NCV8851B ON Semiconductor, NCV8851B Datasheet - Page 14

• Manufacturer Part Number: NCV8851B
• Description: Automotive Grade Synchronous Buck Controller
• Manufacturer: ON Semiconductor

is at a minimum value and vice versa, as follows:
Where: i
increases as L decreases, emphasizing the trade−off between
dynamic response and ripple current. The peak and valley
values of the triangular current waveform are as follows:
Where: I
as the current at which the inductance value has dropped a
certain percentage from the nominal value, typically 10%.
For stable operation, the output inductor must be chosen so
that the inductance is close to the nominal value even at the
peak output current, I
inductor with saturation current sufficiently higher than the
peak output current, such that the inductance is very close to
the nominal value at the peak output current. This introduces
a safety factor and allows for more optimized compensation.
selecting an output inductor. Inductor losses include dc and
ac winding losses and core losses. Core losses include eddy
current losses, which are very low due to high core
resistance, and magnetic hysteresis losses, which increase
with peak−to−peak ripple current. Core losses also increase
as switching frequency increases.
winding and the RMS ripple current through the inductor,
which is much lower than the dc current. The ac winding
losses are due to skin and proximity effects and are typically
much less than the dc losses, but increase with frequency. Dc
winding losses account for a large percentage of output
inductor losses and are the dominant factor at switching
frequencies at or below 500 kHz. The dc winding losses in
the inductor can be calculated with the following equation:
Where: P
inductor losses, an inductor with very low DCR should be
chosen.
The ripple current is at a maximum when the duty cycle
From this equation it is clear that the ripple current
Saturation current is specified by inductor manufacturers
Inductor efficiency is another consideration when
Ac winding losses are based on the ac resistance of the
As can be seen from the above equation, to minimize
i
I
R
L(max)
L(min)
L(pk)
L(vly)
L(dc)
dc
: dc resistance of the output inductor (DCR)
: peak (maximum) value of ripple current [A]
: valley (minimum) value of ripple current [A]
: minimum inductor current ripple [App]
: dc winding losses in the output inductor
: maximum inductor current ripple [App]
i
i
L(max)
L(min)
P
I
I
L(dc)
L(pk)
L(vly)
+
L(pk)
+
V
V
+ I
OUT
+ I
. It is recommended to choose an
+ I
OUT
OUT
OUT
OUT
L @ F
L @ F
@ (1 * D
@ (1 * D
2
)
*
@ R
SW
SW
i
2
i
2
L
L
dc
MAX
MIN
)
)
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NCV8851B
14
www.DataSheet.co.kr
(5) Output Capacitor Selection
response of the power supply. During a load step, for the first
few microseconds, it supplies the current to the load. The
controller immediately recognizes the load step and
increases the duty cycle, but the current slope is limited by
the inductor’s slew rate. During a load release, the output
voltage will overshoot. The capacitance will dampen this
undesirable response, decreasing the amount of voltage
overshoot.
approaches zero with the worst case initial current at the
current limit and the initial voltage at the output voltage set
point, calculating the voltage overshoot as follows:
chosen for a maximum allowed output voltage overshoot:
Where: C
the inrush current and current limit. To calculate the input
startup current, the following equation can be used:
Where: I
current with the maximum load, then the input fuse should
be rated accordingly, if one is used. During soft−start, the
inductor current must provide current to the load, as well as
current to charge the output capacitor. The maximum current
which the inductor is allowed to conduct is the current limit.
Setting the inrush current to the current limit, this puts a limit
on the maximum capacitor size, as follows:
Where: C
output voltage ripple with a dc load, in addition to limiting
voltage overshoot during a dynamic response. Key
specifications are equivalent series resistance (ESR) and
equivalent series inductance (ESL). The output capacitors
must have very low ESL for best transient response. The
PCB traces will add to the ESL, but by putting the output
capacitors close to the load, this effect can be minimized and
ESL neglected in determining output voltage ripple.
The output capacitor is a basic component for the fast
In the case of stepping into a short, the inductor current
Accordingly, a minimum amount of capacitance can be
A maximum amount of capacitance can be found based on
If the inrush current is higher than the steady−state input
Capacitors should also be chosen to provide acceptable
DV
I
INRUSH
OUT(i)
MIN
C
MAX
DV
voltage overshoot to DV
during a short [V]
MIN
OS(max)
I
: minimum amount of capacitance to minimize
OS
INRUSH
: maximum output capacitance [F]
: initial output current
+
C
: input current during startup
+
MAX
(V
: maximum allowed voltage overshoot
OUT
+
+
L @ I
C
(I
) DV
C
OUT
CL
CL
* I
t
2
SS
L @ I
@ V
) V
OS(max)
V
OUT(i)
OUT
OUT
CL
OUT
OS(max)
2
) I
) @ t
)
2
2
* V
* V
OUT(i)
SS
[F]
OUT
OUT
2
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