Datasheet

LT8705
37
8705fb
For more information www.linear.com/LT8705
applicaTions inForMaTion
Now calculate the maximum R
SENSE
values in the boost
and buck regions to be:
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
=
2•107mV •8V
2• 5A •12V
(
)
+ 3.75A •8V
(
)
=11.4mΩ
R
SENSE(MAX,BUCK)
=
2• 86mV
2•I
OUT(MAX,BUCK)
(
)
–∆I
L(MIN,BUCK)
Ω
=
2• 86mV
2• 5A
(
)
–0.53A
=18.2mΩ
Adding an additional 30% margin, choose R
SENSE
to be
11.4mΩ/1.3 = 8.7mΩ.
Inductor Selection: With R
SENSE
known, we can now
determine the minimum inductor value that will provide
adequate load current in the boost region using:
L
(MIN1,BOOST)
≅
V
IN(MIN)
•
DC
(MAX,M3,BOOST)
100%
2• f•
V
RSENSE(MAX,BOOST,MAX)
R
SENSE
–
I
OUT(MAX)
• V
OUT(MAX)
V
IN(MIN)




H
=
8V •
33%
100%







2• 350kHz•
107mV
8.7mΩ
–
5A •12V
8V



=0.8µH
To avoid subharmonic oscillations in the inductor current,
choose the minimum inductance according to:
L
(MIN2,BOOST)
=
V
OU X)
–
V
IN )
• V
OUT(MA
UT(MA
T(MA
X)
V
O X) )
– V
IN(MIN
(MIN













•R
SENSE
.00 8• f
H
=
12V –
•12V8V
12V –8V













•8.7mΩ
0.08•350kHz
= –3.7µH
L
(MIN1,BUCK)
=
V
IN(MAX)
• 1–
V
O
U
T(MAX)
V
IN(MAX
– V
OUT(MIN))





•R
SENSE
0.08• f
=
25V • 1–
12V
25V –12V





•8.7mΩ
0.08•350kHz
=0.6µH
The inductance must be higher than all of the minimum
values calculated above. We will choose a 10μH standard
value inductor for improved margin.
MOSFET Selection: The MOSFETs are selected based on
voltage rating, C
RSS
and R
DS(ON)
value. It is important to
ensure that the part is specified for operation with the
available gate voltage amplitude. In this case, the amplitude
is 6.35V and MOSFETs with an R
DS(ON)
value specified at
V
GS
= 4.5V can be used.
Select M1 and M2: With 25V maximum input voltage,
MOSFETs with a rating of at least 30V are used. As we do
not yet know the actual thermal resistance (circuit board
design and airflow have a major impact) we assume that
the MOSFET thermal resistance from junction to ambient
is 50°C/W.
If we design for a maximum junction temperature, T
J(MAX)
= 125°C, the maximum allowable power dissipation can be
calculated. First, calculate the maximum power dissipation:
P
D(MAX)
=
T
J(MAX)
–T
A(MAX)
R
TH(JA)
P
D(MAX)
=
125°C– 60°C
50°C/W
=1.3W
Since maximum I
2
R power dissipation in the boost region
happens when V
IN
is minimum, we can determine the
maximum allowable R
DS(ON)
for the boost region using:
P
M1
=P
I R
2
≅
V
OUT
V
IN
•I
OUT




2
•R
DS(ON)
•ρ
τ




W
1.3W ≅
12V
8V
•5A




2
•R
DS(ON)
•1.5









Wand therefore
R
DS(ON)
<15.4mΩ