ISL6312A_15 Datasheet, PDF (32/36 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR10,VR11, and AMD Applications

ISL6312A
output voltage deviation than the leading edge. Equation 47
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
L  -2----^---N---_--_-^---IC--_--₂--^---V----^O---  V_MAX – I  ESR
(EQ. 47)
0.3 _{IL(P-P) = 0}
I_L(P-P) = 0.25 I_O
0.2
0.1
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
L  -1---.--2--_-5--_---^-I--N_--₂----^---C--  V_MAX – I  ESR  _V_IN – V_{O}
(EQ. 48)
Switching Frequency
There are a number of variables to consider when choosing the
switching frequency, as there are considerable effects on the
upper MOSFET loss calculation. These effects are outlined in
MOSFETs, and they establish the upper limit for the switching
frequency. The lower limit is established by the requirement for
fast transient response and small output-voltage ripple. Choose
the lowest switching frequency that allows the regulator to meet
the transient-response requirements.
Switching frequency is determined by the selection of the
frequency-setting resistor, R_T. Figure 21 and Equation 49
are provided to assist in selecting the correct value for R_T.
RT = 10^{10.61 – 1.035  log fS}
(EQ. 49)
1000
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER
For a four-phase design, use Figure 22 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (I_O), and the ratio
of the peak-to-peak inductor current (I_L,P-P) 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
1.25x greater than the maximum input voltage. Figures 23
and 24 provide the same input RMS current information for
three-phase and two-phase designs respectively. Use the
same approach for selecting the bulk capacitor type and
number.
0.3
I_L(P-P) = 0
I_L(P-P) = 0.25 I_O
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
100
0.2
10
10
100
1k
10k
SWITCHING FREQUENCY (Hz)
FIGURE 21. R_T vs SWITCHING FREQUENCY
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.
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN/V_O)
FIGURE 23. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 3-PHASE CONVERTER
32
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FN9290.6
January 22, 2015