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SC3303 Datasheet(PDF) 17 Page - Semtech Corporation

Part # SC3303
Description  3A EcoSpeed Step-Down Regulator with LDO and Ultrasonic Power Save
PDF  26 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC3303 Datasheet(HTML) 17 Page - Semtech Corporation

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17
SC3303
17
Applications Information (continued)
The following values are used in this design example.
V
IN
= 24V + 10%
V
OUT
= 3.3V + 4%
f
SW
= 300kHz
Load = 3A maximum
Frequency Selection
Selection of the switching frequency requires making a
trade-off between the size and cost of the external filter
components (inductor and output capacitor) and the
power conversion efficiency.
The desired switching frequency is 500kHz 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.
2
.
1
V
V
400
f
25pF
1
R
OUT
IN
SW
TON
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 = 417 ns at 26.4VIN, 3.3VOUT, 300kHz
Substituting for R
TON
results in the following solution.
R
TON = 157kΩ, use RTON = 154kΩ
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
efficiency. Higher inductor values will reduce the ripple
current/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 power-
save operation. The switching will typically enter power-
save mode when the load current decreases to 1/2 of the
•
•
•
•
ripple current. For example, if ripple current is 2A then
power-save operation will typically start for loads less than
1A. If ripple current is set at 40% of maximum load current,
then power-save will start for loads less than 20% of
maximum current.
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
33% of the maximum load current. Therefore ripple
current will be 33% x 3A or 1A. To find the minimum
inductance needed, use the V
IN
and T
ON
values that corre-
spond to V
INMAX
.
H
A
ns
V
V
L
7
.
8
1
417
)
3
.
3
4
.
26
(
A standard value of 10μH is selected. This gives a maximum
I
RIPPLE
of 963mA. The peak ripple can be calculated by the
equation, below where L
TOL
is assumed to be an inductor
tolerance of 20%.
I
RIPPLE_PEAK = IRIPPLE_MAX / (1 - LTOL) = 1.203A
Note that the inductor must be rated for the maximum DC
load current plus 1/2 of the ripple current.
I
LSAT_MIN = IRIPPLE_PEAK X 0.5 + IOUT = 3.602A
The ripple current under minimum V
IN
conditions is also
checked using the following equations.
L
t
)
V
V
(
I
ON
OUT
IN
RIPPLE
mA
H
ns
V
V
I
VINMIN
RIPPLE
763
10
417
)
3
.
3
6
.
21
(
_



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