SC427MLTRT Semtech, SC427MLTRT Datasheet - Page 24

• Manufacturer Part Number: SC427MLTRT
• Description: IC BUCK SYNC ADJ 10A 32MLPQ
• Manufacturer: Semtech
• Series: EcoSpeed™, SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC427MLTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.25 V
• Current - Output: 10A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC427MLTR

Applications Information (continued)
The desired switching frequency is 250kHz 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
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 4A then
Power-save operation will typically start for loads less than
2A. If ripple current is set at 40% of maximum load current,
then power-save 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.
T
R
R
T
ON
TON
TON
ON
= 318 ns at 13.2V
= 154.9kΩ, use R
TON
V
T (
TON
INMAX
ON
, use the maximum value for V
25
V
) is used to program the on-time (indirectly
OUT
pF
10
TON
f
SW
ns
results in the following solution.
V
)
OUT
V
IN
IN
TON
, 1.05V
= 154kΩ
OUT
, 250kHz
IN
.
IN
, and for T
ON
During the DH on-time, voltage across the inductor is
(V
shown next.
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 10A or 5A. To find the minimum
inductance needed, use the V
spond to V
A slightly larger value of 0.88µH is selected. This will
decrease the maximum 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.
Output Capacitor Selection
The output capacitors are chosen based on required ESR
and capacitance. The maximum ESR requirement is con-
trolled by the output ripple requirement and the DC toler-
ance. 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. Change in the output ripple voltage will lead to a
change in DC voltage at the output.
The design goal is that the output voltage regulation be
±4% under static conditions. The internal 500mV refer-
ence tolerance is 1%. Allowing 1% tolerance from the FB
resistor divider, this allows 2% tolerance due to V
IN
L
L
T
I
I
- V
RIPPLE
RIPPLE
ON
OUT
_
(
(
VINMIN
13
V
). The equation for determining inductance is
_
IN
INMAX
VINMIN
2 .
(
V
I
V
RIPPLE
.
IN
. 1
OUT
25
5
05
A
V
(
)
pF
10
L
OUT
)
T
8 .
ON
318
V
)
R
INMIN
RIPPLE
TON
088
. 1
T
ns
ON
05
to 4.4A.
IN
V
)
H
and T
. 0
OUT
384
77
ns
ON
10
IN
H
values that corre-
conditions is also
ns
. 4
25
384
A
OUT
ns
ripple.
24