ISL6446IAZ-TK Datasheet, PDF (14/19 Pages) Intersil Corporation – Dual (180° Out-of-Phase) PWM and Linear Controller

ISL6446
Application Guidelines
PWM Controller
DISCUSSION
The PWM must be compensated such that it achieves the
desired transient performance goals, stability, and DC regulation
requirements.
The first parameter that needs to be chosen is the switching
frequency, F_SW. This decision is based on the overall size
constraints and the frequency plan of the end equipment.
Smaller space requires higher frequency. This allows the output
inductor, input capacitor bank, and output capacitor bank to be
reduced in size and/or value. The power supply must be designed
such that the frequency and its distribution over component
tolerance, time and temperature causes minimal interference in
RF stages, IF stages, PLL loops, mixers, etc.
INDUCTOR SELECTION
The output inductor is selected to meet the output voltage ripple
requirements and minimize the converter’s response time to the
load transient. The inductor value determines the converter’s
ripple current, and the ripple voltage is a function of the ripple
current. The ripple current and voltage are approximated by the
following Equations 7 and 8, where ESR is the output
capacitance ESR value.
I
=
-^V----^I--^N------^-----^V----^O----^U-----^T-
F_SW • L
•
V_{-----O----U----T--}
V_IN
(EQ. 7)
ΔV_OUT = ΔI x ESR
(EQ. 8)
Increasing the value of inductance reduces the ripple current and
voltage. However, the large inductance value reduces the
converter’s response time to a load transient (and usually
increases the DCR of the inductor, which decreases the
efficiency). Increasing the switching frequency (F_SW) for a given
inductor also reduces the ripple current and voltage.
One of the parameters limiting the converter’s response to a load
transient is the time required to change the inductor current.
Given a sufficiently fast control loop design, the ISL6446 will
provide either 0% or 100% duty cycle in response to a load
transient. The response time is the time required to slew the
inductor current from an initial current value to the transient
current level. During this interval, the difference between the
inductor current and the transient current level must be supplied
by the output capacitor. Minimizing the response time can
minimize the output capacitance required.
The response time to a transient is different for the application of
load and the removal of load. The following Equations give the
approximate response time interval for application and removal
of a transient load:
t_RISE
=
L_{----O-----U----T-----}×_----I_{--T----R----A----N--}
V_IN – V_OUT
(EQ. 9)
t_FALL
=
L_{----O-----U----T----}×_-----I_{--T----R----A----N--}
V_OUT
(EQ. 10)
where: I_TRAN is the transient load current step, t_RISE is the
response time to the application of load, and t_FALL is the
response time to the removal of load. With a +5V input source,
the worst case response time can be either at the application or
removal of load and dependent upon the output voltage setting.
Be sure to check both of these equations at the minimum and
maximum output levels for the worst case response time.
Finally, check that the inductor I_sat rating is sufficiently above the
maximum output current (DC load plus ripple current).
OUTPUT CAPACITOR SELECTION
An output capacitor is required to filter the output and supply the
load transient current. The filtering requirements are a function
of the switching frequency and the ripple current. The load
transient requirements are a function of the slew rate (di/dt) and
the magnitude of the transient load current. These requirements
are generally met with a mix of capacitors and careful layout.
Modern microprocessors produce transient load rates above
1A/ns. High frequency capacitors initially supply the transient
and slow the current load rate seen by the bulk capacitors. The
bulk filter capacitor values are generally determined by the ESR
(effective series resistance) and voltage rating requirements
rather than actual capacitance requirements.
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 on specific decoupling requirements. Keep
in mind that not all applications have the same requirements; some
may need many ceramic capacitors in parallel; others may need
only one.
Use only specialized low-ESR capacitors intended for switching-
regulator applications for the bulk capacitors. The bulk
capacitor’s ESR will determine the output ripple voltage and the
initial voltage drop after a high slew-rate transient. An aluminum
electrolytic capacitor's ESR value is related to the case size with
lower ESR available in larger case sizes. However, the equivalent
series inductance (ESL) of these capacitors increases with case
size and can reduce the usefulness of the capacitor to high slew-
rate transient loading. Unfortunately, ESL is not a specified
parameter. Work with your capacitor supplier and measure the
capacitor’s impedance with frequency to select a suitable
component. In most cases, multiple electrolytic capacitors of
small case size perform better than a single large case capacitor.
14
FN7944.1
October 15, 2013