Datasheet

LT3757/LT3757A
15
3757afd
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:
∆I
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 and greater core losses. It is recommended
that
c
fall within the range of 0.2 to 0.6.
Given an operating input voltage range, and having chosen
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)
∆I
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 conver-
ter 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 absolute
maximum ratings are not exceeded.