LT8705
APPLICATIONS INFORMATION
To avoid subharmonic oscillations in the inductor current,
choose the minimum inductance according to:
2
Since maximum I R power dissipation in the boost region
happens when V is minimum, we can determine the
IN
maximum allowable R
for the boost region using:
DS(ON)
2
V
IN(MIN) •VOUT(MAX)
VOUT
V
–
•R
SENSE
2
PM1 = P
≅
•IOUT •RDS(ON) •ρ
W
OUT(MAX)
I R
τ
V
– V
V
IN(MIN)
OUT(MAX)
IN
L(MIN2,BOOST)
=
=
H
0.08•f
2
12V
8V
8V •12V
12V –8V
0.08•350kHz
1.3W ≅
•5A •RDS(ON) •1.5 Wand therefore
12V –
•8.7mΩ
= –3.7µH
RDS(ON) < 15.4mΩ
VOUT(MAX)
IN(MAX) – V
V
IN(MAX) • 1–
•R
SENSE
The Fairchild FDMS7672 meets the specifications with a
maximum R of ~6.9mΩ at V = 4.5V (~10mΩ at
V
OUT(MIN)
L(MIN1,BUCK)
=
DS(ON)
GS
0.12•f
125°C). Checking the power dissipation in the buck region
with V maximum and V
minimum yields:
12V
25V –12V
IN
OUT
25V • 1–
•8.7mΩ
2
PM1 = PI R + PSWITCHING
=
= 0.6µH
0.08•350kHz
2
VOUT
The inductance must be higher than all of the minimum
values calculated above. We will choose a 10μH standard
value inductor for improved margin.
≅
≅
•IOUT •RDS(ON) •ρ + V •I
•f•tRF1 W
(
)
IN
OUT
V
τ
IN
12V
25V
2
PM1
•5A •6.9mΩ •1.5 + 25V •5A •350k •20ns
(
)
MOSFET Selection: The MOSFETs are selected based on
voltage rating, C
and R
value. It is important to
RSS
DS(ON)
ensure that the part is specified for operation with the
availablegatevoltageamplitude.Inthiscase,theamplitude
= 0.06W+ 0.88W = 0.94W
The maximum switching power of 0.88W can be reduced
by choosing a slower switching frequency. Since this
calculation is approximate, measure the actual rise and
fall times on the PCB to obtain a better power estimate.
is 6.35V and MOSFETs with an R
value specified at
DS(ON)
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.
ThemaximumdissipationinM2occursatmaximuminput
voltage when the circuit is operating in the buck region.
Using the 6.9mΩ Fairchild FDMS7672 the dissipation is:
V – V
IN
OUT
P
≅
≅
•IOUT(MAX)2 •RDS(ON) •ρτ
W
(M2,BUCK)
V
If we designfor amaximumjunction temperature, T
IN
J(MAX)
=125°C,themaximumallowablepowerdissipationcanbe
25V –12V
25V
calculated.First,calculatethemaximumpowerdissipation:
P
• 5A 2 •6.9mΩ •1.5 = 0.13W
(
)
(M2,BUCK)
T
J(MAX) –TA(MAX)
PD(MAX)
=
=
RTH(JA)
125°C–60°C
50°C/W
PD(MAX)
= 1.3W
8705p
38
For more information www.linear.com/8705
Linear [ Linear ]
