ncp5423dr2g ON Semiconductor, ncp5423dr2g Datasheet - Page 10

• Manufacturer Part Number: ncp5423dr2g
• Description: Dual Outofphase Synchronous Buck Controller With Current Limit
• Manufacturer: ON Semiconductor

Selecting the Switching Frequency
component size and power losses. Operation at higher
switching frequencies allows the use of smaller inductor and
capacitor values. Nevertheless, it is common to select lower
frequency operation because a higher frequency results in
lower efficiency due to MOSFET gate charge losses.
Additionally, the use of smaller inductors at higher
frequencies results in higher ripple current, higher output
voltage ripple, and lower efficiency at light load currents.
linearly related to the switching period. If the designer
prefers not to use Figure 8 to select the necessary resistor, the
following equation quite accurately predicts the proper
resistance for room temperature conditions.
where:
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.
Selecting the switching frequency is a trade−off between
The value of the oscillator resistor is designed to be
The inductor should be selected based on its inductance,
There are a variety of materials and types of magnetic
800
700
600
500
400
300
200
100
R
f
SW
10
OSC
= switching frequency in kHz.
= oscillator resistor in kW;
Figure 8. Switching Frequency
20
R OSC +
30
21700 * f SW
R
2.31
OSC (kW)
40
f SW
50
NCP5422A, NCP5423
http://onsemi.com
60
10
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
= switching frequency;
= inductor ripple current;
= switching frequency
MAX
= inductor ripple current;
CAP
= inductor ripple current.
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(VALLEY) + I OUT *
I L(PEAK) + I OUT )
DI L +
f SW
(V IN(MIN) * V OUT )
ESR MAX +
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
DI L
2
V OUT