ISL9444_1107 Datasheet, PDF (22/24 Pages) Intersil Corporation – Triple, 180° Out-of-Phase, Synchronous Step-Down PWM Controll

ISL9444
voltage, V_O is output voltage, and DV_OUT is the drop in output
voltage allowed during the load transient.
High frequency capacitors initially supply the transient current
and slow the load rate-of-change seen by the bulk capacitors. The
bulk filter capacitor values are generally determined by the ESR
(Equivalent Series Resistance) and voltage rating requirements
as well as actual capacitance requirements.
The output voltage ripple is due to the inductor ripple current and
the ESR of the output capacitors as defined by:
V_RIPPLE = ΔI_L(ESR)
(EQ. 18)
Where I_L is calculated in the “Input Capacitor Selection” on
page 22.
High frequency decoupling capacitors should be placed as close
to the power pins of the load as physically possible. Be careful
not to add inductance in the circuit board wiring that could
cancel the usefulness of these low inductance components.
Consult with the manufacturer of the load circuitry for specific
decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. In most
cases, multiple small-case electrolytic capacitors perform better
than a single large-case capacitor.
The stability requirement on the selection of the output capacitor
is that the ‘ESR zero’ (f_Z) be between 2kHz and 60kHz. This range
is set by an internal, single compensation zero at 8.8kHz. The
ESR zero can be a factor of five on either side of the internal zero
and still contribute to increased phase margin of the control loop.
Therefore:
C_OUT
=
----------------¹-----------------
2π(ESR)(f_Z)
(EQ. 19)
In conclusion, the output capacitors must meet three criteria:
1. They must have sufficient bulk capacitance to sustain the
output voltage during a load transient while the output
inductor current is slewing to the value of the load transient.
2. The ESR must be sufficiently low to meet the desired output
voltage ripple due to the output inductor current.
3. The ESR zero should be placed, in a rather large range, to
provide additional phase margin.
The recommended output capacitor value for the ISL9444 is
between 100µF to 680µF, to meet stability criteria with external
compensation. Use of aluminum electrolytic (POSCAP) or
tantalum type capacitors is recommended. Use of low ESR
ceramic capacitors is possible with loop analysis to ensure
stability.
Input Capacitor Selection
The important parameters for the bulk input capacitor(s) are the
voltage rating and the RMS current rating. For reliable operation,
select bulk input capacitors with voltage and 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 and
1.5 times is a conservative guideline. The AC RMS input current
varies with the load. The total RMS current supplied by the input
capacitance is:
I_RMS = I_{R2 MS1} + I_{R2 MS2}
(EQ. 20)
Where DC is duty cycle of the respective PWM.
I_RMSx = DC – DC² ⋅ I_O
(EQ. 21)
Depending on the specifics of the input power and its
impedance, most (or all) of this current is supplied by the input
capacitor(s). Figure 30 shows the advantage of having the PWM
converters operating out-of-phase. If the converters were
operating in phase, the combined RMS current would be the
algebraic sum, which is a much larger value as shown. The
combined out-of-phase current is the square root of the sum of
the square of the individual reflected currents and is significantly
less than the combined in-phase current.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
IN PHASE
OUT-OF-PHASE
3.3V
1
2
3
4
3.3V AND 5V LOAD CURRENT
5V
5
FIGURE 30. INPUT RMS CURRENT vs LOAD
Use a mix of input bypass capacitors to control the voltage ripple
across the MOSFETs. Use ceramic capacitors for the high
frequency decoupling and bulk capacitors to supply the RMS
current. Small ceramic capacitors can be placed very close to the
upper MOSFET to suppress the voltage induced in the parasitic
circuit impedances.
For board designs that allow through-hole components, the
Sanyo OS-CON™ series offer low ESR and good temperature
performance. 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. The
TPS series available from AVX is surge current tested.
22
FN7665.1
July 19, 2011