ISL6226 Datasheet, PDF (15/17 Pages) Intersil Corporation – Advanced PWM and Linear Power Controller for Portable Applications

ISL6226
Selection of the output capacitors is also dependent on the
output inductor so some inductor analysis is required to
select the output capacitors.
One of the parameters limiting the converter’s response to a
load transient is the time required for the inductor current to
slew to it’s new level. Given a sufficiently fast control loop
design, the ISL6226 will provide either 0% or 93% duty cycle
in response to a load transient. The response time is the
time interval required to slew the inductor current from an
initial current value to the load current level. During this
interval the difference between the inductor current and the
transient current level must be supplied by the output
capacitor(s). Minimizing the response time can minimize the
output capacitance required. Also, if the load transient rise
time is slower than the inductor response time, as in a hard
drive or CD drive, this reduces the requirement on the output
capacitor.
The maximum capacitor value required to provide the full,
rising step, transient load current during the response time of
the inductor is:
C_OUT
=
(^---V^----_I^-L-_N^{---O--–---×--}V^---I-_O^{-T----RU----AT---})^-N---×^-----2^-
×
--^I--^T----^R----^A----^N----
DV_OUT
Where: C_OUT is the output capacitor(s) required, L_O is the
output inductor, I_TRAN is the transient load current step, V_IN
is the input voltage, V_OUT 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 = ∆IL × ESR
where, ∆IL is calculated in the Inductor Selection section.
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, at 300kHz, for the bulk
capacitors. In most cases, multiple electrolytic capacitors of
small case size 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 1.8kHz and
45kHz. This range is set by an internal, single compensation
zero at 9kHz. 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----------------------
2 × π × ESR × f_Z
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,
and
3. The ESR zero should be placed, in a rather large range,
to provide additional phase margin.
Output Inductor Selection
The output inductor is selected to meet the output voltage
ripple requirements. The inductor value determines the
converter’s ripple current and the ripple voltage is a function
of the ripple current and output capacitor(s) ESR. The ripple
voltage expression is given in the capacitor selection section
and the ripple current is approximated by the following
equation:
∆IL
=
-^V----^I--^N-_F----–_S----^V-_×----^O-_L---^U----^T--
×
V_{-----O----U----T--}
V_IN
where F_s is the switching frequency.
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 load. Depending on
the specifics of the input power and it’s impedance, most (or
all) of this current is supplied by the input capacitor(s).
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
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