ISL6398 Datasheet, PDF (52/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
inductor AC ripple current (see “Interleaving” on page 15 and
Equation 2), a voltage develops across the 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, as shown in
Equation 48.
L  ESR  -F---_S----_W----V---_-^O--V--^U--_I--_N-^T----_---VK----_P-^R---_P-^C---_-^M-_M-----_A---_X----_
(EQ. 48)
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 DV_MAX. This
places an upper limit on inductance.
Equation 49 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 50 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----I--_--₂-V----^O-----^U----^T-- V_MAX – I  ESR
(EQ. 49)
L  -1---.--2--_-5--_---^-I--N_--₂----^---C-- V_MAX – I  ESR _V_IN – V_{OUT}
(EQ. 50)
Switching Frequency Selection
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 49, 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” on page 51. Choose the lowest switching
frequency that allows the regulator to meet the transient-response
and output-voltage ripple requirements.
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 51.
I_{IN RMS} = K_{I2N CM}  Io² + K_{R2 AMP CM}  I_{L2o p-p}
(EQ. 51)
K_{IN CM} = ^---^N------^----^D------–----^m-------+-----^1N----^₂---^----^---^m------–-----^N------^----^D-----^
(EQ. 52)
K_{RAMP CM} = ^m------²---^---^N-----^-----^D------–----^m-------+----¹----₁^---³₂----_N^+----₂-^--^m-_D----₂--–----¹----^--²----^--^m-------–----^N------^----^D-----^--³-- _{(EQ. 53)}
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
00
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_OUT/V_IN)
FIGURE 37. NORMALIZED INPUT-CAPACITOR RMS CURRENT vs
DUTY CYCLE FOR 2-PHASE CONVERTER
For a 2-phase design, use Figure 37 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(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 1.25 times
greater than the maximum input voltage.
0.3 _{IL(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
0.2
0.1
00
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_OUT/V_IN)
FIGURE 38. NORMALIZED INPUT-CAPACITOR RMS CURRENT vs
DUTY CYCLE FOR 3-PHASE CONVERTER
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August 13, 2015