CM2596

2005/03/04

Champion Microelectronic Corporation

Page 9

Output Voltage 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 ripple voltage of a switching power supply will contain a sawtooth ripple voltages at the switcher frequency, typically about

1% of the output voltages, and may also contain short voltage spikes 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 additional small LC filter (30uH & 100uF) (as shown in Figure 3) will possibly provide a 10X

reduction in output ripple voltage and transients.

3uF

470uF

Cin

L1

68uH

220uF

Cout

Inductor Selection

The CM2596 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 300mA) 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, toriod, 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.