参数资料
| 型号： | NCP5332ADW |
| 厂商： | ON Semiconductor |
| 文件页数： | 18/30页 |
| 文件大小： | 0K |
| 描述： | IC CTLR BUCK 2PH DRVR/DAC 28SOIC |
| 产品变化通告： | Product Obsolescence 30/Sept/2009 |
| 标准包装： | 26 |
| 应用： | 控制器，高性能处理器 |
| 输入电压： | 4.5 V ~ 14 V |
| 输出数： | 2 |
| 输出电压： | 可调 |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-SOIC（0.295"，7.50mm 宽） |
| 供应商设备封装： | 28-SOIC |
| 包装： | 管件 |
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NCP5332A
transient performance of the converter. When selecting an
inductor the designer must consider factors such as DC
current, peak current, output voltage ripple, core material,
magnetic saturation, temperature, physical size, and cost
(usually the primary concern).
In general, the output inductance value should be as low
and physically small as possible to provide the best transient
difficult for the converter to stay within the regulation limits
when the load is removed than when it is applied ? excessive
overshoot may result.
The output voltage ripple can be calculated using the
output inductor value derived in this Section (Lo MIN ), the
number of output capacitors (N OUT,MIN ) and the per
capacitor ESR determined in the previous Section:
response and 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, too low an inductance
VOUT,P?P + (ESR per cap NOUT,MIN) @
(VIN * #Phases @ VOUT) @ D (LoMIN @ fSW)
(4)
value will result in very large ripple currents in the power
components (MOSFETs, capacitors, etc) resulting in
increased dissipation and lower converter efficiency. Also,
increased ripple currents will force the designer to use
higher rated MOSFETs, oversize the thermal solution, and
use more, higher rated input and output capacitors ? the
converter cost will be adversely effected.
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 3 may be used to calculate the minimum
inductor value to produce a given maximum ripple current
( α ) 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 maximum ripple current.
This formula assumes steady?state conditions with no
more than one phase on at any time. The second term in
Equation 4 is the total ripple current seen by the output
capacitors. The total output ripple current is the “time
summation” of the two individual phase currents that are
180 degrees out?of?phase. As the inductor current in one
phase ramps upward, current in the other phase ramps
downward and provides a canceling of currents during part
of the switching cycle. Therefore, the total output ripple
current and voltage are reduced in a multi?phase converter.
3. Input Capacitor Selection
The choice and number of 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 one must first determine the total
LoMIN +
(VIN * VOUT) @ VOUT
( a @ IO,MAX @ VIN @ fSW)
(3)
RMS input ripple current. To this end, begin by calculating
the average input current to the converter:
IIN,AVG + IO,MAX @ D h
α is the ripple current as a percentage of the maximum
output current per phase ( α = 0.15 for ± 15%, α = 0.25 for
± 25%, etc). If the minimum inductor value is used, the
inductor current will swing ± α % about its value at the center
(1/2 the DC output current for a two?phase converter).
Therefore, for a two?phase converter, the inductor must be
designed or selected such that it will not saturate with a peak
current of (1 + α ) ? I O,MAX /2.
The maximum inductor value is limited by the transient
(5)
where:
D is the duty cycle of the converter, D = V OUT /V IN .
η 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 15.
response of the converter. If the converter is to have a fast
transient response then the inductor should be made as small
I C,MAX
? I C,IN = I C,MAX ? I C,MIN
as possible. 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
I C,MIN
0A
t ON
T/2
be increased. For a given inductor value, its interesting to
determine the times required to increase or decrease the
current.
For increasing current:
?I IN,AVG
FET Off,
Caps Charging
D tINC + Lo @ D IO (VIN * VOUT)
For decreasing current:
D tDEC + Lo @ D IO (VOUT)
(3.1)
(3.2)
FET On,
Caps Discharging
Figure 15. Input Capacitor Current for a
Two?Phase Converter
For typical processor applications with output voltages
less than half the input voltage, the current will be increased
The following equations will determine the maximum and
minimum currents delivered by the input capacitors:
much more quickly than it can be decreased. It may be more
http://onsemi.com
18
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