参数资料
| 型号： | NCP5214AMNR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 20/31页 |
| 文件大小： | 0K |
| 描述： | IC CTLR NOTEBOOK DDR PWR 22-DFN |
| 产品变化通告： | Product Discontinuation 01/Oct/2008 |
| 标准包装： | 2,500 |
| 应用： | 控制器，DDR |
| 输入电压： | 4.5 V ~ 24 V |
| 输出数： | 2 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 22-VFDFN 裸露焊盘 |
| 供应商设备封装： | 22-DFN（6x5） |
| 包装： | 带卷 (TR) |
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NCP5214A
Then the required output capacitor capacitance can be
estimated by:
where I L(peak) is the peak inductor current at maximum load
current which is determined by:
COUT w
ISTEP(peak) + D ILOAD )
L I2STEP(peak)
(Vovershoot ) VOUT)2?V2OUT
(VIN?VOUT) VOUT
2L fSW VIN
(eq. 11)
(eq. 12)
IL(peak) + ILOAD(max) )
+ ILOAD(max) )
IL(ripple)
2
(VIN?VOUT)
2 L fSW
VOUT
VIN
(eq. 17)
Since the excessive energy stored in the inductor
where I STEP(peak) is the load current step plus half of the
ripple current at the load release and D I LOAD is the change
in the output load current.
Besides, the ESR and the capacitance of the output filter
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.
Other parameters about output filter capacitor that
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:
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:
COUT ( (Vovershoot ) VOUT)2?V2OUT )
L v
I2STEP(peak)
(eq. 18)
In addition, the inductor also needs to have low enough
DCR to obtain good conversion efficiency. In general,
inductors with about 2.0 m W to 3.0 m W per m H 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. Moreover, it should be
Vrating w 1.25
ICOUT(RMS) w IL(ripple) +
VOUT
(VIN?VOUT)
L fSW
(eq. 13)
VOUT
VIN
(eq. 14)
noted that using too small inductance value will cause very
large inductor ripple current in CCM in S0 mode and
extremely large peak inductor current in DCM in
power?saving mode during S3 mode. For both cases,
output capacitors with smaller ESR and larger capacitance
SP?Cap, POSCAP and OS?CON capacitors are suitable
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
The inductor should be chosen according to the inductor
ripple current, inductance, maximum current rating,
transient load release, and DCR.
In general, the inductor ripple current is 20% to 40% of
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:
are required to keep the output ripple voltage small. It
should also be noted that the peak inductor current under
DCM light?load condition in power?saving mode in S3
mode will be larger than the peak inductor current under
heavy?load condition in S0 mode when very small
inductance value is used. Besides, using smaller
inductance will achieve lower efficiency and
require larger minimum load to maintain nominal voltage
regulation in power?saving mode in S3 state. Therefore, it
is recommended that the inductance value should be at
least 0.56 m H or above to obtain optimum performance.
MOSFET Selection
External N?channel MOSFETs are used as the switching
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 DS(on) ), maximum drain?to?source
voltage (V DSS ), maximum drain current rating, and gate
L w
0.3
(VIN?VOUT)
ILOAD(max)
VOUT
VIN
fSW
(eq. 15)
charges (Q G , Q GD , Q GS ) are the key parameters to be
considered when choosing the MOSFETs.
For on?resistance, it should be the lower; the better is the
where I LOAD(max) is the maximum load current.
The DC current rating of the inductor should be about 1.2
times of the peak inductor current at maximum output load
current. Therefore, the maximum DC current rating of the
inductor can be obtained by:
performance in terms of efficiency and power dissipation.
Check the MOSFET’s rated R DS(on) at V GS = 4.5 Vwhen
selecting the MOSFETs. The low?side MOSFET should
have lower R DS(on) than the high?side MOSFET since the
turn?on time of the low?side MOSFET is much longer than
IL(rating) + 1.2
IL(peak)
(eq. 16)
the high?side MOSFET in high V IN and low V OUT buck
http://onsemi.com
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