LT8708
29
Rev 0
Otherwise, if the inductance is already known then
∆I
L(MAX,BOOST,FWD)
can be more accurately calculated as
follows:
∆I
L(MAX,BOOST)
=
DC
(MAX,M3,BOOST)
100%
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
• V
IN(MIN,BOOST)
ƒ •L
A
where:
DC
(MAX,M3,BOOST)
is the maximum duty cycle percent-
age in the boost region as calculated previously
ƒ 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 while in forward con-
duction (V
IN
to V
OUT
) can be calculated as follows:
R
SENSE(MAX,BOOST,FWD)
=
2 • V
RSENSE(MAX,BOOST,MAXDC)
• V
IN(MIN,BOOST)
2 •I
OUT(MAX,FWD)
• V
OUT(MAX,BOOST)
(
)
+ ∆I
L(MAX,BOOST)
• V
IN(MIN,BOOST)
(
)
Ω
where:
V
RSENSE(MAX,BOOST,MAXDC)
is the maximum inductor
current sense voltage as discussed in the previous
section.
Using values from the previous examples:
R
SENSE(MAX,BOOST,FWD)
=
2 • 68mV •12V
2 • 2A • 36V
(
)
+ 3A • 12V
(
)
= 9.1mΩ
Reverse Conduction: In this section R
SENSE(MAX,BOOST,RVS)
is calculated which is the maximum allowed R
SENSE
re-
sistance when operating in the boost region with reverse
conduction (V
OUT
to V
IN
). Skip this section and assume
R
SENSE(MAX,BOOST,RVS)
= ∞ when this operating condition
does not apply to the application.
In the boost region, the maximum reverse V
IN
current
capability is the lowest when operating at the minimum
duty cycle. See Switch Control: Boost Region (V
) section for the equation to calculate the minimum
duty cycle DC
(ABSMIN, M3, BOOST)
.
Before calculating the maximum R
SENSE
resistance allowed
during reverse operation, however, the inductor ripple
current must be determined. If the main inductor L is not
known, the ripple current ∆I
L(MIN,BOOST)
can be estimated
by choosing ∆I
L(MIN,BOOST)
to be 10% of the minimum
peak inductor current in the boost region as follows:
∆I
L(MIN,BOOST)
I
IN(MAX,RVS)
100%
10%
– 0.5
A
≅
where:
I
IN(MAX,RVS)
is the maximum V
IN
load current required
in the boost region in the reverse direction
If the inductance is already known then ∆I
L(MIN,BOOST)
can
be calculated as follows:
∆I
L MIN,BOOST
(
)
=
DC
(ABSMIN,M3,BOOST)
100%
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
• V
IN(MIN,BOOST)
ƒ •L
A
where:
DC
(ABSMIN,M3,BOOST)
is the minimum duty cycle
percentage in the boost region (see Switch Control:
) section)
ƒ is the switching frequency
L is the inductance of the main inductor
Now that the inductor ripple current is known, the maxi-
mum allowed R
SENSE
in the boost region while in reverse
conduction can be calculated as follows:
R
SENSE(MAX,BOOST,RVS)
=
2•| V
RSENSE(MIN,BOOST,MINDC)
|
2 •I
IN(MAX,RVS)
(
)
– ∆I
L(MIN,BOOST)
Ω
APPLICATIONS INFORMATION
