NCP5331_05 ONSEMI [ON Semiconductor], NCP5331_05 Datasheet - Page 25

• Manufacturer Part Number: NCP5331_05
• Description: Two-Phase PWM Controller with Integrated Gate Drivers
• Manufacturer: ONSEMI [ON Semiconductor]

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.
output inductor value derived in this Section (Lo
number of output capacitors (N
capacitor ESR determined in the previous Section.
V OUT,P−P + (ESR per cap
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 multiphase converter.
3. Input Capacitor Selection
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.
where
is ON and charge when the control FET is OFF as shown in
Figure 30.
minimum currents delivered by the input capacitors.
For increasing current
For decreasing current
For typical processor applications with output voltages
The output voltage ripple can be calculated using the
This formula assumes steady−state conditions with no
The choice and number of input capacitors is primarily
The input capacitors will discharge when the control FET
The following equations will determine the maximum and
I
Lo,MAX
D
I
O,MAX
(V IN * #Phases @ V CORE ) @ D
is the maximum output inductor current.
Dt INC + Lo @ DI O (V IN * V CORE )
I C,MAX + I Lo,MAX h * I IN,AVG
is the duty cycle of the converter,
D = V
is the specified minimum efficiency,
is the maximum converter output current.
I C,MIN + I Lo,MIN h * I IN,AVG
Dt DEC + Lo @ DI O (V CORE )
I IN,AVG + I O,MAX @ D h
CORE
/V
IN
,
N OUT,MIN ) @
OUT,MIN
(Lo MIN @ f SW )
) and the per
MIN
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), the
(3.1)
(3.2)
(4)
(5)
(6)
(7)
NCP5331
25
inductor of value L
current is then
I CIN,RMS + [2D @ (I C,MIN 2 ) I C,MIN @ DI C,IN
rms input current (I
rating per capacitor (I
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 two−phase input ripple current will be
approximately 20% of the converter’s dc output current.
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
and reduce capacitor heating. The life of an electrolytic
capacitor is reduced 50% for every 10 C rise in the
capacitor’s temperature.
I
DI
For the two−phase converter, the input capacitor(s) rms
Select the number of input capacitors (N
For a two−phase converter with perfect efficiency ( = 1),
In general, capacitor manufacturers require derating to the
Low ESR capacitors are recommended to minimize losses
Lo,MIN
−I
I
IN,AVG
Lo
C,MAX
I
C,MIN
P CIN + I CIN,RMS 2 @ ESR_per_capacitor N IN
0 A
is the peak−to−peak ripple current in the output
Figure 30. Input Capacitor Current for a
DI Lo + (V IN * V CORE ) @ D (Lo @ f SW )
is the minimum output inductor current.
FET On,
Caps Discharging
I Lo,MAX + I O,MAX 2 ) DI Lo 2
N IN + I CIN,RMS I RMS,RATED
I Lo,MIN + I O,MAX 2 * DI Lo 2
) DI C,IN 2 3) ) I IN,AVG 2 @ (1 * 2D)] 1 2
Two−Phase Converter
CIN,RMS
o
.
RMS,RATED
t
ON
FET Off,
Caps Charging
DI
) based on the rms ripple current
C,IN
= I
).
C,MAX
− I
T/2
C,MIN
IN
) to provide the
(10)
(12)
(13)
(11)
(8)
(9)