Datasheet

LT8705
22
8705fb
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Otherwise, if the inductor value is already known then
∆I
L(MAX,BOOST)
can be more accurately calculated as
follows:
∆I
L (MAX,BOOST)
=
DC
(MAX,M3,BOOST)
100%




• V
IN(MIN)
f•L
A
where:
DC
(MAX,M3,BOOST)
is the maximum duty cycle percent-
age in the boost region as calculated previously.
f is th
e 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 can be calculated as
follows:
R
SENSE(MAX,BOOST)
=
2• V
RSENSE(MAX,BOOST,MAX)
• V
IN(MIN)
2•I
OUT(MAX,BOOST)
• V
OUT(MIN)
(
)
+ ∆I
L(MAX,BOOST)
• V
IN(MIN)
(
)
Ω
where V
RSENSE(MAX,BOOST,MAX)
is the maximum inductor
current sense voltage as discussed in the previous section.
Using values from the previous examples:
R
SENSE(MAX,BOOST)
=
2•93mV •12
2•2A •36V
(
)
+ 3A•12V
(
)
=12.4mΩ
Buck Region: In the buck region, the maximum output cur-
rent capability is the least when operating at the minimum
du
ty cycle. This is because the slope compensation ramp
increases the maximum R
SENSE
voltage with increasing
duty cycle. The minimum duty cycle for buck operation
can be calculated using:
DC
(MIN,M2,BUCK)
≅ t
ON(M2,MIN)
• f • 100%
where t
ON(M2,MIN)
is 260ns (typical value, see Electrical
Characteristics)
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 inductor current in the buck
region as follows:
∆I
L(MIN,BUCK)
≅
I
OUT(MAX,BUCK)
100%
10%
–0.5




A
where:
I
OUT(MAX,BUCK)
is the maximum output load current
required in the buck region.
If the inductor value is already known then ∆I
L(MIN,BUCK)
can be calculated as follows:
∆I
L MIN,BUCK
(
)
=
DC
(MIN,M2,BUCK)
100%




• V
OUT(MIN)
f•L
A
where:
DC
(MIN,M2,BUCK)
is the minimum duty cycle percentage
in the buck region as calculated previously.
f 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 can be calculated as
follows:
R
SENSE(MAX,BUCK)
=
2• 86mV
2•I
OUT(MAX,BUCK)
(
)
–∆I
L(MIN,BUCK)
applicaTions inForMaTion