LT3837EFE-PBF LINER [Linear Technology], LT3837EFE-PBF Datasheet - Page 21

• Manufacturer Part Number: LT3837EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Short-Circuit Conditions
Loss of current limit is possible under certain conditions
such as an output short circuit. If the duty cycle exhib-
ited by the minimum on-time is greater than the ratio of
secondary winding voltage (referred-to-primary) divided
by input voltage, then peak current is not controlled at
the nominal value. It ratchets up cycle-by-cycle to some
higher level. Expressed mathematically, the requirement
to maintain short-circuit control is:
where:
Other variables as previously defi ned.
Trouble is typically encountered only in applications with a
relatively high product of input voltage times secondary-
to-primary turns ratio and/or a relatively long minimum
switch on time. Additionally, several real world effects such
as transformer leakage inductance, AC winding losses, and
output switch voltage drop combine to make this simple
theoretical calculation a conservative estimate. Prudent
design evaluates the switcher for short-circuit protection
and adds any additional circuitry to prevent destruction.
Output Voltage Error Sources
The LT3837’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.
S
t
I
DC
ON(MIN)
SC
/N
F
MIN
= short-circuit output current
) from its ideal value. This increases the output volt-
=
= primary side switch minimum on-time
t
ON MIN
(
)
•
f
OSC
<
I
SC
•
(
R
V
SEC
IN
•
+
N
S S P
R
DS ON
(
)
)
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 LT3837 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 volt-
ages.
Power MOSFET Selection
The power MOSFETs are selected primarily on the criteria
of “on” resistance R
source breakdown voltage (BV
(VGS) and maximum drain current (I
For the primary-side power MOSFET, the peak current
is:
where X is peak-to-peak current ratio as defi ned earlier.
For each secondary-side power MOSFET, the peak cur-
rent is:
Select a primary-side power MOSFET with a B
than:
DS(ON)
I
I
BV
PK
PK
DSS
=
=
1
1
and output capacitor ESR. The DC equivalent
≥
–
–
I
I
I
DC
DC
OUT
OUT
PK
MAX
MAX
L
C
LKG
P
•
•
DS(ON)
⎛
⎝ ⎜
⎛
⎝ ⎜
1
1
+
+
+
V
X
X
IN MAX
, input capacitance, drain-to-
MIN
MIN
2
2
(
DSS
⎞
⎠ ⎟
⎞
⎠ ⎟
)
), maximum gate voltage
+
V
D(MAX)
OUT MAX
N
SP
(
LT3837
).
VDSS
)
21
greater
3837fa