X RP7 6 6 5
3 A 1 8 V S y n c h r o n o u s S t e p - D o w n Co n v e r t e r
OUTPUT INDUCTOR
Select the output inductor for inductance L, DC
current rating I
DC
and saturation current rating
I
SAT
. I
DC
should be larger than regulator output
current. I
SAT
, as a rule of thumb, should be
50% high er than the regulator output current.
Since the regulator is rated at
3A then I
DC
3A
and I
SAT
4.5 A. Calculate the inductance from:
of thumb, should be at least twice the output
voltage. When calculating
the
required
capacitance,
usually
the
overriding
requirement is current load
- step transient. If
the unloading transient (i.e., when load
transitions from a high to a low
current ) is
met, then usually the loading transient (when
load transitions from a low to a high
cu rrent )
is met as well.
Therefore c alculate the C
OUT
based on the unloading transient requirement
from:
Where:
is peak - to - peak inductor current ripple
nominally set to 30%
- 40% of I
OUT
L
Where:
L is the inductance calculated in the preceding
step
I
High
is the value of load - step prior to
unloading. This is nominally set equal to
regulator current rating (
3A).
I
Low
is the value of load - step after unloading.
This is nominally set equal to
50% of regulator
current rating ( 1.5 A).
V
transient
is the maximum permissible voltage
transient corresponding to the load step
mentioned above. V
transient
is typically specified
from 3% to 5% of
V
OUT
.
ESR of the capacitor ha s to be selected such
that the output voltage ripple requirement
OUT
, nominally 1% of V
OUT
, is met. Voltage
ripple
is mainly composed of t wo
OUT
components : the resistive ripple due to ESR
and capacitive ripple due to
C
OUT
charge
transfer. For applications requiring low
voltage
ripple , ceramic capacitors are recommended
because of their low ESR
which is typically
in
the range of
. Therefore
OUT
is mainly
capacitive. For ceramic capacitors c alculate
the
OUT
from:
f
S
is nominal switching frequency (340kHz)
As an example , inductor values for
several
common output voltages are shown in table
s1
and 2 . Note that example inductors shown in
table s 1 and 2 are Wurth shielded inductors. If
the target application is not
sensitive to EMI
then unshielded inductors may be used.
VOUT(V)
5.0
3.3
2.5
1.8
1.5
1.2
L( p - p )
(A)
L( µ H)
10
10
10
7.6
7.6
4.9
Inductor
Example
744314101
744314101
744314101
744314 760
744314 760
744314 490
IN
=12V
0.9
0.7
0.6
0.6
0.5
0.6
Table 1 : Suggested inductor values for V
I
OUT
= 3A
and
VOUT(V)
3.3
2.5
1.8
1.5
1.2
L(p - p)
(A)
L(µH)
4.9
4.9
4.9
4.9
4.9
Inductor
Example
744314
744314
744314
744314
744314
IN
=5V
0.7
0.8
0.7
0.6
0.5
490
490
490
490
490
and
Table 2 : Suggested inductor values for V
I
OUT
= 3A
Where:
L
is from table 1
or 2
uency (340kHz)
OUTPUT CAPACITOR
C
OUT
C
OUT
is the value calculated above
f
s
is nominal switching freq
Select the output capacitor for
voltage rating,
capacitance C
OUT
and
Equivalent Series
Resistance ESR . The voltage rating, as a rule
© 2013 Exar Corporation
8/ 12
Rev. 2.0. 1
EXAR [ EXAR CORPORATION ]
