LT8708
30
Rev 0
For more information
where:
V
RSENSE(MIN,BOOST,MINDC)
is the minimum inductor cur-
rent sense voltage in the boost region at the minimum
duty cycle. Typical value is –93mV.
Negative result from the above equation indicates that any
R
SENSE
value can meet the requirement. Substitute the
calculated result with ∞ and move onto the next section.
R
SENSE
Selection: Max R
SENSE
in the Buck Region
Forward Conduction: In this section R
SENSE(MAX,BUCK,FWD)
is calculated which is the maximum allowed R
SENSE
re-
sistance when operating in the buck region with forward
conduction (V
IN
to V
OUT
).
In the buck region, the maximum V
OUT
current capability
is the lowest when operating at the minimum duty cycle.
See Switch Control: Boost Region (V
for the equation to calculate the minimum duty cycle
DC
(ABSMIN, M2,BUCK)
.
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 peak inductor current in the
buck region as follows:
∆I
L(MIN,BUCK)
I
OUT(MAX,FWD)
100%
10%
– 0.5
A
≅
where:
I
OUT(MAX,FWD)
is the maximum V
OUT
load current
required in the buck region in the forward direction.
If the inductance is already known then ∆I
L(MIN,BUCK)
can
be calculated as follows:
∆I
L MIN,BUCK
(
)
=
DC
(ABSMIN,M2,BUCK)
100%
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• V
OUT(MIN,BUCK)
ƒ •L
A
where:
DC
(ABSMIN,M2,BUCK)
is the minimum duty cycle per-
centage in the buck region as calculated previously
ƒ 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 while in forward con-
duction can be calculated as follows:
R
SENSE(MAX,BUCK,FWD)
=
2 • V
RSENSE(MAX,BUCK,MINDC)
2 •I
OUT(MAX,FWD)
(
)
– ∆I
L(MIN,BUCK)
Ω
where:
V
RSENSE(MAX,BUCK,MINDC)
is the maximum inductor
current sense voltage at the minimum duty cycle.
Typical value is 82mV.
Negative result from the above equation indicates that any
R
SENSE
value can meet the requirement. Substitute the
calculated result with ∞ and move onto the next section.
Reverse Conduction: In this section R
SENSE(MAX, BUCK,
RVS)
is calculated which is the maximum allowed R
SENSE
resistance when operating in the buck region with reverse
conduction (V
OUT
to V
IN
). Skip this section and assume
R
SENSE(MAX, BUCK, RVS)
= ∞ when this operating condition
does not apply to the application.
In the buck region, the maximum reverse V
IN
current ca-
pability is the least when V
IN
is at its maximum and V
OUT
is at its minimum for buck operation. Therefore R
SENSE
must be chosen to meet the V
IN
current requirements
under these conditions.
Start by finding the buck region duty cycle when V
IN
is
minimum and V
OUT
is maximum using:
DC
(MAX,M2,BUCK)
≅
1–
V
OUT(MIN,BUCK)
V
IN(MAX,BUCK)
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⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
•100%
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