SC414MLTRT Semtech, SC414MLTRT Datasheet - Page 22

• Manufacturer Part Number: SC414MLTRT
• Description: IC BUCK SYNC ADJ 6A 28MLPQ
• Manufacturer: Semtech
• Series: SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC414MLTRT

• Mfg Application Notes: SC414 Basic Design Procedure
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.5 V
• Current - Output: 6A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC414MLTR

Applications Information (continued)
There are two values of load current to evaluate — con-
tinuous load current and peak load current. Continuous
load current relates to thermal stresses which drive the
selection of the inductor and input capacitors. Peak load
current determines instantaneous component stresses and
fi ltering requirements such as inductor saturation, output
capacitors, and design of the current limit circuit.
The following values are used in this design.
Frequency Selection
Selection of the switching frequency requires making a
trade-off between the size and cost of the external fi lter
components (inductor and output capacitor) and the
power conversion effi ciency.
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 fi rst be defi ned. 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/voltage and for a given DC resistance are more
•
•
•
•
T
R
R
T
ON
V
V
f
Load = 6A maximum
TON
TON
ON
SW
IN
OUT
= 303 ns at 13.2V
= 12V + 10%
= 250kHz
= 130.9kΩ, use R
= 1V + 4%
TON
V
25
TON
INMAX
, use the maximum value for V
pF
V
) is used to program the on-time (indirectly
OUT
1
TON
f
f
SW
SW
results in the following solution.
400
IN
TON
, 1V
V
= 130kΩ
V
OUT
OUT
IN
, 250kHz
IN
.
IN
, and for T
ON
effi cient. However, larger inductance translates directly
into larger packages and higher cost. Cost, size, output
ripple, and effi ciency 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,
effi ciency, and transient performance.
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 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 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.
IN
L
L
T
- V
ON
OUT
_
(
(
VINMIN
13
V
). The equation for determining inductance is
IN
INMAX
2 .
V
I
V
RIPPLE
.
OUT
25
3
1
V
A
)
pF
)
T
318
ON
V
R
INMIN
RIPPLE
TON
ns
to 2.53A.
IN
V
and T
. 1
OUT
26
SC414/SC424
ON
10
H
IN
values that corre-
conditions is also
ns
311
ns
22