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LSM2 Series
Single Output, Non-Isolated
Selectable-Output POL DC/DC Converters
Start Up Considerations
When power is ?rst applied to the DC/DC converter, operation is different than
when the converter is running and stabilized. There is some risk of start up
dif?culties 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:
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 insuf?cient 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 speci?ed 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.
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 dif?culties may include failed started, output oscilla-
tion 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.
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 re?ected switching current pulses
from the converter.
The capacitors will not help if the input source has poor regulation. A
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 re-
actance in the load. Certain kinds of electronic loads include “constant current”
characteristics which destabilize the output with insuf?cient 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 ?rst in the off setting (for those converters with an
On/Off Control). After the speci?ed 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 ?del-
ity. 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 dif?cult to give cap values
which serve all applications. Some experimentation may be required.
Pre-Biased Startup
Newer systems with multiple power voltages have an additional problem
besides startup sequencing. Some sections have power already partially ap-
plied (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.
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/DCs 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 recti?er
converter sinks current at start up.
The LSM2 series includes a pre-bias startup mode to prevent these initializa-
tion problems. Essentially, the converter acts as a simple buck converter until the
output reaches its set point voltage at which time it converts to a synchronous
recti?er design. This feature is variously called “monotonic” because the voltage
does not decay (from low side MOSFET shorting) or produce a negative transient
once the input power is applied and the startup sequence begins.
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.
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M D C_LSM2 Series.C01 Δ Page 7 of 17
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制造商:Murata Power Solutions 功能描述:DC/DC Converter
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制造商:Murata Power Solutions 功能描述:Module DC-DC 1-OUT 0.75V to 5V 16A 80W 8-Pin Case C62
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功能描述:DC/DC转换器 52.8W SINGLE OUTPUT 12V / 0.75-5V16A RoHS:否 制造商:Murata 产品: 输出功率: 输入电压范围:3.6 V to 5.5 V 输入电压（标称）: 输出端数量:1 输出电压（通道 1）:3.3 V 输出电流（通道 1）:600 mA 输出电压（通道 2）: 输出电流（通道 2）: 安装风格:SMD/SMT 封装 / 箱体尺寸:
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制造商:Murata Power Solutions 功能描述:Module DC-DC 1-OUT 0.75V to 3.3V 16A 52.8W 8-Pin Case 62
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功能描述:DC/DC转换器 52.8W SINGLE OUTPUT 5V / 0.75-3.3V16A
RoHS:否 制造商:Murata 产品: 输出功率: 输入电压范围:3.6 V to 5.5 V 输入电压（标称）: 输出端数量:1 输出电压（通道 1）:3.3 V 输出电流（通道 1）:600 mA 输出电压（通道 2）: 输出电流（通道 2）: 安装风格:SMD/SMT 封装 / 箱体尺寸:
LSM2-T/16-W3G
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LSM2-T/16-W3G-C
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