ISL6563 Datasheet, PDF (18/19 Pages) Intersil Corporation – Two-Phase Multiphase Buck PWM Controller with Integrated MOSFET Drivers

ISL6563
The output inductor of each power channel controls the ripple
current. The control IC is stable for channel ripple current
(peak-to-peak) up to twice the average current. A single
channel’s ripple current is approximated using Equation 25:
I_{L, PP}
=
-^V----^I--^N-----–----^V-----^O----^U----^T--
F_SW ⋅ L
×
V_{-----O----U----T--}
V_IN
(EQ. 25)
The current from multiple channels tend to cancel each other
and reduce the total ripple current. The total output ripple
current can be determined using the curve in Figure 10; it
provides the total ripple current as a function of duty cycle
and number of active channels, normalized to the parameter
K_NORM at zero duty cycle.
K_NORM = _L---V--â‹--^O-F---^U-_S----^TW----
(EQ. 26)
where L is the channel inductor value.
Find the intersection of the active channel curve and duty
cycle for your particular application. The resulting ripple
current multiplier from the y-axis is then multiplied by the
normalization factor, K_NORM, to determine the total output
ripple current for the given application.
ΔI_TOTAL = K_NORM ⋅ K_CM
(EQ. 27)
1.0
0.8
0.6
0.4
0.2
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O/V_IN)
FIGURE 10. RIPPLE CURRENT vs DUTY CYCLE
Input Capacitor Selection
The important parameters for the bulk input capacitors 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. The input RMS current required for
a multiphase converter can be approximated with the aid of
Figure 11.
0.3
0.2
0.1
0
0
I_L,PP = 0
I_L,PP = 0.5 x I_O
I_L,PP = 0.75 x I_O
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O /V_IN)
FIGURE 11. 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 11 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,PP) to the maximum sustained load current,
I_O. Figure 11 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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FN9126.8
June 10, 2010