ISL6364 Datasheet, PDF (38/44 Pages) Intersil Corporation – Dual 4-Phase + 1-Phase PWM Controller for VR12/IMVP7 Applications

ISL6364
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. The input RMS current can be calculated with
Equation 49.
I_{IN, RMS} = K_{I2N, CM} • Io² + K_{R2AMP, CM} • I_{L2o, PP}
(EQ. 49)
0.3 _{IL,PP = 0}
I_L,PP = 0.25 I_O
0.2
0.1
I_L,PP = 0.5 I_O
I_L,PP = 0.75 I_O
K_{IN, CM} =
-⁽--^N-----^•-----^D-----–-----^m------+-----¹----⁾----^•----⁽--^m-------–-----^N-----^•----^D-----⁾
N²
(EQ. 50)
K_{RAMP, CM} =
m_------²_----(_--N_-----•_-----D_{-----–-----}m_------+-----1_----)_--³_-----+-----(_--m_{-------–-----}1_----)_--²_---(_---m_{-------–----}N_-----•_-----D_----)_--³_--
12 N ² D ²
(EQ. 51)
0.3
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_OUT/V_IN)
FIGURE 30. 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
0.2
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_OUT/V_IN)
FIGURE 29. NORMALIZED INPUT-CAPACITOR RMS CURRENT vs
DUTY CYCLE FOR 2-PHASE CONVERTER
For a 2-phase design, use Figure 29 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.
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_OUT/V_IN)
FIGURE 31. NORMALIZED INPUT-CAPACITOR RMS CURRENT vs
DUTY CYCLE FOR 4-PHASE CONVERTER
Figures 27 and 28 provide the same input RMS current
information for 3 and 4-phase designs respectively. Use the
same approach to selecting the bulk capacitor type and number
as previously described.
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 noise suppression.
38
FN6861.0
December 22, 2010