LT3757EDD#TRPBF Linear Technology, LT3757EDD#TRPBF Datasheet - Page 23

• Manufacturer Part Number: LT3757EDD#TRPBF
• Description: IC CTRLR DC/DC ADJ 10-DFN
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Inverting, Flyback, Sepicr

Specifications of LT3757EDD#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 100kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

applicaTions inForMaTion
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the induc-
tor, the inductor value (L1 and L2 are independent) of the
SEPIC converter can be determined using the following
equation:
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.
By making L1 = L2, and winding them on the same core, the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
I
I
The RMS inductor currents are:
where:
where:
L
L
I
I
c
c
L1(PEAK)
L2(PEAK)
L RMS
L RMS
1
L
L
=
1
2
=
(
1
(
2
∆
V
=
L
=
IN MIN
I
2
I
SW
I
L MAX
(
L MAX
)
=
)
1
= I
= I
∆
2
=
=
∆
(
0 5
(
I
•
I
I
I
L
. •
L MAX
L
L MAX
)
f
L1(MAX)
L2(MAX)
1
1
2
2
V
•
(
(
IN MIN
)
D
)
∆
(
MAX
I
SW
)
)
•
+ 0.5 • ∆I
+ 0.5 • ∆I
•
)
•
1
f
1
+
•
+
D
c
c
MAX
12
2
12
2
L
L
L1
L2
1
2
Based on the preceding equations, the user should choose
rent ratings.
In a SEPIC converter, when the power switch is turned on,
the current flowing through the sense resistor (I
the switch current.
Set the sense voltage at I
The sense resistor value can then be calculated to be:
SEPIC Converter: Power MOSFET Selection
For the SEPIC configuration, choose a MOSFET with a
V
input voltage by a safety margin (a 10V safety margin is
P
+ 2 • (V
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. C
usually specified in the MOSFET characteristics.
For maximum efficiency, R
minimized. From a known power dissipated in the power
the following equation:
T
T
maximum ratings are not exceeded.
the inductors having sufficient saturation and RMS cur-
of the SENSE current limit threshold with a 20% margin.
usually sufficient).
The power dissipated by the MOSFET in a SEPIC con-
verter is:
MOSFET, its junction temperature can be obtained using
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
J
DC
must not exceed the MOSFET maximum junction
R
rating higher than the sum of the output voltage and
J
FET
SENSE
= T
= I
RSS
A
+ P
2
IN(MIN)
is the reverse transfer capacitance, which is
=
SW(MAX)
I
SW PEAK
FET
80
(
• θ
+ V
mV
• R
JA
OUT
)
= T
DS(ON)
)
2
SENSE(PEAK)
A
• I
+ P
DS(ON)
L(MAX)
• D
FET
MAX
• (θ
and C
• C
to be the minimum
JC
RSS
+ θ
RSS
• f / 1A
LT3757
CA
should be
)
SENSE

3757fb
) is