lt3825 Linear Technology Corporation, lt3825 Datasheet - Page 22

• Manufacturer Part Number: lt3825
• Description: Isolated No-opto Synchronous Flyback Controller With Wide Input Supply Range
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3825
Output Voltage Error Sources
The LT3825’s feedback sensing introduces additional
sources of errors. The following is a summary list.
The internal bandgap voltage reference sets the reference
voltage for the feedback amplifi er. The specifi cations detail
its variation.
The external feedback resistive divider ratio proportional
directly affects regulated voltage. Use 1% components.
Leakage inductance on the transformer secondary reduces
the effective secondary-to-feedback winding turns ratio
(N
age target by a similar percentage. Since secondary leakage
inductance is constant from part to part (with a tolerance)
adjust the feedback resistor ratio to compensate.
The transformer secondary current fl ows through the im-
pedances of the winding resistance, synchronous MOSFET
R
current for these errors is higher than the load current
because conduction occurs only during the converter’s
“off” time. So divide the load current by (1 – DC).
If the output load current is relatively constant, the feedback
resistive divider is used to compensate for these losses.
Otherwise, use the LT3825 load compensation circuitry
(see Load Compensation).
If multiple output windings are used, the fl yback winding will
have a signal that represents an amalgamation of all these
windings impedances. Take care that you examine worst-
case loading conditions when tweaking the voltages.
Power MOSFET Selection
The power MOSFETs are selected primarily on the criteria
of “on” resistance R
source breakdown voltage (BV
(V
22
DS(ON)
GS
S
/N
) and maximum drain current (I
F
) from its ideal value. This increases the output volt-
and output capacitor ESR. The DC equivalent
DS(ON)
, input capacitance, drain-to-
DSS
), maximum gate voltage
D(MAX)
).
For the primary-side power MOSFET, the peak current is:
where X
earlier.
For each secondary-side power MOSFET, the peak current
is:
Select a primary-side power MOSFET with a BV
than:
where N
mary winding. L
and C
C
be added to reduce the leakage inductance as discussed
earlier.
For each secondary-side power MOSFET, the BV
be greater than:
Choose the primary side MOSFET R
gate drive voltage (7.5V). The secondary side MOSFET
gate drive voltage depends on the gate drive method.
Primary side power MOSFET RMS current is given by:
OSS
BV
I
I
BV
I
PK(PRI)
PK(SEC)
RMS PRI
of the primary-side power MOSFET). A snubber may
DSS
DSS
P
(
is the primary-side capacitance (mostly from the
SP
MIN
≥ V
≥I
refl ects the turns ratio of that secondary-to-pri-
=
)
=
PK
=
OUT
V
is peak-to-peak current ratio as defi ned
1– DC
IN(MIN)
V
LKG
IN MIN
I
L
OUT
+ V
(
C
LKG
is the primary-side leakage inductance
P
MAX
P
IN(MAX)
IN
• DC
P
)
+ V
IN
DC
• 1+
IN(MAX)
MAX
MAX
• N
X
• 1+
SP
MIN
2
+
V
DS(ON)
OUT(MAX)
X
MIN
N
2
SP
at the nominal
DSS
DSS
greater
should
3525fa