LT3837EFE#TRPBF Linear Technology, LT3837EFE#TRPBF Datasheet - Page 13

• Manufacturer Part Number: LT3837EFE#TRPBF
• Description: IC CNTRLR SYNC ISO 16TSSOP
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT3837EFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.5 ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

APPLICATIONS INFORMATION
Primary Winding Feedback
The previous work was developed using a separate wind-
ing for voltage feedback. It is possible to use the primary
winding as the feedback winding as well. This can simplify
the design of the transformer.
When using the primary winding the feedback voltage
will be added to the V
where N
primary winding turns ratio. Use the circuit of Figure 2
to get more accurate output regulation. In this case the
regulation equations becomes:
where V
mately 0.7V).
Likewise the load compensation equation needs to be
changed to use N
R
R
V
FLYBK
1
CMP
=
R1
R2
V
R
BE
SP
=
FB
2
=
K
is the base emitter drop of the PNP (approxi-
FB
•
1
V
is the transformer effective secondary to
V
FLYBK
⎡
⎢
⎢
⎣
LT3837
•
OUT
V
R
OUT
ESR R
SENSE
3837 F02
SP
+
PG
I
OUT
instead of N
+
+
IN
I
OUT
• –
voltage so:
(
DS ON
Figure 2
•
PRIMARY
1
N
(
MP
(
ESR R
SP
•
N
V
DC
(
IN
ESR R
SP
)
)
+
•
SF
•
R N
+
•
DS ON
so:
1
SECONDARY
MS
•
(
DS ON
SF
(
)
)
)
)
−
V
C
BE
OUT
⎤ ⎤
⎥
⎥
⎦
Transformer Design
Transformer design/specifi cation is the most critical part
of a successful application of the LT3837. The following
sections provide basic information about designing the
transformer and potential tradeoffs.
If you need help, the LTC Applications group is available to
assist in the choice and/or design of the transformer.
Turns Ratios
The design of the transformer starts with determining
duty cycle (DC). DC impacts the current and voltage stress
on the power switches, input and output capacitor RMS
currents and transformer utilization (size vs power).
The ideal turns ratio is:
Avoid extreme duty cycles as they, in general, increase
current stresses. A reasonable target for duty cycle is
50% at nominal input voltage.
For instance, if we wanted a 9V to 3.3V converter at 50%
DC then:
In general, better performance is obtained with a lower
turns ratio. A DC of 52% yields a 1:3 ratio.
Note the use of the external feedback resistive divider
ratio to set output voltage provides the user additional
freedom in selecting a suitable transformer turns ratio.
Turns ratios that are the simple ratios of small integers;
e.g., 1:1, 2:1, 3:2 help facilitate transformer construction
and improve performance.
When building a supply with multiple outputs derived
through a multiple winding transformer, lower duty cycle
can improve cross regulation by keeping the synchronous
rectifi er on longer, and thus, keep secondary windings
coupled longer.
For a multiple output transformer, the turns ratio between
output windings is critical and affects the accuracy of the
voltages. The ratio between two output voltages is set with
the formula V
N
N
IDEAL
IDEAL
=
=
3 3
V
9
V
.
OUT2
OUT
IN
•
1 0 5
•
= V
– .
0 5
– 1
.
DC
OUT1
DC
=
2 72
• N21 where N21 is the turns
.
1
LT3837
13
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