cs5422 ON Semiconductor, cs5422 Datasheet - Page 11

• Manufacturer Part Number: cs5422
• Description: Dual Outofphase Synchronous Buck Controller With Current Limit
• Manufacturer: ON Semiconductor

Selection of the Output Inductor
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.
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.
inductor saturation or exceeding the rated FET current can
be calculated as follows:
where:
where:
The inductor should be selected based on its inductance,
There are a variety of materials and types of magnetic
The minimum value of inductance which prevents
The inductor ripple current can then be determined:
800
700
600
500
400
300
200
100
L
V
V
f
I
ΔI
V
L = inductor value;
D = duty cycle.
f
10
SW
SW(MAX)
SW
MIN
IN(MIN)
OUT
OUT
L
= inductor ripple current;
= switching frequency;
= switching frequency
L MIN +
= minimum inductance value;
= output voltage;
= output voltage;
Figure 8. Switching Frequency
20
= minimum design input voltage;
− maximum design switch current.
DI L +
f SW
(V IN(MIN) * V OUT )V OUT
30
V OUT
V IN(MIN)
R
L
OSC (kW)
f SW
40
(1 * D)
I SW(MAX)
50
http://onsemi.com
60
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capacitors will provide an acceptable output voltage ripple
(1.0% of output voltage is common). The formula below is
used:
yields reasonable inductor peak and valley currents (the
inductor current is a triangular waveform):
where:
Selection of the Output Capacitors
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.
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.
The designer can now verify if the number of output
Rearranging we have:
where:
The number of output capacitors is determined by:
where:
The designer must also verify that the inductor value
where:
These components must be selected and placed carefully
In order to determine the number of output capacitors the
ESR
ΔV
ΔI
V
ESR
I
I
ΔI
I
L(PEAK)
OUT
L(VALLEY)
OUT
L
L
OUT
= inductor ripple current;
= inductor ripple current.
MAX
CAP
= load current;
= output voltage.
Number of capacitors +
= 1.0% × V
= maximum ESR per capacitor (specified in
= inductor peak current;
= maximum allowable ESR;
voltage ripple ( budgeted by the designer );
manufacturer’s data sheet).
I L(VALLEY) + I OUT *
= inductor valley current.
I L(PEAK) + I OUT )
ESR MAX +
DI L +
OUT
ESR MAX
DV OUT
= maximum allowable output
DV OUT
DI L
ESR CAP
ESR MAX
DI L
2
DI L
2