ISL6322G Datasheet, PDF (35/39 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers, I2C Interface and Phase Dropping

ISL6322G
bulk capacitor ESR equal to I_C(P-P) (ESR). Thus, once the
output capacitors are selected, the maximum allowable
ripple voltage, V_(P-P)(MAX), determines the lower limit on the
inductance.
L
≥ ESR ⋅
-^⎝⎛--V-----^I-^N------^–----N-------^â‹---V----^O----^U-----^T---^⎠⎞----^â‹----V----^O----^U-----^T-
f_S ⋅ V_IN ⋅ V_(P-P)(MAX)
(EQ. 38)
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
ΔV_MAX. This places an upper limit on inductance.
Equation 39 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 40
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
≤
---------------------------------
(ΔI)²
⋅
ΔV_MAX – (ΔI ⋅ ESR)
(EQ. 39)
L
≤
-1---.--2---5-----^â‹---N------^â‹---C--
(ΔI)²
⋅
ΔV_MAX – (ΔI ⋅ ESR)
⋅ _⎝⎛V_IN – V_{O⎠⎞}
(EQ. 40)
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” on page 31, 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
“Compensation” on page 34. 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 23 and Equation 41
are provided to assist in selecting the correct value for R_T.
RT
=
[10.61
10
–
(1.035
⋅
log
(fS))]
(EQ. 41)
1000
100
10
10k
100k
1000k
SWITCHING FREQUENCY (Hz)
FIGURE 23. R_T vs SWITCHING FREQUENCY
10000k
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 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,(P-P)) to I_O.
0.3
0.2
0.1
I_L(P-P) = 0
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
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 2-PHASE CONVERTER
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. Figure 25 provides the same input
RMS current information for single-phase designs. Use the
same approach for selecting the bulk capacitor type and
number.
35
FN6715.0
May 22, 2008