8
Mode rn micr oprocess ors produce tran sient load rates ab ove
1A/ns. High frequency capacitors initially supply the
transient and slow the current load rate seen by the bulk
capacitors. The bulk filter capac itor values are generally
determ ined by the E SR (effective series resist ance) and
voltage rating requirements rather than actual capacitance
requirements.
High frequency decoupling capacitors should be placed as
clos e to the po wer pi ns of t he load as p hysical ly pos si ble. 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 requ ir em ents. For example, Intel
recommends that the hig h 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 l ow-ESR capacitors intended for
switching- regul ator appli cati ons for the bulk capaci tor s.
The bulk c apa ci tor’s ESR will det ermine the o u tput ripple
voltage and the i n itial vo ltage drop aft er a hi gh sl ew-rate
transient. An aluminum electrolytic capacitor's ESR value is
related to the ca se si ze wi th l ower ESR avai la ble in lar ger
case sizes. However, the equivalent series inductance
(ESL ) of these capacitors inc reases with case size and can
reduce the usefulness o f the capacitor to high slew-rate
transient loadi ng. U nf ortunat ely, ESL is not a spe cified
paramet er. Work with your capacitor supplier and measure
the capac itor’s im pedance wi th freq uency t o sel ect a
suitable component. In most cases, multiple electrolytic
capac itors of small case size pe rfo rm better th an a s ingle
large case cap aci to r.
Outp ut Indu ct or S e lection
The out pu t in du ct or is se le c ted to meet the out pu t v o lt a ge
ripple requirements and minimize the converter’s response
ti me to t he load tr ans ient. The i nduct or value det ermi nes the
converter’s ripple current and the ripple voltage is a function
of the ripple current. The ripple voltage and current are
approximated by the following equations:
Incr easing the value of induc tance r educe s the r ipple current
and voltage. However, the large inductance values reduce
the converter’s response time to a load transient.
One of the paramete rs limit ing the convert er’s respons e to a
load transient is the time required to change the inductor
current. Given a suffic iently fast control loop design, the
HIP6006 will provi de eith e r 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 transi ent current l evel. During this interva l the
difference between the inductor current and the transient
current l evel 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
appli cati on and removal of a transient load:
where: ITRAN is the transient load current step, tRISE is the
respon se tim e to th e ap plicatio n of load, and tFALL 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 leve ls for
the worst case response time.
Input Capaci tor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capaci tors for high freque ncy decoupling and bulk capacitor s
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 im portant paramet er s for the bulk input capaci tor are th e
voltage rating and the R MS curren t 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 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 l oad current.
For a through hole design, several electrolytic capacitors
(Panasonic HFQ series or Nichicon PL series or Sanyo MV-
GX or equivalent) may be needed. For surface mount
designs, solid tantalum capacitors can be used, but caution
must be exercised with regard to the capacitor surge cu rrent
rating. These capacitors must be capable of handling the
surge-curr ent at power-up. The TPS series avail able from
AVX, and the 593D series from Sprague are bo th sur ge
current tested.
MOSFET Selecti on/Considerations
The HIP6006 requi res 2 N-Channel power MOSFETs. These
should be selected based upon rDS(ON), gate supply
requirements, and thermal management requirements.
In hi gh- cur ren t a pp li cat io ns, the MOSFET power dissipation,
package selection and heatsink are the dominant design
fac tors . T he p o wer dissi patio n in cludes two loss
components; conduction loss and switching loss. The
∆VOUT=∆I x ESR
∆I = VIN - VOUT
Fs x L
--------------------------------VOUT
VIN
----------------
•
tFALL LOITRAN
×
VOUT
-------------------------------=tRISE LOITRAN
×
VIN VOUT
–
--------------------------------=
HIP6006