LT8708
52
Rev 0
For more information
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,BOOST)
•
DC
(MAX,M3,BOOST)
100%
2 • ƒ •
V
RSENSE(MAX,BOOST,MAXDC)
R
SENSE
–
I
OUT(MAX,BOOST)
• V
OUT(MAX,BOOST)
V
IN(MIN,BOOST)
H
=
8V •
33%
100%
2 • 150kHz •
83mV
6.3mΩ
–
5A •12V
8V
=
1.55µH
To avoid subharmonic oscillations in the inductor current,
choose the minimum inductance according to:
L
(MIN2,BOOST)
=
V
OUT(MAX,BOOST)
–
V
IN(MIN,BOOST)
• V
OUT(MAX,BOOST)
V
OUT(MAX,BOOST)
– V
IN(MIN,BOOST)
•R
SENSE
0.08 • ƒ
H
=
12V –
8V •12V
12V – 8V
• 6.3mΩ
0.08 • 150kHz
=
–6.3µH
L
(MIN1,BUCK)
=
V
IN(MAX,BUCK)
1–
V
OUT(MAX,BUCK)
V
IN(MAX,BUCK)
– V
OUT(MIN,BUCK)
•R
SENSE
0.08 • ƒ
H
=
25V • 1–
12V
25V – 12V
• 6.3mΩ
0.08 •150kHz
=
1.01µH
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
OSS
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 6.3V 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, MOS-
FETs 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:
PD
(MAX)
=
T
J(MAX)
– T
A(MAX)
R
TH(JA)
PD
(MAX)
=
125°C – 60°C
50
°C
W
=
1.3W
Since maximum I
2
R power in the boost region with positive
inductor current happens when V
IN
is minimum, we can
determine the maximum allowable R
DS(ON)
for the boost
region using (see Table 7):
PM1
=
P
I2R
≅
V
OUT
V
IN
• I
OUT(MAX,FWD)
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
2
• R
DS(ON)
•
ρ
τ
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
W
and therefore
R
DS(ON)
<
13W
12V
8V
•5
2
•1.5
=
15.4mΩ
A
The Fairchild FDMS7672 meets the specifications with a
maximum R
DS(ON)
of ~6.9mΩ at V
GS
= 4.5V (~10mΩ at
125°C).
The maximum dissipation in M2 occurs at maximum V
IN
voltage when the circuit is operating in the buck region in the
reverse direction. Using the 6.9mΩ Fairchild FDMS7672,
the dissipation is (see Table 7):
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