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參數(shù)資料
| 型號： | LLT3837EFE |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 110 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 14/28頁 |
| 文件大?。?/td> | 255K |
| 代理商： | LLT3837EFE |
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LT3837
21
3837fa
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:
DC
t
f
IR
R
VN
MIN
ON MIN
OSC
SC
SEC
DS ON
IN
=<
+
()
SSP
where:
tON(MIN) = primary side switch minimum on-time
ISC = short-circuit output current
Other variables as previously dened.
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 amplier. The specications 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
(NS/NF) from its ideal value. This increases the output volt-
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 ows through the im-
pedances of the winding resistance, synchronous MOSFET
RDS(ON) and output capacitor ESR. The DC equivalent
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 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 RDS(ON), input capacitance, drain-to-
source breakdown voltage (BVDSS),maximumgatevoltage
(VGS) and maximum drain current (ID(MAX)).
For the primary-side power MOSFET, the peak current
is:
I
DC
X
PK
OUT
MAX
MIN
=+
1
2
–
where X is peak-to-peak current ratio as dened earlier.
For each secondary-side power MOSFET, the peak cur-
rent is:
I
DC
X
PK
OUT
MAX
MIN
=+
1
2
–
Select a primary-side power MOSFET with a BVDSS greater
than:
BV
I
L
C
V
N
DSS
PK
LKG
P
IN MAX
OUT MAX
SP
≥+
+
()
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