AMC2576
APPLICATION INFORMATION (contd.)
Output Voltage Ripple and Transients
The output ripple voltage is due mainly to the inductor saw tooth ripple current multiplied by the ESR of the output
capacitor.
The output voltage of a switching power supply will contain a saw tooth 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 saw tooth
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 saw tooth 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 additional small LC filter (20µH & 100µF) (as shown in
Figure 3) will possibly provide a 10X reduction in output ripple voltage and transients.
AMC2576-ADJ
FB
4
7V – 40V
DC INPUT
1
V
IN
C
IN
100µF
GND
V
OUT
2
ENABLE
L1
100µH
R2
50K
R1
1.21K
L2
20µH
OUTPUT
C
OUT
1000µF
C
1
100µF
3
5
Figure 3. LC Filter for Low Output Ripple
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.
Copyright
©
2006 ADDtek Corp.
9
DD007_E --
DECEMBER 2006
ADDTEK [ ADDTEK CORP ]
