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PDF D1067 Datasheet ( Hoja de datos )

Número de pieza D1067
Descripción monolithic step-down switch mode converter
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D1067 datasheet

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D1067 pdf
TM
D1067
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive volt-
age divider from the output voltage to FB (see
Typical Application circuit on page 1). The volt-
age divider divides the output voltage down by
the ratio:
Where VFB is the feedback voltage and VOUT is
the output voltage.
Thus the output voltage is:
R2 can be as high as 100kQ, but a typical value
is 10k. Using that value, R1 is determined by:
For example, for a 3.3V output voltage, R2 is
10k, and R1 is 16.9k.
Inductor
The inductor is required to supply constant cur-
rent to the output load while being driven by the
switched input voltage. A larger value inductor
results in less ripple current that results in lower
output ripple voltage. However, the larger value
inductor has a larger physical size, higher series
resistance and/or lower saturation current.
Choose an inductor that does not saturate under
the worst-case load conditions. A good rule for
determining the inductance is to allow the peak-
to-peak ripple current in the inductor to be ap-
proximately 30% of the maximum load current.
Also, make sure that the peak inductor current
(the load current plus half the peak-to-peak in-
ductor ripple current) is below the 2.4A minimum
current limit.
The inductance value can be calculated by the
equation:
Where VIN is the input voltage, f is the switching
frequency, and Δl is the peak-to-peak inductor
ripple current.
Table 1 lists a number of suitable inductors from vari-
ous manufacturers.
Table 1—Inductor Selection Guide
Vendor/ Model Core
Core Ma- Package
Type
terial Dimensions
(mm)
W LH
Sumida
CR75
CDH74
CDRH5D28
CDRH5D28
CDRH6D28
CDRH104R
Open
Open
Shielded
Shielded
Shielded
Shielded
Ferrite 7.0 7.8 5.5
Ferrite 7.3 8.0 5.2
Ferrite 5.5 5.7 5.5
Ferrite 5.5 5.7 5.5
Ferrite 6.7 6.7 3.0
Ferrite 10.1 10.0 3.0
Toko
D53LC Type A Shielded
D75C
Shielded
D104C
Shielded
D10FL
Open
Ferrite 5.0 5.0 3.0
Ferrite 7.6 7.6 5.1
Ferrite 10.0 10.0 4.3
Ferrite 9.7 1.5 4.0
Coilcraft
DO3308
D03316
Open
Open
Ferrite 9.4
Ferrite 9.4
13.0 3.0
13.0 5.1
Input Capacitor
The input current to the step-down converter is dis-
continuous, and therefore an input capacitor C1 is
required to supply the AC current to the step-down
converter while maintaining the DC input voltage. A
low ESR capacitor is required to keep the noise at the
IC to a minimum. Ceramic capacitors are preferred,
but tantalum or low-ESR electrolytic capacitors may
also suffice.
The input capacitor value should be greater than
10uF. The capacitor can be electrolytic, tantalum or
ceramic. However, since it absorbs the input switch-
ing current it requires an adequate ripple current rat-
ing. Its RMS current rating should be greater than ap-
proximately 1/2 of the DC load current.
For insuring stable operation, C2 should be placed as
close to the IC as possible. Alternately a smaller high
quality ceramic 0.1 uF capacitor may be placed closer
to the IC and a larger capacitor placed further away. If
using this technique, it is recommended that the lar-
ger capacitor be a tantalum or electrolytic type. All
ceramic capacitors should be placed close to the
DMS1067.
REV:A2FYJ 2011-01-01
5
DMS Microelectronic Limited

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