Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
rise to potential sources of noise. This noise manifests itself
on the output voltage as spikes at the switching frequency.
limited but usually adequate for most applications. When
noise sources. Figure 1 illustrates two recommended meth-
ods for testing output voltage ripple and noise.
output filter capacitor.
50鈩?/div>
(b) Alternate Method
Scope
ESR
ESL
It is important that the inductor wire size can carry the load
current, including a safety factor, and that the core does not
saturate. Also notice that the DC resistance of the inductor
is outside the feedback loop for regulated units and subse-
quently degrades that regulation.
LC filters are generally used only where very accurate
analog measurements are being taken, and the power supply
rejection is poor at the ripple frequency. A much more
common filtering technique is the output filter capacitor.
CHOOSING AN OUTPUT CAPACITOR
The saying 鈥渢he more the better鈥?is definitely not applicable
to output capacitors of switching type power supplies. The
basic design equations of the supply rules out any brute force
approach. The parameter of major concern is the Effective
Series Resistance (ESR). ESR is due to stray resistance
inside the electrolytic capacitor that becomes significant at
switching power supply frequencies and higher. Together
with Effective Series Inductance (ESL), ESR can be mod-
eled as the equivalent circuit shown in Figure 3.
50鈩?/div>
Equivalent Circuit
Impedance (鈩?
(1)
FIGURE 1. Recommended Noise Measurement Methods.
LC FILTERING
For applications requiring higher accuracy, such as analog
measurements, LC filters should be used on each channel to
attenuate high-frequency noise. Because of the output filter
capacitor already present in the DC-to-DC converter, adding
an inductor and capacitor to the output creates a pi filter at
half the use-capacitor cost (Figure 2).
X
C
100
1k
10k
100k
X
L
(ESL)
1M
10M
Frequency (Hz)
NOTE: (1) Improvement from paralleling several capacitors with equal
total capacitance.
PWRXXX
+
+V
OUT
FIGURE 3. Typical Log/Log Impedance Plot for Aluminum
Electrolytic Capacitor.
ESR is also a function of temperature and actually decreases
as temperature is increased. This temperature dependency is
particularly strong in the below zero range as shown in
Figure 4. This, together with decreasing capacitance (Figure
5), can prove fatal to a switcher design that performed well
on the bench.
Internal
Output
Capacitors
C
Common
鈥?/div>
鈥揤
OUT
FIGURE 2. Output 鈥淧I鈥?Filters.
漏
1987 Burr-Brown Corporation
AB-162
Printed in U.S.A. March, 1994
Application Bulletin Number 162
(ESR)
1
2
3
