ncp5424adr2 ON Semiconductor, ncp5424adr2 Datasheet - Page 11

• Manufacturer Part Number: ncp5424adr2
• Description: Dual Synchronous Buck Controller With Input Current Sharing
• Manufacturer: ON Semiconductor

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:
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):
The minimum value of inductance which prevents
The inductor ripple current can then be determined:
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:
L
V
V
f
I
DI
V
L = inductor value;
D = duty cycle.
f
ESR
DV
DI
V
ESR
I
I
DI
SW
SW(MAX)
SW
L(PEAK)
OUT
MIN
IN(MIN)
OUT
OUT
OUT
L
L
L
OUT
= inductor ripple current;
= inductor ripple current;
= inductor ripple current.
= switching frequency;
= switching frequency
MAX
CAP
L MIN +
= load current;
= minimum inductance value;
= output voltage;
= output voltage;
= output voltage.
Number of capacitors +
= 1.0% V
= maximum ESR per capacitor (specified in
= inductor peak current;
= minimum design input voltage;
= maximum allowable ESR;
− maximum design switch current.
voltage ripple ( budgeted by the designer );
manufacturer’s data sheet).
I L(PEAK) + I OUT )
DI L +
f SW
ESR MAX +
(V IN(MIN) * V OUT )V OUT
DI L +
V OUT
OUT
V IN(MIN)
ESR MAX
L
DV OUT
= maximum allowable output
DV OUT
f SW
(1 * D)
DI L
ESR MAX
ESR CAP
DI L
I SW(MAX)
2
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NCP5424
11
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.
where:
change in output voltage due to ESR, ESL, and output
capacitor discharging or charging. Empirical data indicates
that most of the output voltage change (droop or spike
depending on the load current transition) results from the
total output capacitor ESR.
according to the formula:
where:
number of output capacitors can be found by using the
formula:
where:
These components must be selected and placed carefully
In order to determine the number of output capacitors the
The voltage change during the load current transient is:
The designer has to independently assign values for the
The maximum allowable ESR can then be determined
Once the maximum allowable ESR is determined, the
I
DI
DI
Dt = load transient duration time;
ESL = Maximum allowable ESL including capacitors,
ESR = Maximum allowable ESR including capacitors
t
DV
ESR
ESR
TR
L(VALLEY)
DV OUT + DI OUT
OUT
OUT
ESR
= output voltage transient response time.
CAP
MAX
/ Dt = load current slew rate;
= load transient;
circuit traces, and vias;
and circuit traces;
Number of capacitors +
= change in output voltage due to ESR (assigned
by the designer)
= maximum ESR per capacitor (specified in
= maximum allowable ESR.
manufacturer’s data sheet).
I L(VALLEY) + I OUT *
= inductor valley current.
ESR MAX +
ESL
Dt
DV ESR
DI OUT
) ESR )
ESR MAX
ESR CAP
DI L
2
C OUT
t TR