46 The PCB Design Magazine • September 2016
but there is very minimal change with input
voltage and load current, especially above
0.2A DC load.
There are many other parameters we may
want to test. In the frequency domain we could
check the gain-phase plot to assess the stability
margin of the converter. In the time domain we
can measure the output ripple, high-frequency
burst noise and transient response. And finally
we could tie all that data back to simulations
to see how good correlation we get. We will ad-
dress some of those items in future columns,
and also things that don't make sense to check
on evaluation boards.
So next time when you evaluate an evalua-
tion board, be prepared to know what to expect.
Double checking the test data always helps to
avoid wrong and potentially misleading con-
clusions.
PCBDESIGN
References
1. Quietpower column, Do not measure
PDN noise across capacitors!
2. LM20143 data sheet
3. AN-1691 LM20143 evaluation board
4. DesignCon East 2011 paper, DC and AC
Bias Dependence of Capacitors Including Tem-
perature Dependence
Dr. Istvan Novak is a distinguished
engineer at Oracle, working on
signal and power integrity designs
of mid-range servers and new
technology developments. With
25 patents to his name, Novak is
co-author of "Frequency-Domain Characteriza-
tion of Power Distribution Networks." To contact
Istvan, click here.
EVALUATING EVALUATION BOARDS
Figure 7: Variation of impedance surface as a function of input voltage. Left plot: 3.5V;middle plot:
4.5V; right plot: 5.5V DC input voltage. For sake of simplicity, axis labels and titles are not shown, but
are the same as in Figure 2.
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