ncp3121 ON Semiconductor, ncp3121 Datasheet - Page 14

• Manufacturer Part Number: ncp3121
• Description: Dual 3.0 A, Step-down Dc/dc Switching Regulator
• Manufacturer: ON Semiconductor

Inductor
in the converter because it will directly affect the choice of
other components and dictate both the steady state and
transient performance of the converter. When choosing
inductors, one might have to consider maximum load
current, core and copper losses, component height, output
ripple, EMI, saturation and cost. Lower inductor values are
chosen to reduce the physical size of the inductor. A higher
value cuts down the ripple current and core losses and allows
more output current. In general, the output inductance value
should be as low and the output inductor physically as small
as possible to provide the best transient response and
minimum cost. If a large inductance value is used, the
converter will not respond quickly to rapid changes in the
load current. On the other hand, an inductance value that is
too low will result in very large ripple currents in the power
components, resulting in increased dissipation and lower
converter efficiency.
to select the inductor peak-to-peak ripple current to be
approximately 25% of the maximum switch current. Also,
make sure that the inductor peak current is below the
maximum switch current limit and the selected inductor type
saturation current specification is higher than the peak
current through the switch.
Table 1. Calculated Inductor Values
The output inductor may be the most critical component
A good standard for determining the inductance to use is
12 V
12 V
5 V
5 V
5 V
12 V
in
in
in
in
in
in
to 3.3 V
to 2.5 V
to 1.8 V
to 7.5 V
to 3.3 V
to 5 V
out
out
out
out
out
out
I
f [kHz]
out
2 A
3 A
2 A
3 A
2 A
3 A
2 A
3 A
2 A
3 A
2 A
3 A
APPLICATION & DESIGN INFORMATION
[A]
Calculated coils, I ripple peak-peak 20%
http://onsemi.com
NCP3121
10.4 mH
9.4 mH
36 mH
24 mH
36 mH
24 mH
30 mH
20 mH
14 mH
16 mH
15 mH
10 mH
200
14
the continuous current mode is defined as the load current
plus one half of the DI
calculated by:
where;
only on the calculated inductance value, saturation current
rating and parasitic serial resistance, but also on the required
physical dimensions, EMI requirements (shielded or open
inductor) and the price. Examples of suitable inductors from
various manufacturers are shown in the table below.
The maximum current in the inductor while operating in
The inductance value can be calculated by:
I
V
V
DI
f
The choice of the appropriate inductor type depends not
LOAD
OSC
Therefore, the inductor peak current, I
OUT
IN
L
is the peak-to-peak inductor ripple current
is the input voltage
is the switching frequency of the oscillator
is the output voltage
is the output load current
5.4 mH
8.2 mH
5.5 mH
20 mH
14 mH
20 mH
14 mH
17 mH
12 mH
8 mH
9 mH
6 mH
350
I
LP
+ I
L +
I
LOAD
LP
L
+ I
V
V
currrent:
OUT
)
IN
LOAD
V
@ DI
2 @ V
V
5.6 mH
3.7 mH
6.3 mH
4.2 mH
5.8 mH
3.8 mH
14 mH
10 mH
15 mH
10 mH
12 mH
OUT
8 mH
IN
500
) 1
L
* V
V
@ f
IN
2
IN
@ L @ f
DI
OSC
OUT
* V
L
OUT
OSC
6.5 mH
6.5 mH
5.4 mH
3.7 mH
2.5 mH
2.8 mH
3.8 mH
2.6 mH
10 mH
10 mH
LP
8 mH
4 mH
750
, can be