ISL6420A Datasheet, PDF (18/21 Pages) Intersil Corporation – Advanced Single Synchronous Buck Pulse-Width Modulation PWM Controller

ISL6420A
The compensation gain uses external impedance
networks Z_FB and Z_IN to provide a stable, high
bandwidth (BW) overall loop. A stable control loop has
a gain crossing with -20dB/decade slope and a phase
margin greater than 45°. Include worst case
component variations when determining phase margin.
Component Selection
Guidelines
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. For example, Intel recommends that the
high frequency decoupling for the Pentium Pro be
composed of at least forty (40) 1.0µF ceramic
capacitors in the 1206 surface-mount package.
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.
Output Inductor Selection
The output inductor is selected to meet the output
voltage ripple requirements and minimize the
converter’s response time to the load transients. The
inductor value determines the converter’s ripple
current and the ripple voltage is a function of the ripple
current and the output capacitors ESR. The ripple
voltage and current are approximated by Equations 10
and 11:
ΔI_L =
V_{-----I--N------}-_----V_{----O----U-----T-}
Fs x L
⋅
V_{-----O----U----T--}
V_IN
(EQ. 10)
ΔV_OUT = ΔI_L ⋅ ESR
(EQ. 11)
Increasing the value of inductance reduces the ripple
current and voltage. However, larger inductance values
reduce the converter’s response time to a load
transient.
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 ISL6420A 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. Equations
12 and 13 give the approximate response time interval
for application and removal of a transient load:
t_RISE
=
-^L---^O------^×-----^I--^T---^R----^A----^N---
V_IN – V_OUT
(EQ. 12)
t_FALL
=
L_----O------×_----I_{--T----R----A----N--}
V_OUT
(EQ. 13)
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.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the
voltage overshoot across the MOSFETs. Use small
ceramic capacitors for high frequency decoupling and
bulk capacitors to supply the current needed each
time Q₁ turns on. Place the small ceramic capacitors
physically close to the MOSFETs and between the
drain of Q₁ and the source of Q₂.
The important parameters for the bulk input capacitor
are the voltage rating and the RMS current rating. For
reliable operation, select the bulk capacitor 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 x
18
FN9169.4
December 4, 2009