ISL6308 Datasheet, PDF (24/27 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308
Equation 31 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output-voltage deviation than the leading edge. Equation 32
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.
2 ⋅ N ⋅ C ⋅ V_O
L
≤
---------------------------------
(∆I)²
⋅
∆V_MAX – (∆I ⋅ ESR)
(EQ. 31)
L
≤
(_{---1---.--2---5----})_----⋅_---N-----⋅_---C---
(∆I)²
⋅
∆V_MAX – (∆I ⋅ ESR)
⋅


V_IN
–
V_{O}
(EQ. 32)
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 as outlined in Output Filter Design. 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_FS. Figure 23 and Equation 33
are provided to assist in selecting the correct value for R_FS
R_FS
=
[10.61
10
–
1.035
⋅
log
(FSW
)]
(EQ. 33)
200
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
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_IN/V_O)
FIGURE 24. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 3-PHASE CONVERTER
For a three-phase design, use Figure 24 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,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 25 and 26 provide the same input
RMS current information for two-phase and single-phase
designs respectively. Use the same approach for selecting
the bulk capacitor type and number.
0.3
100
0.2
50
20
10
100K
200K
500K
1M
2M
SWITCHING FREQUENCY (Hz)
FIGURE 23. R_FS 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
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_IN/V_O)
FIGURE 25. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 2-PHASE CONVERTER
24
FN9208.2
October 19, 2005