SC173MLTRT Semtech, SC173MLTRT Datasheet - Page 16

• Manufacturer Part Number: SC173MLTRT
• Description: IC BUCK SYNC ADJ 3A MLPD-10
• Manufacturer: Semtech
• Series: EcoSpeed™r
• Type: Step-Down (Buck)r

Specifications of SC173MLTRT

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.75 ~ 5.23 V
• Current - Output: 3A
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MLPD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: SC173MLTR

Applications Information (continued)
Example
In this example, the inductor ripple current is set equal
to 30% of the maximum load current. Therefore ripple
current will be 30% x 3A or 0.9A. To find the minimum
inductance needed, use the V
respond to V
A larger value of 2µH 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.
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 tol-
erance. 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 750mV refer-
ence tolerance is 1%. Assuming a 1% tolerance from the
FB resistor divider, this allows 2% tolerance due to V
ripple. Since this 2% error comes from 1/2 of the ripple
voltage, the allowable ripple is 4%, or 40mV for a 1V out-
put.
© 2010 Semtech Corporation
I
RIPPLE_VIN
RIPPLE
INMAX
T
ON_VINMIN
T
L
ON_VINMAX
to 0.511A.
I
RIPPLE
=
MIN
.
(5.5V
=
=
(4.5V
=
(V
4.5V
=
IN
-
5.5V
0.9A
1V)
-
-
1V)
V
2
×
1V
m
OUT
L
800kHz
H
IN
•
⋅
1V
×
800
)
and T
227ns
277ns
×
T
kHz
ON
IN
=
ON
=
conditions is also
277ns
=
=
0.485A
values that cor-
1.14
227ns
m
H
OUT
The maximum ripple current of 0.511A 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 re-
quirements. A worst-case load release, from maximum
load to no load at the exact moment when inductor cur-
rent is at the peak, determines the required capacitance.
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 3A load release, the required capaci-
tance is shown by the next equation.
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 750mV 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 much 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 inductive energy that must be absorbed by the
output capacitor, therefore a smaller capacitance can be
used.
The following can be used to calculate the needed ca-
pacitance for a given dI
shown by the next equation.
OUT
ESR
ESR
. This causes a down-slope or falling di/dt in the
COUT
MAX
MAX
=
COUT
= 78.3 mΩ
MIN
I
RIPPLEMAX
V
RIPPLE
=
2
MIN
m
H
=
(1.05V)
×
=
L
(3A
LOAD
40
0.51A
×
(I
(V
PEAK
mV
OUT
+
/dt. Peak inductor current is
2
PEAK
1
2
-
of 1.050V (50mV rise upon
+
(1.0V)
×
)
0.511A)
1
2
2
×
-
(V
I
RIPPLEMAX
2
OUT
2
)
2
=
207
)
2
SC173
m
F
16