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參數(shù)資料
| 型號： | NCP5331FTR2G |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Two-Phase PWM Controller with Integrated Gate Drivers |
| 中文描述： | 1.5 A SWITCHING CONTROLLER, 750 kHz SWITCHING FREQ-MAX, PQFP32 |
| 封裝： | LEAD FREE, LQFP-32 |
| 文件頁數(shù)： | 25/36頁 |
| 文件大?。?/td> | 443K |
| 代理商： | NCP5331FTR2G |
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NCP5331
http://onsemi.com
25
For increasing current
tINC
Lo
IO(VIN
VCORE)
(3.1)
For decreasing current
tDEC
Lo
IO(VCORE)
(3.2)
For typical processor applications with output voltages
less than half the input voltage, the current will be increased
much more quickly than it can be decreased. It may be more
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.
VOUT,PP
(ESR per cap
#Phases
NOUT,MIN)
D
(VIN
VCORE)
(LoMIN
fSW)
(4)
This formula assumes steadystate 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 outofphase. 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 multiphase 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
rms input ripple current. To this end, begin by calculating the
average input current to the converter.
IIN,AVG
IO,MAX
D
(5)
where
D
is the duty cycle of the converter,
D = V
CORE
/V
IN
,
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 30.
The following equations will determine the maximum and
minimum currents delivered by the input capacitors.
η
IC,MAX
ILo,MAX
IIN,AVG
(6)
IC,MIN
ILo,MIN
IIN,AVG
(7)
I
Lo,MAX
is the maximum output inductor current.
ILo,MAX
IO,MAX2
ILo2
(8)
I
C,MAX
I
C,MIN
0 A
I
IN,AVG
FET On,
Caps Discharging
FET Off,
Caps Charging
t
ON
T/2
I
C,IN
= I
C,MAX
I
C,MIN
Figure 30. Input Capacitor Current for a
TwoPhase Converter
I
Lo,MIN
is the minimum output inductor current.
ILo,MIN
IO,MAX2
ILo2
(9)
I
Lo
is the peaktopeak ripple current in the output
inductor of value L
o
.
ILo
(VIN
VCORE)
D (Lo
fSW)
(10)
For the twophase converter, the input capacitor(s) rms
current is then
ICIN,RMS
[2D
(IC,MIN2
IC,MIN
IC,IN
IC,IN23)
IIN,AVG2
(1
2D)]1 2
(11)
Select the number of input capacitors (N
IN
) to provide the
rms input current (I
CIN,RMS
) based on the rms ripple current
rating per capacitor (I
RMS,RATED
).
NIN
ICIN,RMSIRMS,RATED
(12)
For a twophase converter with perfect efficiency (
η
= 1),
the worst case input ripple current will occur when the
converter is operating at a 25% duty cycle. At this operating
point, the parallel combination of input capacitors must
support an rms ripple current equal to 25% of the converter’s
dc output current. At other duty cycles, the ripple current
will be less. For example, at a duty cycle of either 10% or
40%, the twophase input ripple current will be
approximately 20% of the converter’s dc output current.
In general, capacitor manufacturers require derating to the
specified ripple current based on the ambient temperature.
More capacitors will be required because of the current
derating. The designer should be cognizant of the ESR of the
input capacitors. The input capacitor power loss can be
calculated from
PCIN
ICIN,RMS2
ESR_per_capacitor NIN
(13)
Low ESR capacitors are recommended to minimize losses
and reduce capacitor heating. The life of an electrolytic
capacitor is reduced 50% for every 10
°
C rise in the
capacitor’s temperature.
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