Datasheet
LT3757/LT3757A
15
3757afe
For more information www.linear.com/LT3757
applicaTions inForMaTion
Boost Converter: Inductor and Sense Resistor Selection
For the boost topology, the maximum average inductor
current is:
I
L(MAX)
=I
O(MAX)
•
1
1−D
MAX
Then, the ripple current can be calculated by:
ΔI
L
= c • I
L(MAX)
= c • I
O(MAX)
•
1
1−D
MAX
The constant
c
in the preceding equation represents the
percentage peak-to-peak ripple current in the inductor,
relative to I
L(MAX)
.
The inductor ripple current has a direct effect on the
choice of the inductor value. Choosing smaller values of
∆I
L
requires large inductances and reduces the current
loop gain (the converter will approach voltage mode).
Accepting larger values of ∆I
L
provides fast transient
response and allows the use of low inductances, but
results in higher input current ripple, greater core losses,
and in some cases, sub-harmonic oscillation. A good
starting point of
c
is 0.2 and careful evaluation of system
stability should be made to ensure adequate design
margin.
Given an operating input voltage range, and having cho-
sen the operating frequency and ripple current in the
inductor, the inductor value of the boost converter can
be determined using the following equation:
L =
V
IN(MIN)
ΔI
L
• f
• D
MAX
The peak and RMS inductor current are:
I
L(PEAK)
=I
L(MAX)
• 1+
c
2
⎛
⎝
⎜
⎞
⎠
⎟
I
L(RMS)
=I
L(MAX)
• 1+
c
2
12
Based on these equations, the user should choose the
inductors having sufficient saturation and RMS current
ratings.
Set the sense voltage at I
L(PEAK)
to be the minimum of the
SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
R
SENSE
=
80mV
I
L(PEAK)
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-source voltage rating (V
DS
), the threshold voltage
(V
GS(TH)
), the on-resistance (R
DS(ON)
), the gate to source
and gate to drain charges (Q
GS
and Q
GD
), the maximum
drain current (I
D(MAX)
) and the MOSFET’s thermal
resistances (R
θJC
and R
θJA
).
The power MOSFET will see full output voltage, plus a
diode forward voltage, and any additional ringing across
its drain-to-source during its off-time. It is recommended
to choose a MOSFET whose B
VDSS
is higher than V
OUT
by
a safety margin (a 10V safety margin is usually sufficient).
The power dissipated by the MOSFET in a boost converter
is:
P
FET
= I
2
L(MAX)
• R
DS(ON)
• D
MAX
+ 2 • V
2
OUT
• I
L(MAX)
• C
RSS
• f /1A
The first term in the preceding equation represents the
conduction losses in the device, and the second term, the
switching loss. C
RSS
is the reverse transfer capacitance,
which is usually specified in the MOSFET characteristics.
For maximum efficiency, R
DS(ON)
and C
RSS
should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
J
= T
A
+ P
FET
• θ
JA
= T
A
+ P
FET
• (θ
JC
+ θ
CA
)
T
J
must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that abso-
lute maximum ratings are not exceeded.
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