION
In order to provide adequate load current at low V volt-
In the boost region, if V
can be greater than twice V ,
IN
OUT IN
as follows:
ages in the boost region, L should be at least:
calculate L
(MIN2,BOOST)
L(MIN1,BOOST)
≅
L(MIN2,BOOST)
=
DC
(MAX,M3,BOOST)
V
IN(MIN) •VOUT(MAX)
V
•
IN(MIN)
V
–
•R
SENSE
OUT(MAX)
100%
V
– V
IN(MIN)
OUT(MAX)
H
VRSENSE(MAX,BOOST,MAX) IOUT(MAX) •VOUT(MAX)
0.08•f
2•f•
–
RSENSE
V
IN(MIN)
In the buck region, if V can be greater than twice V
,
IN
OUT
calculate L
as follows:
where:
DC
(MIN1,BUCK)
L
=
is the maximum duty cycle per-
(MAX,M3,BOOST)
(MIN1,BUCK)
centage of the M3 switch (see R
Maximum Current section).
Selection and
SENSE
V
OUT(MAX)
V
• 1–
•R
SENSE
IN(MAX)
V
– V
OUT(MIN)
f is the switching frequency
IN(MAX)
H
0.08 • f
V
isthemaximumcurrentsense
RSENSE(MAX,BOOST,MAX)
voltage in the boost region at maximum duty cycle (see
Selection and Maximum Current section)
R
Inductor Selection: Maximum Current Rating
SENSE
Negative values of L
indicate that the output
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:
(MIN1,BOOST)
can’t be delivered in the boost region
load current I
OUT
becausetheinductorcurrentlimitistoolow.IfL
(MIN1,BOOST)
is too large or is negative, consider reducing the R
SENSE
resistor value to increase the inductor current limit.
V
OUT(MAX)
I
≅ I
•
L(MAX,BOOST) OUT(MAX)
V
Inductor Selection: Subharmonic Oscillations
IN(MIN)
The LT8705’s internal slope compensation circuits will
prevent subharmonic oscillations that can otherwise oc-
DC
(MAX,M3,BOOST
V
•
IN(MIN)
100%
cur when V /V
is less than 0.5 or greater than 2. The
IN OUT
+
A
slopecompensationcircuitswillpreventtheseoscillations
provided that the inductance exceeds a minimum value
(seetheearliersectionInductorCurrentSensingandSlope
Compensation for more information). Choose an induc-
2 •L • f
where DC
percentage of the M3 switch (see R
Maximum Current section).
is the maximum duty cycle
(MAX,M3,BOOST)
tancegreaterthanalloftherelevantL
limitsdiscussed
(MIN)
Selection and
SENSE
below. Negative results can be interpreted as zero.
8705p
24
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