LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 17

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The constant χ in the preceding equation represents the
percentage peak-to-peak ripple current in the inductor,
relative to I
The inductor ripple current has a direct effect on the choice
of the inductor value. Choosing smaller values of ΔI
requires large inductances and reduces the current loop
gain (the converter will approach voltage mode). Accepting
larger values of ΔI
allows the use of low inductances, but results in higher input
current ripple and greater core losses. It is recommended
that χ 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:
The peak and RMS inductor current are:
Based on these equations, the user should choose the
inductors having suffi cient saturation and RMS current
ratings.
Set the sense voltage at I
SENSE current limit threshold with a 20% margin. The
sense resistor value can then be calculated to be:
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-source voltage rating (V
(V
and gate to drain charges (Q
drain current (I
resistances (R
GS(TH)
L
I
I
R
L PEAK
L RMS
=
(
(
SENSE
V
Δ
IN MIN
), the on-resistance (R
I
)
)
(
L
=
=
L(MAX)
=
•
I
I
L MAX
I
f
L MAX
L PEAK
80
(
)
θJC
(
(
•
D
mV
.
D(MAX)
MAX
and R
L
)
)
provides fast transient response and
•
)
•
⎛
⎝ ⎜
1
1 1
θJA
+
+
) and the MOSFET’s thermal
L(PEAK)
χ
2
χ
12
).
2
⎞
⎠ ⎟
GS
DS
DS(ON)
to be the minimum of the
and Q
), the threshold voltage
), the gate to source
GD
), the maximum
L
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
a safety margin (a 10V safety margin is usually suffi cient).
The power dissipated by the MOSFET in a boost converter is:
The fi rst term in the preceding equation represents the
conduction losses in the device, and the second term, the
switching loss. C
which is usually specifi ed in the MOSFET characteristics.
For maximum effi ciency, R
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
Boost Converter: Output Diode Selection
To maximize effi ciency, a fast switching diode with low
forward drop and low reverse leakage is desirable. The
peak reverse voltage that the diode must withstand is
equal to the regulator output voltage plus any additional
ringing across its anode-to-cathode during the on-time.
The average forward current in normal operation is equal
to the output current, and the peak current is equal to:
It is recommended that the peak repetitive reverse voltage
rating V
safety margin is usually suffi cient).
The power dissipated by the diode is:
and the diode junction temperature is:
J
P
• C
T
P
T
I
must not exceed the MOSFET maximum junction
D PEAK
J
J
FET
D
(
RSS
= T
= T
= I
= I
RRM
A
O(MAX)
A
• f/1A
)
2
+ P
+ P
=
L(MAX)
is higher than V
I
FET
L PEAK
D
(
• R
• V
RSS
• θ
• R
D
θJA
JA
)
DS(ON)
=
is the reverse transfer capacitance,
= T
⎛
⎝ ⎜
1
A
+
+ P
OUT
χ
2
• D
DS(ON)
VDSS
⎞
⎠ ⎟
FET
MAX
•
by a safety margin (a 10V
I
L MAX
(
• (θ
is higher than V
and C
+ 2 • V
JC
)
+ θ
RSS
2
LT3758
OUT
CA
should be
)
• I
17
L(MAX)
OUT
3758f
by