AM C DOC. #: AMC2576_B AMC2576
November 2002
Copyright 2002, ADD Microtech Corp. 9 www.addmtek.com
A
TEP-D
LTA
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LAT
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Application Information (contd.)
Output Volt age Ripple and Transients
The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output
capacitor.
The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency,
typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth
waveform.
Due to the fast switching action, and the parasitic inductance of the output filter capacitor, there is voltage spikes
presenting at the peaks of the sawtooth waveform. Cautions must be taken for stray capacitance, wiring inductance,
and even the scope probes used for transients evaluation. To minimize these voltage spikes, shortening the lead length
and PCB traces is always the first thought. Further more, an ad ditional small LC filter (20µH & 100µF) (as shown in
Figure 3) will possibly provide a 10 X reduction in output ripple voltage and transients.
Inductor Selection
The AMC2576 can be used for either continuous or discontinuous modes of operation. Each mode has distinctively
different operating characteristics, which can affect the regulator performance and requirements.
With relatively heavy load currents, the circuit operates in the continuous mode (inductor current always flowing), but
under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a
period of time). For light loads (less than approximately 300 mA) it may be desirable to operate the regulator in the
discontinuous mode, primarily because of the lower inductor values required for the discontinuous mode.
Inductors are available in different styles such as pot core, toroid, E-frame, bobbin core, et., as well as different core
materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire wrapped on
a ferrite rod core. This type of construction makes for an inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more electromagnetic interference (EMI). This EMI can cause
problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe.
An inductor should not be operated beyond its maximum rated current because it may saturate. When an inductor
begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC
resistance of the winding). This will cause the switch current to rise very rapidly. Different inductor types have
different saturation characteristics, and this should be well considered when selecting as inductor.
COUT
1000µF
AMC2576-ADJ
OUTPUT
L1
100
H
5
2
4
VIN VOUT
FB
GND
CIN
100µF
7V – 40V
DC INPUT
R2
50K
R1
1.21K
3ENABLE
Figure 3. LC Filter for Low Output Ripple
1L2
20
H
C1
100µF