ISL6559 Datasheet, PDF (18/21 Pages) Intersil Corporation – Multi-Phase PWM Controller

ISL6559
Application on page 3). Figure 15 and Equation 29 are
provided to assist in the selecting the correct value for R_T.
1000
100
Select a bulk capacitor with a ripple current rating which will
minimize the total number of input capacitors required to
support the RMS current calculated. The voltage rating of
the capacitors should also be at least 1.25 times greater
than the maximum input voltage.
Figures 16 and 17 provide the same input RMS current
information for three and four phase designs respectively.
Use the same approach to selecting the bulk capacitor type
and number as described above.
0.3
I_C,PP = 0
I_C,PP = 0.5 I_O
I_C,PP = 0.25 I_O
I_C,PP = 0.75 I_O
0.2
10
10
100
1000
10000
SWITCHING FREQUENCY (kHz)
0.1
FIGURE 14. R_T vs SWITCHING FREQUENCY
R_T = 10^{[11.09 – 1.13log(fS)]}
(EQ. 29)
Input Capacitor Selection
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 which is related to duty cycle and the number of
active phases.
0.3
0.2
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN / V_O)
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS CURRENT
VS DUTY CYCLE FOR 3-PHASE CONVERTER
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes.The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
close as possible to each upper MOSFET drain to minimize
board parasitics and maximize suppression.
0.3
I_C,PP = 0
I_C,PP = 0.25 I_O
I_C,PP = 0.5 I_O
I_C,PP = 0.75 I_O
0.1
I_C,PP = 0
I_C,PP = 0.5 I_O
I_C,PP = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN / V_O)
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS CURRENT
VS DUTY CYCLE FOR 2-PHASE CONVERTER
For a two phase design, use Figure 15 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I_O), and the ratio
of the combined peak-to-peak inductor current (I_C,PP) to I_O.
18
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN / V_O)
FIGURE 17. NORMALIZED INPUT-CAPACITOR RMS CURRENT
VS DUTY CYCLE FOR 4-PHASE CONVERTER
FN9084.8
December 29, 2004