ISL6443
selected to meet the dynamic regulation requirements
including ripple voltage and load transients. Selection of
output capacitors is also dependent on the output inductor,
so some inductor analysis is required to select the output
capacitors.
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,
1
------------------------------------
C
=
OUT
2Π(ESR)(f )
Z
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. The ISL6443 will provide either 0% or
71% duty cycle in response to a load transient.
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
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, it reduces the
requirement on the output capacitor.
3. The ESR zero should be placed, in a rather large range,
to provide additional phase margin.
The recommended output capacitor value for the ISL6443 is
between 150µF 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 but would take
more rigorous loop analysis to ensure stability.
The maximum capacitor value required to provide the full,
rising step, transient load current during the response time of
the inductor is:
2
(L )(I
)
O
TRAN
----------------------------------------------------------
=
C
OUT
2(V – V )(DV
)
Output Inductor Selection
IN
O
OUT
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:
where, C
OUT
is the output capacitor(s) required, L is the
O
output inductor, I
is the transient load current step, V
TRAN
IN
is the
is the input voltage, V is output voltage, and DV
drop in output voltage allowed during the load transient.
O
OUT
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.
(V – V
)(V
)
IN
OUT
OUT
---------------------------------------------------------
=
∆I
L
(f )(L)(V
)
S
IN
For the ISL6443, Inductor values between 6.4µH to 10µH is
recommended when using the Typical Application
Schematic. Other values can be used but a thorough stability
study should be done.
The output voltage ripple is due to the inductor ripple current
and the ESR of the output capacitors as defined by:
V
= ∆I (ESR)
L
RIPPLE
Input Capacitor Selection
where, I is calculated in the Inductor Selection section.
L
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
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.
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:
Use only specialized low-ESR capacitors intended for
switching-regulator applications at 300kHz for the bulk
capacitors. In most cases, multiple small-case electrolytic
capacitors perform better than a single large-case capacitor.
2
2
I
=
I
+ I
RMS
RMS1
RMS2
where,
The stability requirement on the selection of the output
capacitor is that the ‘ESR zero’, f , be between 1.2kHz and
2
I
=
DC – DC
Z
RMSx
FN9044.1
16
October 4, 2005
INTERSIL [ Intersil ]
