12 Watt LV Single Series DC/DC Converters
When using the LV Single be sure that the impedance at
the decision of how you define how big is big enough.
the input to the converter is less than 0.05 ohms from DC to
about 100 kHz, this is usually not a problem in battery
powered systems when the converter is connected directly to
the battery. If the converter is located more than about 1 inch
from the input source an added capacitor is required directly
at the input pins for proper operation.
Suitable capacitors for use at the input of the converter are
given at the end of this section.
Startup Current Demand
Because the LV Single appears as a constant power load to
your source and operation starts at about 3 volts, you should
besurethatyoursourcecansupplytherequiredcurrentatlow
voltages when starting. If this presents a problem the ON/OFF
pin and a simple voltage detector (comparator) may be used
to prevent startup until some higher steady state voltage.
The maximum source impedance is a function of output
power and line voltage. The impedance can be higher when
operating at less than full power. The minimum impedance is
required when operating with a 9 volt input. The impedance
reduces as the input voltage is raised or lowered or the power
is reduced. In general you should keep the peak to peak
voltage measured across the input pins less than 0.15 volts
peak to peak (not including the high frequency spikes) for
maximum converter performance and life.
Generally this is not a problem with battery powered
circuits and only appears when the LV Single is powered by
marginally sized 5 or 12 volt linear supplies that can’t supply
the required startup current. See the”Input Current Vs. Line
Input” curve for the low voltage current requirements of the LV
Single.
There is no lower limit on the allowed source impedance,
it can be any physically realizable value, even approaching 0.
If the source impedance is too large in your system you
should choose an external input capacitor as detailed below.
Very Low Noise Input Circuit
Figure 2 shows a very low noise input circuit that may be used
with the converters. This circuit will reduce the input reflected
ripple current to less than 5 mA RMS (Vin = 5 V, 10 kHz to 1
MHz bw). See the discussion above for the optimum selection
of C2.
Picking An External Input Capacitor
If an input capacitor is needed at the input to the converter it
must be sized correctly for proper converter operation. The
curve “RMS Input Current Vs Line Input” shows the RMS
ripple current that the input capacitor must withstand with
varying loading conditions and input voltages.
Several system tradeoffs must be made for each particular
system application to correctly size the input capacitor.
Theprobableresultofundersizingthecapacitorisincreased
self heating, shortening it’s life. Oversizing the capacitor can
have a negative effect on your products cost and size,
although this kind of overdesign does not result in shorter life
of any components.
L1 = 10µH
C1 = 10µF / 25V, TANTALUM
C2 = SEE TEXT
There is no one optimum value for the input capacitor. The
size and capacity depend on the following factors:
Figure 2.
1) Expected ambient temperature and your temperature
derating guidelines.
This circuit will reduce the input reflected ripple current to less than
5 mA RMS. See the discussion in the text for help on the optimum
selection of C2. L1 should be sized to handle the maximum input
current at your lowest operating voltage and maximum expected
output power.
2) Your ripple current derating guidelines.
A
3) The maximum anticipated load on the converter.
4) The input operating voltage, both nominal and
excursions.
Suggested Capacitor Sources
5) The statistical probability that your system will spend
a significant time at any worst case extreme.
These capacitors may be used to lower your sources input
impedance at the input of the converter. These capacitors will
work for 100% load, worst case input voltage and ambient
temperature extremes. They however, may be oversized for
your exact usage, see “Picking An External Input Capacitor”
above for more information. You may also use several smaller
capacitorsinparalleltoachievethesameripplecurrentrating.
This may save space in some systems.
Factors 1 and 2 depend on your system design guidelines.
These can range from 50 to 100% of the manufacturers listed
maximum rating, although the usual derating factor applied is
about 70%. 70% derating means if the manufacturer rated the
capacitor at 1 A RMS you would not use it over 0.7 A RMS in
your circuit.
Factors3and4realisticallydeterminetheworstcaseripple
current rating required for the capacitor along with the RMS
ripple current curve.
United Chemi-Con SXE, RXC, RZ and RZA series
Suggested Part:
SXE025VB820M12.5X20LL
820µF, 25V, 105°C Rated
ESR=0.085 ohms
Factor 5 is not easy to quantify. At CALEX we can make no
assumptions about a customers system so we leave to you
Allowable Ripple at 85 °C = 1.96 A
2401 Stanwell Drive
•
Concord, California 94520
•
Ph: 925/687-4411 or 800/542-3355
•
Fax: 925/687-3333
•
www.calex.com
•
Email: sales@calex.com
3/2001
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