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CS5422 Datasheet(PDF) 11 Page - ON Semiconductor

Part No. CS5422
Description  Dual Out−of−Phase Synchronous Buck Controller with Current Limit
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5422 Datasheet(HTML) 11 Page - ON Semiconductor

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CS5422
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11
Figure 8. Switching Frequency
10
20
30
40
50
60
100
200
300
400
500
600
700
800
ROSC (kW)
Selection of the Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response. There are many factors to
consider in selecting the inductor including cost, efficiency,
EMI and ease of manufacture. The inductor must be able to
handle the peak current at the switching frequency without
saturating, and the copper resistance in the winding should
be kept as low as possible to minimize resistive power loss.
There are a variety of materials and types of magnetic
cores that could be used for this application. Among them
are ferrites, molypermalloy cores (MPP), amorphous and
powdered iron cores. Powdered iron cores are very
commonly used. Powdered iron cores are very suitable due
to its high saturation flux density and have low loss at high
frequencies, a distributed gap and exhibit very low EMI.
The minimum value of inductance which prevents
inductor saturation or exceeding the rated FET current can
be calculated as follows:
LMIN +
(VIN(MIN) * VOUT)VOUT
fSW
VIN(MIN)
ISW(MAX)
where:
LMIN = minimum inductance value;
VIN(MIN) = minimum design input voltage;
VOUT = output voltage;
fSW = switching frequency;
ISW(MAX) − maximum design switch current.
The inductor ripple current can then be determined:
DIL +
VOUT
(1 * D)
L
fSW
where:
ΔIL = inductor ripple current;
VOUT = output voltage;
L = inductor value;
D = duty cycle.
fSW = switching frequency
The designer can now verify if the number of output
capacitors will provide an acceptable output voltage ripple
(1.0% of output voltage is common). The formula below is
used:
DIL + D
VOUT
ESRMAX
Rearranging we have:
ESRMAX + D
VOUT
DIL
where:
ESRMAX = maximum allowable ESR;
ΔVOUT = 1.0% × VOUT = maximum allowable output
voltage ripple ( budgeted by the designer );
ΔIL = inductor ripple current;
VOUT = output voltage.
The number of output capacitors is determined by:
Number of capacitors +
ESRCAP
ESRMAX
where:
ESRCAP = maximum ESR per capacitor (specified in
manufacturer’s data sheet).
The designer must also verify that the inductor value
yields reasonable inductor peak and valley currents (the
inductor current is a triangular waveform):
IL(PEAK) + IOUT )
DIL
2
where:
IL(PEAK) = inductor peak current;
IOUT = load current;
ΔIL = inductor ripple current.
IL(VALLEY) + IOUT *
DIL
2
where:
IL(VALLEY) = inductor valley current.
Selection of the Output Capacitors
These components must be selected and placed carefully
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the regulator output voltage.
Key specifications for output capacitors are their ESR
(Equivalent Series Resistance), and ESL (Equivalent Series
Inductance). For best transient response, a combination of
low value/high frequency and bulk capacitors placed close
to the load will be required.
In order to determine the number of output capacitors the
maximum voltage transient allowed during load transitions
has to be specified. The output capacitors must hold the
output voltage within these limits since the inductor current
can not change with the required slew rate. The output
capacitors must therefore have a very low ESL and ESR.


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