LT8708
28
Rev 0
For more information
inductor current in the buck region. For example, refer to
the Maximum Inductor Current Sense Voltage vs Duty Cycle
graph in the Typical Performance Characteristics section.
The graph shows that, with V
C
at its maximum voltage, the
maximum peak inductor sense voltage V
RSENSE
is between
47mV and 93mV depending on the duty cycle. It also shows
that the maximum inductor valley current in the buck region
is 82mV increasing to ~130mV at higher duty cycles.
R
SENSE
SELECTION AND MAXIMUM CURRENT
The R
SENSE
resistance must be chosen properly to
achieve the desired amount of output current (forward
conduction) and input current (reverse conduction). Too
much resistance can limit the input/output current below
the application requirements. Start by determining the
maximum allowed R
SENSE
resistances in the forward
and reverse boost regions (R
SENSE(MAX,BOOST,FWD)
and R
SENSE(MAX,BOOST,RVS)
). Follow this by finding the
maximum allowed R
SENSE
resistances in the forward
and reverse buck regions (R
SENSE(MAX,BUCK,FWD)
and
R
SENSE(MAX,BUCK,RVS)
). The selected R
SENSE
resistance
must be less than all four values.
R
SENSE
Selection: Max R
SENSE
in the Boost Region
Forward Conduction: In this section R
SENSE(MAX,BOOST,FWD)
is calculated which is the maximum allowed R
SENSE
re-
sistance when operating in the boost region with forward
conduction (V
IN
to V
OUT
). Skip this section and assume
R
SENSE(MAX,BOOST,FWD)
= ∞ when this operating condition
does not apply to the application.
In the boost region, the maximum positive V
OUT
current
capability is the lowest when V
IN
is at its minimum and
V
OUT
is at its maximum. Therefore, R
SENSE
must be cho-
sen to meet the output current requirements under these
conditions.
Start by finding the maximum boost region duty cycle which
occurs when V
IN
is minimum and V
OUT
is maximum using:
DC
(MAX,M3,BOOST)
≅
1–
V
IN(MIN,BOOST)
V
OUT(MAX,BOOST)
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
•100%
For example, an application with a V
IN
range of 12V to
48V and V
OUT
set to 36V will have:
DC
(MAX,M3,BOOST)
≅
1–
12V
36V
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•100% = 67%
Referring to the Maximum Inductor Current Sense Voltage
graph in the Typical Performance Characteristics section,
the maximum R
SENSE
voltage at 67% duty cycle is 68mV, or:
V
RSENSE(MAX,BOOST,MAXDC)
≅
68mV
for V
IN
= 12V, V
OUT
= 36V.
Next, the inductor ripple current in the boost region must
be determined. If the main inductor L is not known, the
maximum ripple current ∆I
L(MAX,BOOST)
can be estimated
by choosing ∆I
L(MAX,BOOST)
to be 30% to 50% of the maxi-
mum peak inductor current in the boost region as follows:
∆I
L(MAX,BOOST)
V
OUT(MAX,BOOST)
•I
OUT(MAX,FWD)
V
IN(MIN,BOOST)
•
100%
%Ripple
– 0.5
A
≅
where:
I
OUT(MAX,FWD)
is the maximum V
OUT
load current
required in the boost region.
%Ripple is 30% to 50%
For example, using V
OUT(MAX)
= 36V, V
IN(MIN)
= 12V,
I
OUT(MAX,FWD)
= 2A and %Ripple = 40% we can calculate:
I
L(MAX,BOOST)
36V •2A
12V • 100%
40%
– 0.5
= 3A
≅
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