11
One of the parameters limiting the converter’s response to a
load transient is the time required for the inductor current to
slew to its new level. Given a sufficiently fast control loop
design, the IPM6220A will provide either 0% or 94% 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:
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 V
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:
where, I
L
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 1.2kHz and
30kHz. This range is set by an internal, single compensation
zero at 6kHz. 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:
In conclusion, the output capacitors must meet three criteria: By
varying the values of the soft-start capacitors, it is possible to
provide sequencing of the main outputs at start-up.
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.
3.3V ALWAYS and 5V ALWAYS Output Capacitors
The output capacitors for the linear regulators insure stability
and provide dynamic load current. The 3.3V ALWAYS and
the 5V ALWAYS linear regulators should have, as a
minimum, 10F capacitors on their outputs.
3.3V Main and 5V Main PWM Output Inductor
Selection
The PWM converters require output inductors. 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:
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 as shown in
Figure 9. 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 9 also 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.
C
OUT
L
O
I
TRAN
V
IN
V
OUT
–2
----------------------------------------------
I
TRAN
DV
OUT
--------------------
=
C
OUT
1
2 ESR f
Z
-------------------------------------------
=
I
L
V
IN
V
OUT
–
F
S
L
--------------------------------
V
OUT
V
IN
----------------
=
IPM6220A
OS-CON® is a registered trademark of Sanyo Electric Company, Ltd. (Japan)
