ncp5306dw ON Semiconductor, ncp5306dw Datasheet - Page 17

• Manufacturer Part Number: ncp5306dw
• Description: Threephase Vrm 9.0 Buck Controller
• Manufacturer: ON Semiconductor

less than half the input voltage, the current will be increased
much more quickly than it can be decreased. Thus, it may be
more difficult for the converter to stay within the regulation
limits when the load is removed than when it is applied and
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 three individual phase currents that are
120 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
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 21.
minimum currents delivered by the input capacitors:
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
D is the duty cycle of the converter, D = V
η is the specified minimum efficiency;
I
The input capacitors will discharge when the control FET
The following equations will determine the maximum and
I
O,MAX
Lo,MAX
(V IN * #Phases @ V OUT ) @ D
is the maximum converter output current.
is the maximum output inductor current:
I C,MAX + I Lo,MAX h * I IN,AVG
I Lo,MAX + I O,MAX 3 ) DI Lo 2
I C,MIN + I Lo,MIN h * I IN,AVG
Dt DEC + Lo @ DI O (V OUT )
I IN,AVG + I O,MAX @ D h
N OUT,MIN ) @
OUT,MIN
(Lo MIN @ f SW )
) and the per
OUT
/V
MIN
IN
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), the
;
(3.2)
(4)
(5)
(6)
(7)
(8)
17
inductor of value Lo:
current is then:
I CIN,RMS + [3D @ (I C,MIN 2 ) I C,MIN @ DI LO
RMS input current (I
current rating per capacitor (I
the worst case input ripple−current will occur when the
converter is operating at a 16.7% duty cycle. At this
operating point, the parallel combination of input capacitors
must support an RMS ripple current equal to 16.7% 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 3% or 30%, the three−phase input ripple−current will
be approximately 10% 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 know 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
ΔI
For the three−phase converter, the input capacitor(s) RMS
Select the number of input capacitors (N
For a three−phase converter with perfect efficiency (η = 1),
In general, capacitor manufacturers require derating to the
Low ESR capacitors are recommended to minimize losses
Lo,MIN
−I
Lo
I
IN,AVG
I
C,MAX
P CIN + I CIN,RMS 2 @ ESR_per_capacitor N IN
C,MIN
0 A
is the peak−to−peak ripple current in the output
Figure 21. Input Capacitor Current for a
DI Lo + (V IN * V OUT ) @ D (Lo @ f SW )
is the minimum output inductor current:
FET On,
Caps Discharging
N IN + I CIN,RMS I RMS,RATED
I Lo,MIN + I O,MAX 3 * DI Lo 2
) DI LO 2 3) ) I IN,AVG 2 @ (1 * 3D)] 1 2
Four−Phase Converter
CIN,RMS
t
ON
FET Off,
Caps Charging
ΔI
C,IN
RMS,RATED
) based on the RMS ripple
= I
C,MAX
− I
):
T/3
C,MIN
IN
) to provide the
= ΔI
LO
(10)
(12)
(13)
(11)
(9)