LT3757IMSE#TRPBF Linear Technology, LT3757IMSE#TRPBF Datasheet - Page 18

• Manufacturer Part Number: LT3757IMSE#TRPBF
• Description: IC CTRLR DC/DC ADJ 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Inverting, Flyback, Sepicr

Specifications of LT3757IMSE#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-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

applicaTions inForMaTion
LT3757
Flyback Converter: Switch Duty Cycle and Turns Ratio
The flyback converter conversion ratio in the continuous
mode operation is:
where N
Figure 8 shows the waveforms of the flyback converter
in discontinuous mode operation. During each switching
period T
During DT
D2T
L
The flyback converter conversion ratio in the discontinu-
ous mode operation is:

S
currents are zero during D3T
V
V
S
V
V
OUT
OUT
, M is off, and L
V
I
IN
IN
SW
DS
I
D
Figure 8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
S
S
=
=
/N
, three subintervals occur: DT
S
N
N
N
N
P
, M is on, and D is reverse-biased. During
I
S
P
S
P
is the second to primary turns ratio.
D(MAX)
DT
•
•
S
1
D
D
−
D
2
D
S
is conducting current. Both L
I
SW(MAX)
T
S
D2T
S
S
.
D3T
S
S
, D2T
3757 F07
S
, D3T
t
P
and
S
.
According to the preceding equations, the user has relative
freedom in selecting the switch duty cycle or turns ratio to
suit a given application. The selections of the duty cycle
and the turns ratio are somewhat iterative processes, due
to the number of variables involved. The user can choose
either a duty cycle or a turns ratio as the start point. The
following trade-offs should be considered when select-
ing the switch duty cycle or turns ratio, to optimize the
converter performance. A higher duty cycle affects the
flyback converter in the following aspects:
• Lower MOSFET RMS current I
• Lower diode peak reverse voltage, but higher diode
• Higher transformer turns ratio (N
The choice,
(for discontinuous mode operation with a given D3) gives
the power MOSFET the lowest power stress (the product
of RMS current and peak voltage). However, in the high
output voltage applications, a higher duty cycle may be
adopted to limit the large peak reverse voltage of the
diode. The choice,
(for discontinuous mode operation with a given D3) gives
the diode the lowest power stress (the product of RMS
current and peak voltage). An extreme high or low duty
cycle results in high power stress on the MOSFET or diode,
and reduces efficiency. It is recommended to choose a
duty cycle, D, between 20% and 80%.
MOSFET V
RMS current I
D D
D D
+
+
D
D
2
2
=
=
3
2
3
1
DS
peak voltage
D(RMS)
SW(RMS)
P
/N
S
)
, but higher
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