8
FN4324.2
December 27, 2004
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 transient. The inductor value determines 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:
Increasing the value of inductance reduces the 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
HIP6012 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:
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 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.
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 current rating. These
capacitors must be capable of handling the surge-current at
power-up. The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
MOSFET Selection/Considerations
The HIP6012 requires 2 N-Channel power MOSFETs.
These should be selected based upon r
DS(ON)
, gate supply
requirements, and thermal management requirements.
In high-current applications, the MOSFET power dissipation,
package selection and heatsink are the dominant design
factors. The power 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 factor (see the equations below). Only the
upper MOSFET has switching losses, since the Schottky
rectifier clamps the switching node before the synchronous
rectifier turns on.
∆I =
V
IN
- V
OUT
Fs x L
--------------------------------
V
OUT
V
IN
----------------
•
∆V
OUT
= ∆I x ESR
t
FALL
L
O
I
TRAN
×
V
OUT
-------------------------------=t
RISE
L
O
I
TRAN
×
V
IN
V
OUT
–
--------------------------------=
P
UPPER
= I
O
2
x r
DS(ON)
x D +
1
2
Io x V
IN
x t
SW
x Fs
P
LOWER
= I
O
2
x r
DS(ON)
x (1 - D)
Where: D is the duty cycle = V
O
/ V
IN
,
t
SW
is the switching interval, and
Fs is the switching frequency.
HIP6012
