ISL6443
DC is duty cycle of the respective PWM.
Depending on the specifics of the input power and its
impedance, most (or all) of this current is supplied by the
input capacitor(s). Figure 20 shows the advantage of having
the PWM converters operating out of phase. If the
converters were operating in phase, the combined RMS
current would be the algebraic sum, which is a much larger
value as shown. The combined out-of-phase current is the
square root of the sum of the square of the individual
reflected currents and is significantly less than the combined
in-phase current.
5
4.5
4
IN PHASE
3.5
3
2.5
OUT OF PHASE
2
1.5
5V
3.3V
1
0.5
0
0
1
2
3
4
5
3.3V AND 5V LOAD CURRENT
FIGURE 20. INPUT RMS CURRENT vs LOAD
Use a mix of input bypass capacitors to control the voltage
ripple across the MOSFETs. Use ceramic capacitors for the
high frequency decoupling and bulk capacitors to supply the
RMS current. Small ceramic capacitors can be placed very
close to the upper MOSFET to suppress the voltage induced
in the parasitic circuit impedances.
For board designs that allow through-hole components, the
Sanyo OS-CON® series offer low ESR and good
temperature performance. For surface mount designs, solid
tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating.
These capacitors must be capable of handling the surge-
current at power-up. The TPS series available from AVX is
surge current tested.
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Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
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FN9044.1
17
October 4, 2005
INTERSIL [ Intersil ]
