ISL6443A Datasheet, PDF (17/19 Pages) Intersil Corporation – 300kHz Dual, 180° Out-of-Phase, Step-Down PWM and Single Linear Controller

ISL6443A
Input Capacitor Selection
The important parameters for the bulk input capacitor(s) are
the voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors with voltage and
current ratings above the maximum input voltage and largest
RMS current required by the circuit. The capacitor voltage
rating should be at least 1.25 times greater than the
maximum input voltage and 1.5 times is a conservative
guideline. The AC RMS Input current varies with the load.
The total RMS current supplied by the input capacitance is:
I_RMS = I_{R2 MS1} + I_{R2 MS2}
(EQ.16)
where,
I_RMSx = DC – DC² ⋅ I_O
(EQ.17)
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.
5.0
4.5
4.0
IN-PHASE
3.5
3.0
2.5
OUT-OF-PHASE
2.0
1.5
3.3V
5V
1.0
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.
17
FN6600.1
December 7, 2007