参数资料
| 型号： | NCP5318FTR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 22/32页 |
| 文件大小： | 0K |
| 描述： | IC CTLR CPU 2/3/4 PHASE 32-LQFP |
| 产品变化通告： | Product Obsolescence 08/Apr/2011 |
| 标准包装： | 1 |
| 应用： | 控制器，CPU |
| 输入电压： | 9.5 V ~ 13.2 V |
| 输出数： | 4 |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-LQFP |
| 供应商设备封装： | 32-LQFP（7x7） |
| 包装： | 剪切带 (CT) |
| 其它名称： | NCP5318FTR2GOSCT |
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NCP5318
3. Output Inductor Selection
The output inductor is a very critical component in the
converter because it directly affects the choice of other
components and affects both the steady ? state and transient
performance of the converter. When selecting an inductor,
For increasing current:
D tINC + Lo
For decreasing current:
D IO
(VIN * VOUT)
(eq. 5)
D tDEC + Lo
(VIN * VOUT)
VOUT
LoMIN +
IIN,AVG + IO,MAX
D
h
I C,MAX
the designer must consider factors such as DC current, peak
current, core loss, magnetic saturation, output voltage
ripple, load step and release, temperature, physical size and
cost.
In general, the output inductance value should be
electrically and physically as small as possible in order to
provide the best transient response at minimum cost. If a
large inductance value is used, the converter will not
respond quickly to rapid changes in the load current. On the
other hand, lower inductance requires more parallel ceramic
output capacitors to make the output filter ESL low enough
to avoid excessive output voltage ripple. And the higher
ripple current in the MOSFETs and input capacitors
increases dissipation and lowers converter efficiency
(especially at light loads) ? possibly requiring the use of
higher rated MOSFETs, an oversized thermal solution, and
the use of more or higher current rated input capacitors,
which increases converter cost. Too high a ripple current
may saturate the inductor, further increasing ripple current
and all losses including core loss.
One method of calculating an output inductor value is to
size the inductor to produce a specified maximum ripple
current in the inductor. Lower ripple currents will result in
less core and MOSFET losses and higher converter
efficiency. Equation 4 may be used to calculate the inductor
value to produce a given maximum ripple current ( a ) per
phase. The inductor value calculated by this equation is a
minimum because values less than this will produce more
ripple current than desired. Conversely, higher inductor
values will result in less than the selected maximum ripple
current.
(eq. 4)
( a IO,MAX VIN fSW)
D IO (eq. 6)
(VOUT)
For typical processor applications with output voltages
less than one quarter of the input voltage, the current can be
increased more quickly than it can be decreased. Thus, it
may be more difficult for the converter to avoid
overshooting the regulation limits when the load is removed
than when it is applied.
4. Input Capacitor Selection
Input capacitors must be both bulk electrolytic and
ceramic types. Bulk capacitors are needed to ensure
converter stability, and can provide some buffering of the
ATX power supply from the effects of load step and release.
The ceramic capacitors are needed to provide the input
ripple current. The choice and number of ceramic input
capacitors is primarily determined by their voltage and
ripple current ratings. The designer must choose capacitors
that will support the worst case input voltage with adequate
margin. To calculate the number of input capacitors, the
converter RMS input ripple current must be calculated by
the following procedure:
(eq. 7)
where:
D is the duty cycle of the converter, D = V OUT /V IN ;
h is the specified minimum efficiency;
I O,MAX is the maximum converter output current.
The input capacitors will discharge when the control FET
is ON and charge when the control FET is OFF as shown in
Figure 23.
D I C,IN = I C,MAX ? I C,MIN
a is the ripple current as a percentage of the maximum
output current per phase ( a = 0.15 for ± 15%, a = 0.25 for
± 25%, etc.). If the minimum inductor value is used, the
inductor current will swing ± ( a /2)% about its value at the
center. Therefore, for a four ? phase converter, the inductor
must be designed or selected such that it will not saturate
with a peak current of (1 + a /2) ? I O,MAX /4.
The maximum inductor value is limited by the transient
response required of the converter. If the converter is to have
I C,MIN
0A
? I IN,AVG
t ON
Control FET Off,
Input Caps
Charging
T/4
Control FET On,
Input Caps Discharging
a fast transient response, the inductor should be made as
small as will be allowed by other constraints. If the inductor
is too large, its current will change too slowly, the output
voltage will droop excessively, more bulk capacitors will be
required and the converter cost will be increased. For a given
inductor value (L O ), it is useful to determine the time
required to increase or decrease the current.
http://onsemi.com
22
Figure 23. Input Capacitor Current for a
Four ? Phase Converter
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