ISL6568_06 Datasheet, PDF (25/29 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications

ISL6568
Most capacitor solutions rely on a mixture of high frequency
capacitors with relatively low capacitance in combination
with bulk capacitors having high capacitance but limited
high-frequency performance. Minimizing the ESL of the
high-frequency capacitors allows them to support the output
voltage as the current increases. Minimizing the ESR of the
bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors also creates the majority of
the output-voltage ripple. As the bulk capacitors sink and
source the inductor ac ripple current (See Interleaving and
Equation 2), a voltage develops across the bulk capacitor
ESR equal to I_C,PP (ESR). Thus, once the output capacitors
are selected, the maximum allowable ripple voltage,
V_PP(MAX), determines the lower limit on the inductance.
Switching frequency is determined by the selection of the
frequency-setting resistor, R_T. Figure 21 and Equation 34
are provided to assist in selecting the correct value for R_T.
R_T = 10^{[10.61 – 1.035log(fS)]}
(EQ. 34)
1000
100
L
≥
(ESR)


V_I
N
–
N
V_O
U

T_
V_O
U
T
-----------------------------------------------------------
f _S V _{I N} V _{P P( M A X )}
(EQ. 31)
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
∆V_MAX. This places an upper limit on inductance.
Equation 32 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 33
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 ⋅ C ⋅ V_O
---------------------------------
(∆I)²
∆V_MAX – (∆I ⋅ ESR)
(EQ. 32)
L ≤ ⁽---1---.--2---5----⁾----^â‹---N-----^â‹---C---
(∆I)²
∆V_MAX – (∆I ⋅ ESR)


V_IN
–
V_{O}
(EQ. 33)
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.
10
10
100
1000
10000
SWITCHING FREQUENCY (kHz)
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.
0.3
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_IN/V_O)
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 2-PHASE CONVERTER
For a two-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,PP) to I_O. Select a
bulk capacitor with a ripple current rating which will minimize
25
FN9187.4
March 9, 2006