13
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
s witch ing-regula tor application s for the bulk capacitors . The
bulk capa citor’s ESR will determine the output ripple vo ltage
and the initial voltage drop after a high sle w-r ate tr ansi ent. An
aluminum electrol ytic ca pacitor’s ESR value is related to the
case size with lower ESR av ai lab l e in larger ca se siz es.
How ever, the equivalent series inductance (ESL) of these
capacitors increases with case size and can reduce the
usefulness of the capacito r to high sle w-rate transient loading .
Unf ortunately, ESL is not a specified parameter. Work with
your ca pacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In
most cases, multiple electrolytic capacitors of small case size
perf orm better than a single large case capacito r.
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 transient. Additionally, the output inductor f or
the VTT regulator has to meet the minimum v alue criteria for
loop stability as described in the VTT Feedback
Compensation section. The inductor value determines the
conve rter’s ripple current and the ripple voltage is a functi on
of the ripple current. Th e ripple voltage and current are
appro ximated by the following equations:
Increasing t he v alue of indu ctance reduces t he ripple current
and voltage. However, the large 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
ISL6531 will provide either 0% or 100% duty cycle in
response to a load tra nsi ent. The respo nse 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:
where: ITRAN is the transient load current step, tRISE is the
response time to the application of load, and tFALL is the
response time to the removal of load. The worst case
response time can be either at the application or removal of
load. 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 capa citors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors f or h igh frequency dec oupling and b ulk cap acitors
to supply the current needed each time Q 1 turns on. Place the
small ceramic capacitors physically close to the MOSFETs
and between the dr ain of Q1 and the source of Q2.
The important parameters for the b ulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings abov e 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 a voltage rating of 1.5 times is a
conservative guideline. The RMS current rating requirement
for the input capacitor of a buck regulator is approximately
1/2 the DC load current.
The maximum RMS current required by the regulator ma y be
closely approximated through the following equation:
F or a through hole design, se v er al electrolytic capacitors ma y
be needed. For surface mount designs, solid tantalum
capacitors can be used, but caution must be exercised with
regard to the capacitor surge currentrating. These capacitors
must be capable of handling the surge-current at power-up.
Some capacitor series available from reputab le manufacturers
are surge current tested.
MOSFET Selection/Considerat ions
The ISL6531 requires two N-Channel power MOSFETs f or
each PWM regulator . These should be selected based upon
rDS(ON), gate supply requirements, and thermal management
requirements.
In high-current applications, the MOSFET pow er dissipation,
package selection and heatsink are the dominant design
f actors. The pow er dissipation includes two loss components;
conduction loss and switching loss. The conduction losses are
the largest component of power dissipation for both the upper
and the lower MOSFETs. These losses are distributed between
the two MOSFETs according to duty f actor. The switching
losses seen when sourcing current will be diff erent from the
s witching losses seen whe n sinking curre nt. The VDDQ
regulator will only source current while the VTT regulator can
sink and source. When sourcing current, the upper MOSFET
realizes most of the s witching losses. The lower s witch realiz es
most of the s witching losses when the conv erter is sinking
∆I=VIN - VOUT
fs x L VOUT
VIN ∆VOUT =∆I x ESR
x
tRISE = L x ITRAN
VIN - VOUT tFALL = L x ITRAN
VOUT
RMSMAX
VOUT
VIN
----------------IOUTMAX21
12
------ VIN VOUT
–
Lf
s
×
--------------------------------VOUT
VIN
----------------
×
2
×+
×=
ISL6531