Datasheet

LT3837
13
3837fd
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
IN
voltage so:
V
FLYBK
=
V
OUT
+I
OUT
• ESR +R
DS(ON)
( )
N
SP
where N
SP
is the transformer effective secondary to
primary winding turns ratio. Use the circuit of Figure 2
to get more accurate output regulation. In this case the
regulation equations becomes:
R1=
R2
V
FB
•
V
OUT
+I
OUT
• ESR+R
DS(ON)
( )
N
SP
− V
BE
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
where V
BE
is the base emitter drop of the PNP (approxi-
mately 0.7V).
Likewise the load compensation equation needs to be
changed to use N
SP
instead of N
SF
so:
R
CMP
=K1•
R
SENSE
• 1–DC
( )
ESR+R
DS(ON)
•R1•N
SF
•
MS
MP
R2
R1
SECONDARY
PRIMARY
C
OUT
V
IN
V
FLYBK
3837 F02
•
LT3837
FB PG
Figure 2
Transformer Design
Transformer design/specification 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:
N
IDEAL
=
V
OUT
V
IN
•
1–DC
DC
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:
N
IDEAL
=
3.3
9
•
1–0.5
0.5
=
1
2.72
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
rectifier 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
OUT2
= V
OUT1
• N21 where N21 is the turns