NCP1442, NCP1443, NCP1444, NCP1445
I
L
Magnetic Component Selection
I
IN
When choosing a magnetic component, one must consider
factors such as peak current, core and ferrite material, output
voltage ripple, EMI, temperature range, physical size and
cost. In boost circuits, the average inductor current is the
+
−
V
CC
C
IN
product of output current and voltage gain (V
/V ),
OUT CC
assuming 100% energy transfer efficiency. In continuous
conduction mode, inductor ripple current is:
R
ESR
V
(V
CC OUT
* V
)
CC
I
+
RIPPLE
(f)(L)(V
OUT)
where:
f = 280 kHz for NCP1442/3 and 560 kHz for NCP1444/5.
The peak inductor current is equal to average current plus
half of the ripple current, which should not cause inductor
saturation. The above equation can also be referenced when
selecting the value of the inductor based on the tolerance of
the ripple current in the circuits. Small ripple current
provides the benefits of small input capacitors and greater
output current capability. A core geometry like a rod or
barrel is prone to generating high magnetic field radiation,
but is relatively cheap and small. Other core geometries,
such as toroids, provide a closed magnetic loop to prevent
EMI.
Figure 34. Boost Circuit Effective Input Filter
The situation is different in a flyback circuit. The input
current is discontinuous and a significant pulsed current is
seen by the input capacitors. Therefore, there are two
requirements for capacitors in a flyback regulator: energy
storage and filtering. To maintain a stable voltage supply to
the chip, a storage capacitor larger than 20 mF with low ESR
is required. To reduce the noise generated by the inductor,
insert a 1.0 mF ceramic capacitor between V and ground
CC
as close as possible to the chip.
Input Capacitor Selection
Output Capacitor Selection
In boost circuits, the inductor becomes part of the input
filter, as shown in Figure 34. In continuous mode, the input
current waveform is triangular and does not contain a large
pulsed current, as shown in Figure 33. This reduces the
requirements imposed on the input capacitor selection.
During continuous conduction mode, the peak to peak
inductor ripple current is given in the previous section. As
we can see from Figure 33, the product of the inductor
current ripple and the input capacitor’s effective series
V
OUT
ripple
resistance (ESR) determine the V
ripple. In most
CC
applications, input capacitors in the range of 10 mF to
100 mF with an ESR less than 0.3 W work well up to a full
4.0 A switch current.
I
L
Figure 35. Typical Output Voltage Ripple
V
CC
ripple
By examining the waveforms shown in Figure 35, we can
see that the output voltage ripple comes from two major
sources,
charging/discharging of the output capacitor. In boost
circuits, when the power switch turns off, I flows into the
namely
capacitor
ESR
and
the
I
IN
L
output capacitor causing an instant DV = I × ESR. At the
IN
I
L
same time, current I − I
charges the capacitor and
L
OUT
increases the output voltage gradually. When the power
switch is turned on, I is shunted to ground and I
L
OUT
discharges the output capacitor. When the I ripple is small
L
enough, I can be treated as a constant and is equal to input
L
Figure 33. Boost Input Voltage and Current
Ripple Waveforms
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