SC418ULTRT Semtech, SC418ULTRT Datasheet - Page 23

• Manufacturer Part Number: SC418ULTRT
• Description: IC BUCK SYNC ADJ 20MLPQ
• Manufacturer: Semtech
• Series: EcoSpeed™, SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC418ULTRT

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.5 ~ 5.5 V
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Other names: SC418ULTRTTR

Applications Information (continued)
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 for the output voltage regulation to 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
Since this 2% error comes from 1/2 of the ripple voltage,
the allowable ripple is 4%, or 42mV for a 1.05V output.
The maximum ripple current of 4.4A creates a ripple
voltage across the ESR. The maximum ESR value allowed
is shown by the following equations.
The output capacitance is chosen to meet transient
requirements. A worst-case load release, from maximum
load to no load at the exact moment when inductor
current is at the peak, determines the required capaci-
tance. If the load release is instantaneous (load changes
from maximum to zero in < 1µs), the output capacitor
must absorb all the inductor’s stored energy. This will
cause a peak voltage on the capacitor according to the
following equation.
Assuming a peak voltage V
load release), and a 10A load release, the required capaci-
tance is shown by the next equation.
COUT
ESR
ESR
COUT
COUT
MAX
MAX
MIN
MIN
MIN
= 9.5 mΩ
= 595µF
I
RIPPLEMAX
. 0
V
L
RIPPLE
88
I
OUT
V
. 1
PEAK
15
H
10
1
2
2
2
42
4
I
4 .
RIPPLEMAX
PEAK
. 1
mV
1
2
V
A
05
OUT
of 1.150 (100mV rise upon
4
4 .
2
2
2
2
OUT
ripple.
If the load release is relatively slow, the output capacitance
can be reduced. At heavy loads during normal switching,
when the FB pin is above the 500mV reference, the DL
output is high and the low-side MOSFET is on. During this
time, the voltage across the inductor is approximately
-V
inductor. If the load di/dt is not faster than the -di/dt in
the inductor, then the inductor current will tend to track
the falling load current. This will reduce the excess induc-
tive energy that must be absorbed by the output capaci-
tor, therefore a smaller capacitance can be used.
The following can be used to calculate the needed capaci-
tance for a given dI
by the next equation.
Example
This would cause the output current to move from 10A to
zero in 4µs as shown by the following equation.
Note that C
compared to 595µF based on a worst-case load release. To
OUT
I
I
I
C
Rate
C
C
dl
LPK
LPK
MAX
. This causes a down-slope or falling di/dt in the
OUT
OUT
OUT
LOAD
dt
= I
= 10 + 1/2 x 4.4 = 12.2A
= maximum load release = 10A
= 379µF
of
MAX
12
I
LPK
OUT
change
2
2 .
+ 1/2 x I
5 .
s
is much smaller in this example, 379µF
A
. 0
L
LOAD
88
2
V
I
of
LPK
OUT
/dt. Peak inductor current is shown
2
V
H
RIPPLEMAX
PK
. 1
Load
15
12
. 1
dl
05
V
I
2 .
MAX
LOAD
OUT
Current
. 1
05
2
10
5 .
dt
1
s
dl
LOAD
dt
SC418
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