ISL6556A Datasheet, PDF (21/24 Pages) Intersil Corporation – Optimized Multi-Phase PWM Controller with 6-Bit DAC for VR10.X Application

ISL6556A
1000
0.3
100
10
10
100
1000
SWITCHING FREQUENCY (kHz)
FIGURE 13. R_T vs SWITCHING FREQUENCY
10000
Switching frequency is determined by the selection of the
frequency-setting resistor, R_T (see the figures labeled
Typical Application on pages 4 and 5). Figure 13 and
Equation 26 are provided to assist in selecting the correct
value for R_T.
·
R_T = 1.0203(10)^{[10.6258-(1.03167)log(fS)]} – 1200
(EQ. 26)
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.
For a two phase design, use Figure 14 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I_O), and the ratio
of the per-phase peak-to-peak inductor current (I_L,PP) to I_O.
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 15 and 16 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.
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 parasitic impedances and maximize suppression.
21
0.2
0.1
I_L,PP = 0
I_L,PP = 0.5 I_O
I_L,PP = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 14. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 2-PHASE CONVERTER
0.3 _{IL,PP = 0}
I_L,PP = 0.25 I_O
I_L,PP = 0.5 I_O
I_L,PP = 0.75 I_O
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 3-PHASE CONVERTER
0.3
I_L,PP = 0
I_L,PP = 0.25 I_O
I_L,PP = 0.5 I_O
I_L,PP = 0.75 I_O
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER