20
FN6288.5
October 7, 2008
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 Equation 19:
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
ISL6540A 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. Equation 20
gives 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 lower input
source such as 1.8V or 3.3V, 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 Q
1
turns on. Place the
small ceramic capacitors physically close to the MOSFETs
and between the drain of Q
1
and the source of Q
2
.
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 1.25x greater than the maximum input
voltage and a voltage rating of 1.5x is a conservative
guideline. The RMS current rating requirement for the input
capacitor of a buck regulator is approximated in Equation 21.
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. Figure 11 provides an easy
graphical approximation of the input RMS requirements for a
single-phase buck converter.
MOSFET Selection/Considerations
The ISL6540A 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
ΔI =
V
IN
- V
OUT
F
S
x L
--------------------------------
V
OUT
V
IN
----------------
•
ΔV
OUT
= ΔIESR×
(EQ. 19)
t
FALL
L
O
I
TRAN
×
V
OUT
-------------------------------
=t
RISE
L
O
I
TRAN
×
V
IN
V
OUT
–
------------------------------- -
=
(EQ. 20)
I
IN RMS,
I
O
2
DD
2
–()
IΔ
2
12
--------
D+=
D
V
O
VIN
----------
=
OR
I
INRMS
K
ICM
I
O
•=
(EQ. 21)
FIGURE 11. INPUT-CAPACITOR CURRENT MULTIPLIER FOR
SINGLE-PHASE BUCK CONVERTER
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
K
ICM
DUTY CYCLE (D)
ΔI
LOUT
= 0.25 x I
out
ΔI
LOUT
= 0.5 x I
out
ΔI
LOUT
= 0
0.0
0.2
0.3
ISL6540A
