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LSN2 Series
Non-isolated, DOSA-SIP, 6/10/16A Selectable-Output DC/DC Converters
installed within the ungrounded input path to the converter.
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Safety Considerations
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LSN2 SIPs are non-isolated DC/DC converters. In general, all DC/DC's
must be installed, including considerations for I/O voltages and spacing/sepa-
ration requirements, in compliance with relevant safety-agency speci-
fications (usually UL/IEC/EN60950).
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In particular, for a non-isolated converter's output voltage to meet SELV
(safety extra low voltage) requirements, its input must be SELV compliant.
If the output needs to be ELV (extra low voltage), the input must be ELV.
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Input Overvoltage and Reverse-Polarity Protection
Figure 2. Measuring Input Ripple Current
LSN2 SIP Series DC/DC's do not incorporate either input overvoltage or input
reverse-polarity protection. Input voltages in excess of the specified absolute
maximum ratings and input polarity reversals of longer than "instantaneous"
duration can cause permanent damage to these devices.
Output ripple/noise (also referred to as periodic and random deviations or
PARD) may be reduced below specified limits with the installation of additional
external output capacitors. Output capacitors function as true filter elements
and should be selected for bulk capacitance, low ESR, and appropriate fre-
quency response. Any scope measurements of PARD should be made directly
at the DC/DC output pins with scope probe ground less than 0.5" in length.
Start-Up Time
The VIN to VOUT Start-Up Time is the interval between the time at which a
ramping input voltage crosses the lower limit of the specified input volt-
age range and the fully loaded output voltage enters and remains within its
specified accuracy band. Actual measured times will vary with input source
impedance, external input capacitance, and the slew rate and final value of
the input voltage as it appears to the converter.
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The On/Off to VOUT Start-Up Time assumes the converter is turned off via the
On/Off Control with the nominal input voltage already applied to the converter.
The specification defines the interval between the time at which the converter
is turned on and the fully loaded output voltage enters and remains within its
specified accuracy band. See Typical Performance Curves.
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Remote Sense
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LSN2 Series offer an output sense function. The sense function enables point-
of-use regulation for overcoming moderate IR drops in conductors and/or
cabling. Since these are non-isolated devices whose inputs and outputs usu-
ally share the same ground plane, sense is provided only for the +Output.
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The remote sense line is part of the feedback control loop regulating the
DC/DC converter’s output. The sense line carries very little current and
consequently requires a minimal cross-sectional-area conductor. As such,
it is not a low-impedance point and must be treated with care in layout and
cabling. Sense lines should be run adjacent to signals (preferably ground), and
in cable and/or discrete-wiring applications, twisted-pair or similar techniques
should be used. To prevent high frequency voltage differences between VOUT
and Sense, we recommend installation of a 1000pF capacitor close to the
converter.
Figure 3. Measuring Output Ripple/Noise (PARD)
All external capacitors should have appropriate voltage ratings and be located
as close to the converters as possible. Temperature variations for all relevant
parameters should be taken into consideration.
The most effective combination of external I/O capacitors will be a function
of your line voltage and source impedance, as well as your particular load
and layout conditions. Our Applications Engineers can recommend potential
solutions and discuss the possibility of our modifying a given device’s internal
filtering to meet your specific requirements. Contact our Applications Engineer-
ing Group for additional details.
The sense function is capable of compensating for voltage drops between the
+Output and +Sense pins that do not exceed 10% of VOUT.
[VOUT(+) – Common] – [Sense(+) – Common] ≤ 10%VOUT
Power derating (output current limiting) is based upon maximum output
current and voltage at the converter's output pins. Use of trim and sense
functions can cause the output voltage to increase, thereby increasing output
power beyond the LSN2's specified rating. Therefore:
Input Fusing
Most applications and or safety agencies require the installation of fuses at
the inputs of power conversion components. The LSN2 Series are not inter-
nally fused. Therefore, if input fusing is mandatory, either a normal-blow or a
slow-blow fuse with a value no greater than twice the maximum input current
calculated at low line with the converter's minimum efficiency should be
(VOUT at pins) x (IOUT) ≤ rated output power
LSN2 Series Page 5 of 14
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