ncp5214 ON Semiconductor, ncp5214 Datasheet - Page 20

• Manufacturer Part Number: ncp5214
• Description: 2-in-1 Notebook Ddr Power Controller
• Manufacturer: ON Semiconductor

estimated by:
where I
ripple current at the load release and DI
in the output load current.
capacitor also contribute to double pole and ESR zero
frequencies of the output filter, and the poles and zeros
frequencies of the compensation network for close loop
stability. The compensation network will be discussed in
more detail in the Loop Compensation section.
needed to be considered are the voltage rating and ripple
current rating. The voltage rating should be at least 1.25
times the output voltage and the rms ripple current rating
should be greater than the inductor ripple current. Thus, the
voltage rating and ripple current rating can be obtained by:
for the output capacitor since their ESR is low enough to
meet the ripple voltage and load transient requirements.
Usually, two or more capacitors of the same type,
capacitance and ESR can be used in parallel to achieve the
required ESR and capacitance without change the ESR
zero position for maintaining the same loop stability. Other
than the performance point of view, the physical size and
cost are also the concerned factors for output capacitor
selection.
Inductor Selection
ripple current, inductance, maximum current rating,
transient load release, and DCR.
the maximum load current. A ripple current of 30% of the
maximum load current can be used as a typical value. The
required inductance can be estimated by:
where I
times of the peak inductor current at maximum output load
Then the required output capacitor capacitance can be
Besides, the ESR and the capacitance of the output filter
Other parameters about output filter capacitor that
SP−Cap, POSCAP and OS−CON capacitors are suitable
The inductor should be chosen according to the inductor
In general, the inductor ripple current is 20% to 40% of
The DC current rating of the inductor should be about 1.2
I COUT(RMS) w I L(ripple) +
I STEP(peak) + DI LOAD )
LOAD(max)
STEP(peak)
C OUT w
L w
0.3
V rating w 1.25
(V overshoot ) V OUT ) 2 −V 2 OUT
is the load current step plus half of the
is the maximum load current.
(V IN −V OUT )
I LOAD(max)
L
I 2 STEP(peak)
(V IN −V OUT )
(V IN −V OUT )
2L
V OUT
L
V OUT
V IN
LOAD
f SW
f SW
f SW
is the change
V OUT
V IN
V IN
V OUT
(eq. 12)
(eq. 13)
(eq. 14)
(eq. 15)
(eq. 11)
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NCP5214
20
current. Therefore, the maximum DC current rating of the
inductor can be obtained by:
where I
current which is determined by:
contributed to the output voltage overshoot during load
release, the following inequality can be used to ensure that
the selected inductance value can meet the voltage
overshoot requirement at load release:
DCR to obtain good conversion efficiency. In general,
inductors with about 2.0 mW to 3.0 mW per mH of
inductance can be used. Besides, larger inductance value
can be selected to achieve higher efficiency as long as it
still meets the targeted voltage overshoot at load release
and inductor DC current rating.
MOSFET Selection
elements of the buck controller. Both high−side and
low−side MOSFETs must be logic−level MOSFETs which
can be fully turned on at 5.0 V gate−drive voltage.
On−resistance (R
voltage (V
charges (Q
considered when choosing the MOSFETs.
performance in terms of efficiency and power dissipation.
Check the MOSFET’s rated R
selecting the MOSFETs. The low−side MOSFET should
have lower R
turn−on time of the low−side MOSFET is much longer than
the high−side MOSFET in high V
converter. Generally, high−side MOSFET with R
about 7.0 mW and low−side MOSFET with R
5.0 mW can achieve good efficiency.
MOSFET and low−side MOSFET must be higher than the
peak inductor current at maximum load current. The
low−side MOSFET should have larger maximum drain
current rating than the high−side MOSFET since the
low−side MOSFET have longer turn−on time.
Since the excessive energy stored in the inductor
In addition, the inductor also needs to have low enough
External N−channel MOSFETs are used as the switching
For on−resistance, it should be the lower; the better is the
The maximum drain current rating of the high−side
I L(peak) + I LOAD(max) )
L v
L(peak)
C OUT
DSS
G
, Q
+ I LOAD(max) )
DS(on)
is the peak inductor current at maximum load
), maximum drain current rating, and gate
I L(rating) + 1.2
GD
(
DS(on)
, Q
than the high−side MOSFET since the
(V overshoot ) V OUT ) 2 −V 2 OUT
GS
I 2 STEP(peak)
),
) are the key parameters to be
maximum drain−to−source
DS(on)
(V IN −V OUT )
I L(ripple)
2
I L(peak)
IN
2
L
at V
and low V
f SW
GS
= 4.5 Vwhen
DS(on)
V OUT
OUT
V IN
(eq. 16)
(eq. 17)
(eq. 18)
)
DS(on)
about
buck