sc250wltrt Semtech Corporation, sc250wltrt Datasheet - Page 9

• Manufacturer Part Number: sc250wltrt
• Description: Sc250 Step-down Dc-dc Converter With Bias Ldo For Rf Power Amplifiers
• Manufacturer: Semtech Corporation

Under-Voltage Lockout
Under-voltage lockout protection is used to prevent
erroneous operation. As the input decreases, the device
shuts down when the voltage drops below 2.35V and will
not restart until the input voltage exceeds approximately
2.5V.
Inductor Selection
The SC250 is designed for use with a 4.7μH inductor. The
magnitude of the inductor current ripple is dependent on
the inductor value and can be determined by the following
equation:
This equation demonstrates the relationship between
input voltage, output voltage, and inductor ripple current.
The inductor should have a low DC resistance to minimize
the conduction losses and maximize effi ciency. As a
minimum requirement, the DC current rating of the
inductor should be equal to the maximum load current
plus half of the inductor current ripple as shown by the
following equation:
Final inductor selection depends on various design
considerations such as effi ciency, EMI, size, and cost.
Table 1 lists the manufacturers of practical inductor
options.
C
The source input current to a buck converter is non-
continuous. To prevent large input voltage ripple, a low
© 2006 Semtech Corp.
POWER MANAGEMENT
Applications Information (Cont.)
Manufacturer/Part #
BI Technologies
HM66304R7
Coilcraft
D01608C-472ML
TDK
VLCF4018T- 4R7N1R0-2
IN
Selection
Table 1 — Recommended Inductors
I
Value
L
(μH)
4.7
4.7
4.7
I
LPK
V
0.072
0.101
DCR
0.09
I
(Ω)
OUT
OUT
(
L
MAX
1
Current (A)
)
Saturation
1.32
1.07
1.5
f
OSC
2
I
V
L
V
OUT
I
N
Tolerance
(%)
20
20
30
LxWxH (mm)
4.7 × 4.7 ×3.0
6.6 × 4.5 ×3.0
4.3 × 4.0 ×1.8
Dimensions
9
ESR ceramic capacitor is required. A minimum value of
10μF should be used for suffi cient input voltage fi ltering
and a 22μF should be used for improved input voltage
fi ltering.
C
The internal compensation is designed to work with a
certain output fi lter corner frequency defi ned by the
equation:
This single pole fi lter is designed to operate with an output
capacitor value of 4.7μF.
Output voltage ripple is a combination of the voltage
ripple from the inductor current charging and discharging
the output capacitor, and the voltage created from the
inductor current ripple through the output capacitor ESR.
Selecting an output capacitor with a low ESR will reduce
the output voltage ripple component that is dependent
upon this ESR, as can be seen in the following equation:
Capacitors with X7R or X5R ceramic dielectric are
strongly recommended for their low ESR and superior
temperature and voltage characteristics. Y5V capacitors
should not be used as their temperature coeffi cients
make them unsuitable for this application. Table 2 lists
the manufacturers of recommended capacitor options.
Manufacturer/Part Number
Murata
GRM21BR60J226ME39L
Murata
GRM188R60J106MKE19
TDK
C2012X5R0J106K
Murata
GRM188R60J475KE19D
OUT
Selection
Table 2 — Recommended Capacitors
V
OUT
(
f
ESR
C
)
Value
(μF)
4.7
22
10
10
2
I
( L
ripple
L
1
)
Voltage
Rated
(VDC)
C
6.3
6.3
6.3
6.3
OUT
ESR
(
COUT
PRELIMINARY
)
Type
X5R
X5R
X5R
X5R
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SC250
Case Size
0805
0603
0603
0603