ISL6567_07 Datasheet, PDF (25/26 Pages) Intersil Corporation – Multipurpose Two-Phase Buck PWM Controller with Integrated MOSFET Drivers

ISL6567
current ratings above the maximum input voltage and
largest RMS current required by the circuit. The capacitor
voltage rating should be at least 1.25 times greater than the
maximum input voltage. The input RMS current required for
a multi-phase converter can be approximated with the aid
of Figure 28. For a more exact calculation of the input RMS
current use the following equation:
I_IN(RMS) =
I_O2
⋅
(D
–
D²)
+
I^2L_,
PP
⋅
--^D----
12
0.3
0.2
0.1
0
0
I_L,PP = 0
I_L,PP = 0.5 x I_O
I_L,PP = 0.75 x I_O
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O /V_IN)
FIGURE 28. NORMALIZED INPUT RMS CURRENT vs DUTY
CYCLE FOR A 2-PHASE CONVERTER
As 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. Figure 28 can be used to determine the
input-capacitor RMS current function of duty cycle,
maximum sustained output current (I_O), and the ratio of the
peak-to-peak inductor current (I_L,PP) to the maximum
sustained load current, I_O.
Use a mix of input bypass capacitors to control the input
voltage ripple. Use ceramic capacitance for the high
frequency decoupling and bulk capacitors to supply the
RMS current. Minimize the connection path inductance of
the high frequency decoupling ceramic capacitors (from
drain of upper MOSFET to source of lower MOSFET).
For bulk capacitance, several electrolytic or high-capacity MLC
capacitors may be needed. For surface mount designs, solid
tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating.
These capacitors must be capable of handling the
surge-current at power-up.
APPLICATION SYSTEM DC TOLERANCE
Although the ISL6567 features a tight voltage reference, the
overall system DC tolerance can be affected by the
tolerance of the other components employed. The resistive
divider used to set the output voltage will directly influence
the system DC voltage tolerance. Figure 29 details the
absolute worst case tolerance stack-up for 1% and 0.1%
feedback resistors, and assuming the ISL6567 is regulating
at 0.8% above its nominal reference. Other component
tolerance stack-ups may be investigated using the following
equation, where REF_TM, R_PTM, and R_STM are the tolerance
multipliers corresponding to V_REF, R_S, and R_P, respectively.
TOL
=
-^R----^E-----^F----^T---^M------^â‹----⁽-----^{k--------–----------1--------)------k⋅------R---â‹}------^R-^S-------T-^{P------M--T-------M}---+----------^R---------P--------^T-------^{M---------–----1}--
100
[%]
2.8
2.6 R_STM = 1.01
R_PTM = 0.99
2.4 _{REFTM = 1.008}
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
1
2
3
4
5
6
7
k = V_OUT/V_REF
R_STM = 1.001
R_PTM = 0.999
REF_TM = 1.008
8
9 10
FIGURE 29. WORST CASE SYSTEM DC REGULATION
TOLERANCE (V_REF AT 0.8% ABOVE NOMINAL)
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25
FN9243.2
March 20, 2007