LSM16A-W3 Datasheet, PDF (7/12 Pages) Murata Manufacturing Co., Ltd. – Non-Isolated, Wide Input SMT DC/DC Converters

LSM-16A W3 Models
Non-Isolated, Wide Input SMT DC/DC Converters
Output Reverse Conduction
Many DC/DC’s using synchronous rectification suffer from Output Reverse
Conduction. If those devices have a voltage applied across their output before
a voltage is applied to their input (this typically occurs when another power
supply starts before them in a power-sequenced application), they will either
fail to start or self destruct. In both cases, the cause is the “freewheeling” or
“catch” FET biasing itself on and effectively becoming a short circuit.
LSM W3 SMT DC/DC converters do not suffer from Output Reverse Conduc-
tion. They employ proprietary gate drive circuitry that makes them immune to
moderate applied output overvoltages.
Thermal Considerations and Thermal Protection
The typical output-current thermal-derating curves shown below enable
designers to determine how much current they can reliably derive from each
model of the LSM W3 SMT’s under known ambient-temperature and air-flow
conditions. Similarly, the curves indicate how much air flow is required to reli-
ably deliver a specific output current at known temperatures.
The highest temperatures in LSM W3 SMT’s 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 vary-
ing the load to keep that temperature below +110°C under the assorted condi-
tions 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.
As you may deduce from the derating curves and observe in the efficiency
curves on the following pages, LSM W3 SMT’s maintain virtually constant
efficiency from half to full load, and consequently deliver very impressive
temperature performance even if operating at full load.
Lastly, when LSM W3 SMT’s 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 demanding
temperature conditions, we advise you to monitor the output-inductor tempera-
ture to ensure it remains below +110°C at all times.
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 input volt-
age converters may have more problems here since they tend to have higher
input currents. Operation is most critical with any combination of the following
external factors:
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.
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.
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 depression 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.
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.
Symptoms of start-up difficulties may include failed started, output oscillation or
brief start up then overcurrent shutdown. Since the input voltage is never abso-
lutely constant, the converter may start up at some times and not at others.
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 supplies the
surge current and a smaller parallel low-ESR ceramic cap gives low AC imped-
ance. 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 control-
ler. Make sure the capacitors can tolerate reflected switching current pulses
from the converter.
The capacitors will not help if the input source has poor regulation. A converter
which starts successfully at 3.3 Volts will turn off if the input voltage decays to
below the input voltage theshold, regardless of external capacitance.
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.
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
problems 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.
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).
Finally, it is challenging to model some application circuits with absolute fidelity.
How low is the resistance of your ground plane? What is the inductance (and
distributed capacitance) of external wiring? Even a detailed mathematical
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.
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