ISL6315_07 Datasheet, PDF (19/20 Pages) Intersil Corporation – Two-Phase Multiphase Buck PWM Controller with Integrated MOSFET Drivers

ISL6315
maximum input voltage. The input RMS current required for
a multiphase converter can be approximated with the aid of
Figure 12.
0.3
0.2
0.1
I_L,P-P = 0
I_L,P-P = 0.5 x I_O
I_L,P-P = 0.75 x I_O
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O /V_IN)
FIGURE 12. NORMALIZED INPUT RMS CURRENT vs DUTY
CYCLE FOR A 2-PHASE CONVERTER
As the input capacitors are responsible for sourcing the AC
component of the input current flowing into the upper
MOSFETs, their RMS current capacity must be sufficient to
handle the AC component of the current drawn by the upper
MOSFETs. Figure 12 can be used to determine the input-
capacitor RMS current function of duty cycle, maximum
sustained output current (I_O), and the ratio of the peak-to-
peak inductor current (I_L,P-P) to the maximum sustained
load current, I_O. Figure 12 can also be used as a reference
demonstrating the dramatic reduction in input capacitor RMS
current in a 2-phase DC/DC converter, as compared to a
single-phase regulator.
Use a mix of input bypass capacitors to control the input
voltage ripple. Use ceramic capacitance for the high
frequency decoupling and bulk capacitors to supply the
RMS current. Minimize the connection path inductance of
the high frequency decoupling ceramic capacitors (from
drain of upper MOSFET to source of lower MOSFET).
For bulk capacitance, several electrolytic or high-capacity MLC
capacitors may be needed. 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.
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FN9222.1
July 18, 2007