NCP5318 ON Semiconductor, NCP5318 Datasheet - Page 22

• Manufacturer Part Number: NCP5318
• Description: Two/Three/Four-Phase Buck CPU Controller
• Manufacturer: ON Semiconductor

minimum currents delivered by the input capacitors:
where f is the number of phases in operation.
inductor of value Lo:
current is then:
I CIN,RMS + [4D
the RMS input current (I
current rating per capacitor (I
the worst case input ripple−current will occur when the
converter is operating at a 12.5% duty cycle. At this
operating point, the parallel combination of input capacitors
must support an RMS ripple current equal to 12.5% 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 6% or 19%, the four−phase input ripple−current will
be approximately 10% of the converter’s DC output current.
The following equations will determine the maximum and
I
I
DI
For the four−phase converter, the input capacitor(s) RMS
Select the number of input capacitors (NC
For a four−phase converter with perfect efficiency (h = 1),
Lo,MAX
Lo,MIN
Lo
is the peak−to−peak ripple current in the output
DI Lo + (V IN * V OUT )
is the minimum output inductor current:
is the maximum output inductor current:
I C,MAX +
I Lo,MAX +
I C,MIN +
I Lo,MIN +
)
DI C,IN 2
NC IN +
3
(I C,MIN 2 ) I C,MIN
I Lo,MAX
I Lo,MIN
CIN,RMS
I O,MAX
) ) I IN,AVG 2
I O,MAX
I RMS,RATED
h
h
f
f
I CIN,RMS
RMS,RATED
* I IN,AVG
* I IN,AVG
) based on the RMS ripple
* DI Lo
) DI Lo
(Lo @ f SW )
2
2
D
):
(1 * 4D)] 1 2
DI C,IN
IN
) to provide
(eq. 10)
(eq. 12)
(eq. 13)
(eq. 14)
(eq. 11)
http://onsemi.com
(eq. 8)
(eq. 9)
22
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.
5. Input Inductor Selection
the power source isolates the voltage source and the system
from noise generated by the switching converter, while also
reducing the input current slew rate during load transients.
The worst case input current slew rate will occur during the
first few PWM cycles immediately after a step−load change
is applied as shown in Figure 24. When the load is applied,
the output voltage is pulled down very quickly. Current
through
instantaneously, so the initial transient load current is
conducted by the output capacitors. The output voltage will
step downward depending on the magnitude of the output
current (I
capacitors (ESR
output capacitors (NB
output voltage at full transient load will be:
the input voltage will be applied to the input terminal of the
output inductor (the SWNODE). At that instant, the voltage
across the output inductor can be calculated as:
cause its current to increase linearly with time. The slew rate
of this current can be calculated from:
V OUT,FULL−LOAD +
DV Lo + V IN * V OUT,FULL−LOAD
The use of an inductor between the input capacitors and
When the control MOSFET (Q1 in Figure 24) turns ON,
The differential voltage across the output inductor will
+
O,MAX
the
V IN * V OUT,NO−LOAD
) (I O,MAX )
V OUT,NO−LOAD * (I O,MAX )
OUT
), the per capacitor ESR of the output
output
) and the number of bulk electrolytic
dI Lo
OUT
dt
inductors
ESR OUT
) as shown in Figure 24. The
NB OUT
+ DV Lo
Lo
will
ESR OUT
not
NB OUT
(eq. 15)
(eq. 16)
(eq. 17)
change