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參數(shù)資料
| 型號： | LSM2-T/6-W3-C |
| 元件分類： | 電源模塊 |
| 英文描述： | 1-OUTPUT 19.8 W DC-DC REG PWR SUPPLY MODULE |
| 封裝： | ROHS COMPLIANT |
| 文件頁數(shù)： | 15/17頁 |
| 文件大?。?/td> | 1394K |
| 代理商： | LSM2-T/6-W3-C |
LSM2 Series
Single Output, Non-isolated, Selectable-Output POL DC/DC Converters
MDC_LSM2_A01 Page 7 of 7
P O W E R E L E C T R O N I C S D I V I S I O N
D C / D C C O N V E R T E R S
www.cd4power.com
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.
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/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.
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 synchro-
nous rectifier 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.
Don’t Use Pre-Biasing and Sequencing Together
Normally, you would use startup sequencing on multiple DC/DC’s to solve the
Pre-Bias problem. By causing all power sources to ramp up together, no one
source can dominate and force the others to fail to start. For most applica-
tions, do not use startup sequencing in a Pre-Bias application, especially with
an external active power source.
If you have active source pre-biasing, leave the Sequence input open so that
the output will step up quickly and safely. A symptom of this condition is
repeated failed starts. You can further verify this by removing the existing load
and testing it with a separate passive resistive load which does not exceed full
current. If the resistive load starts successfully, you may be trying to drive an
external pre-biased active source.
It may also be possible to use pre-bias and sequencing together if the Pre-
Bias source is in fact only a small external bypass capacitor slowly charged by
leakage currents. Test your application to be sure.
Output Adjustments
The LSM2 series includes a special output voltage trimming feature which is
fully compatible with competitive units. The output voltage may be varied us-
ing a single trim resistor from the Trim input to Power Common (pin 4) or an
external DC trim voltage applied between the Trim input and Power Common.
The output voltage range for W3 models is 0.75 to 3.3 Volts. For D2 models,
the output range is 0.75 to 5 Volts.
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 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 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.
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.
The capacitors will not help if the input source has poor regulation. A convert-
er 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 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.
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
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