SC403
23
Applications Information (continued)
The desired switching frequency is 300kHz which results
from using components selected for optimum size and
cost.
A resistor (R
TON
) is used to program the on-time (indirectly
setting the frequency) using the following equation.
T
IN
ON
TON
V
pF
25
V
)
ns
10
t
(
R
To select R
TON
, use the maximum value for V
IN
, and for t
ON
use the value associated with maximum V
IN
.
SW
INMAX
OUT
ON
f
V
V
t
t
ON
= 379 ns at 13.2V
IN
, 1.5V
OUT
, 300kHz
Substituting for R
TON
results in the following solution.
R
TON
= 129.9k&, use R
TON
= 130k&
Inductor Selection
In order to determine the inductance, the ripple current
must first be defined. Low inductor values result in smaller
size but create higher ripple current which can reduce effi-
ciency. Higher inductor values will reduce the ripple current
and ripple voltage and for a given DC resistance are more
efficient. However, larger inductance translates directly into
larger packages and higher cost. Cost, size, output ripple,
and efficiency are all used in the selection process.
The ripple current will also set the boundary for PSAVE
operation. The switching will typically enter PSAVE mode
when the load current decreases to 1/2 of the ripple
current. For example, if ripple current is 4A then PSAVE
operation will typically start for loads less than 2A. If ripple
current is set at 40% of maximum load current, then PSAVE
will start for loads less than 20% of maximum current.
The inductor value is typically selected to provide a ripple
current that is between 25% to 50% of the maximum load
current. This provides an optimal trade-off between cost,
efficiency, and transient performance.
During the DH on-time, voltage across the inductor is
(V
IN
- V
OUT
). The equation for determining inductance is
shown next.
RIPPLE
ON
OUT
IN
I
t
)
V
V
(
L
Example
In this example, the inductor ripple current is set equal to
50% of the maximum load current. Therefore ripple
current will be 50% x 6A or 3A. To find the minimum
inductance needed, use the V
IN
and t
ON
values that corre-
spond to V
INMAX
.
H
48
.
1
A
3
ns
379
)
5
.
1
2
.
13
(
L
A slightly larger value of 1.5礖 is selected. This will
decrease the typical I
RIPPLE
to 2.7A.
Note that the inductor must be rated for the maximum DC
load current plus 1/2 of the ripple current.
The ripple current under minimum V
IN
conditions is also
checked using the following equations.
ns
461
ns
10
V
V
R
pF
25
t
INMIN
OUT
TON
VINMIN
_
ON
L
t
)
V
V
(
I
ON
OUT
IN
RIPPLE
A
38
.
2
)
2
.
0
1
(
H
5
.
1
ns
461
)
5
.
1
8
.
10
(
I
MIN
_
RIPPLE
A
7
.
3
)
2
.
0
1
(
H
5
.
1
ns
379
)
5
.
1
8
.
10
(
I
MAX
_
RIPPLE
The value of L has been adjusted by +20% for the equa-
tions above assuming an inductor tolerance of +20%.
Output Capacitor Selection
The output capacitors are chosen based upon required
ESR and capacitance. The maximum ESR requirement is
controlled by the output ripple requirement and the DC
tolerance. The output voltage has a DC value that is equal
to the valley of the output ripple plus 1/2 of the peak-to-
peak ripple. A change in the output ripple voltage will
lead to a change in DC voltage at the output.
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