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CXW8508 View Datasheet(PDF) - Unspecified

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Description
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CXW8508
ETC
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CXW8508 Datasheet PDF : 9 Pages
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深圳市诚芯微科技有限公司                      CX8508
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 VOUT is the output voltage, VIN is the input volt-
age, fS is the switching frequency, and ΔIL is the peak-
to-peak inductor ripple current.
Choose an inductor that will not saturate under the
maximum inductor peak current. The peak inductor
current can be calculated by:
Where VFB is the feedback voltage and VOUT is the
output voltage.
Thus the output voltage is:
R2 can be as high as 100k, but a typical value
is 10k. Using the typical value for R2, R1 is
determined by:
For example, for a 3.3V output voltage, R2 is
10k, and R1 is 26.1k. Table 1 lists recom-
mended resistance values of R1 and R2
for standard output voltages.
Where ILOAD is the load current.
The choice of which style inductor to use mainly de-
pends on the price vs. size requirements and any EMI
requirements.
Optional Schottky Diode
During the transition between high-side switch and
low-side switch, the body diode of the lowside power
MOSFET conducts the inductor current. The forward
voltage of this body diode is high. An optional Schot-
tky diode may be paralleled between the SW pin and
GND pin to improve overall efficiency. Table 2 lists
example Schottky diodes and their Manufacturers.
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
will result in less ripple current that will result in
lower output ripple voltage. However, the larger
value inductor will have a larger physical size,
higher series resistance, and/or lower saturation
current. A good rule for determining the induc-
tance to use is to allow the peak-to-peak ripple
current in the inductor to be approximately 30%
of the maximum switch current limit. Also, make
sure that the peak inductor current is below the
maximum switch current limit. The inductance
value can be calculated by:
Input Capacitor
The input current to the step-down converter is dis-
continuous, therefore a capacitor is required to supply
the AC current to the step-down converter while
maintaining the DC input voltage. Use low ESR ca-
pacitors for the best performance. Ceramic capacitors
are preferred, but tantalum or low-ESR electrolytic
capacitors may also suffice. Choose X5R or X7R di-
electrics when using ceramic capacitors. Since the
input capacitor absorbs the input switching current it
requires an adequate ripple current rating. The RMS
current in the input capacitor can be estimated by:
5
 

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