®
LSN2 Series
Non-isolated, DOSA-SIP, 6/10/16A Selectable-Output DC/DC Converters
The highest temperatures in LSN2 SIPs occur at their output inductor, whose
heat is generated primarily by I2R losses. The derating curves were developed
using thermocouples to monitor the inductor temperature and varying the load
to keep that temperature below +110°C under the assorted conditions of air
flow and air temperature. Once the temperature exceeds +115°C (approx.),
the thermal protection will disable the converter. Automatic restart occurs
after the temperature has dropped below +110°C.
Solutions
To improve start up, review the conditions above. One of the better solutions
is to place a moderate size capacitor very close to the input terminals. You
may need two parallel capacitors. A larger electrolytic or tantalum cap sup-
plies the surge current and a smaller parallel low-ESR ceramic cap gives low
AC impedance. Too large an electrolytic capacitor may have higher internal
impedance (ESR) and/or lower the start up slew rate enough to upset the
DC/DC’s controller. Make sure the capacitors can tolerate reflected switching
current pulses from the converter.
As you may deduce from the derating curves and observe in the efficiency
curves on the following pages, LSN2 SIPs maintain virtually constant
efficiency from half to full load, and consequently deliver very impressive
temperature performance even if operating at full load.
The capacitors will not help if the input source has poor regulation. A con-
verter which starts successfully at 3.3 Volts will turn off if the input voltage
decays to below the input voltage theshold, regardless of external capaci-
tance.
Lastly, when LSN2 SIPs are installed in system boards, they are obviously
subject to numerous factors and tolerances not taken into account here. If you
are attempting to extract the most current out of these units under demand-
ing temperature conditions, we advise you to monitor the output-inductor
temperature to ensure it remains below +110°C at all times.
Increase the input start up voltage if possible to raise the downward voltage
spike. Also, make sure that the input voltage ramps up in a reasonably short
time (less than a few milliseconds). If possible, move the input source closer
to the converter to reduce ohmic losses in the input wiring. Remember that
the input current is carried both by the wiring and the ground plane return.
Make sure the ground plane uses adequate thickness copper. Run additional
bus wire if necessary.
Start Up Considerations
When power is first applied to the DC/DC converter, operation is different than
when the converter is running and stabilized. There is some risk of start up
difficulties if you do not observe several application features. Lower output
voltage converters may have more problems here since they tend to have
higher output currents. Operation is most critical with any combination of the
following external factors:
Any added output capacitor should use just enough capacitance (and no more)
to reduce output noise at the load and to avoid marginal threshold noise prob-
lems with external logic. An output cap will also “decouple” inductive reac-
tance in the load. Certain kinds of electronic loads include “constant current”
characteristics which destabilize the output with insufficient capacitance. If
the wiring to the eventual load is long, consider placing this decoupling cap at
the load. Use the Remote Sense input to avoid ohmic voltage drop errors.
1 - Low initial input line voltage and/or poor regulation of the input source.
2 – Full output load current on lower output voltage converters.
3 – Slow slew rate of input voltage.
An elegant solution to start up problems is to apply the input voltage with the
Remote On/Off control first in the off setting (for those converters with an On/
Off Control). After the specified start-up delay (usually under 20 mSec), turn on
the converter. The controller will have already been stabilized. The short delay
will not be noticed in most applications. Be aware of applications which need
“power management” (phased start up).
4 – Longer distance to input voltage source and/or higher external input
source impedance.
5 - Limited or insufficient ground plane. External wiring that is too small.
6 – Too small external input capacitance. Too high ESR.
7 – High output capacitance causing a start up charge overcurrent surge.
Finally, it is challenging to model some application circuits with absolute fidel-
ity. How low is the resistance of your ground plane? What is the inductance
(and distributed capacitance) of external wiring? Even a detailed mathemati-
cal model may not get all aspects of your circuit. Therefore it is difficult to
give cap values which serve all applications. Some experimentation may be
required.
8 – Output loads with excessive inductive reactance or constant current
characteristics.
If the input voltage is already at the low limit before power is applied, the
start up surge current may instantaneously reduce the voltage at the input
terminals to below the specified minimum voltage. Even if this voltage depres-
sion is very brief, this may interfere with the on-board controller and possibly
cause a failed start. Or the converter may start but the input current load will
now drive the input voltage below its running low limit and the converter will
shut down.
Pre-Biased Startup
Newer systems with multiple power voltages have an additional problem
besides startup sequencing. Some sections have power already partially
applied (possibly because of earlier power sequencing) or have leakage power
present so that the DC/DC converter must power up into an existing voltage.
This power may either be stored in an external bypass capacitor or supplied
by an active source.
If you measure the input voltage before start up with a Digital Voltmeter (DVM),
the voltage may appear to be adequate. Limited external capacitance and/or
too high a source impedance may cause a short downward spike at power
up, causing an instantaneous voltage drop. Use an oscilloscope not a DVM to
observe this spike. The converter’s soft-start controller is sensitive to input
voltage. What matters here is the actual voltage at the input terminals at all
times.
This “pre-biased” condition can also occur with some types of program-
mable logic or because of blocking diode leakage or small currents passed
through forward biased ESD diodes. Conventional DC/DC’s may fail to start up
correctly if there is output voltage already present. And some external circuits
are adversely affected when the low side MOSFET in a synchronous rectifier
converter sinks current at start up.
Symptoms of start-up difficulties may include failed started, output oscillation
or brief start up then overcurrent shutdown. Since the input voltage is never
absolutely constant, the converter may start up at some times and not at
others.
LSN2 Series Page 7 of 14
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