SC403MLTRT Semtech, SC403MLTRT Datasheet - Page 23

• Manufacturer Part Number: SC403MLTRT
• Manufacturer: Semtech

Specifications of SC403MLTRT

• Lead Free Status / Rohs Status: Supplier Unconfirmed

Applications Information (continued)
The desired switching frequency is 300kHz which results
from using components selected for optimum size and
cost.
A resistor (R
setting the frequency) using the following equation.
To select R
use the value associated with maximum V
Substituting for R
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
shown next.
IN
t
R
R
t
- V
ON
ON
TON
TON
OUT
= 379 ns at 13.2V
= 129.9k
V
). The equation for determining inductance is
TON
t (
INMAX
TON
ON
V
, use the maximum value for V
25
) is used to program the on-time (indirectly
OUT
pF
10
TON
f
SW
Ω
ns
, use R
results in the following solution.
V
)
OUT
V
IN
IN
TON
, 1.5V
= 130k
OUT
, 300kHz
Ω
IN
.
IN
, and for t
ON
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
spond to V
A slightly larger value of 1.5µH is selected. This will
decrease the typical I
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
checked using the following equations.
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.
L
I
I
I
L
t
RIPPLE
RIPPLE
RIPPLE
ON
_
(
(
VINMIN
13
V
_
_
IN
INMAX
MIN
MAX
2 .
(
I
V
RIPPLE
V
.
IN
. 1
OUT
25
(
3
(
10
) 5
10
A
1
1
pF
V
)
5 .
8 .
L
5 .
OUT
8 .
RIPPLE
379
t
ON
H
V
H
R
)
. 1
. 1
INMIN
TON
ns
to 2.7A.
) 5
t
1 (
) 5
1 (
ON
461
IN
. 0
379
V
. 1
. 0
OUT
and t
) 2
48
) 2
ns
ns
H
ON
10
IN
. 2
3
conditions is also
values that corre-
ns
38
7 .
A
A
461
SC403
ns
23