ncp3127 ON Semiconductor, ncp3127 Datasheet - Page 10

• Manufacturer Part Number: ncp3127
• Description: Ncp3127 2 A Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

However, when using electrolytic capacitors, a lower ripple
current will result in lower output ripple due to the higher
ESR of electrolytic capacitors. The ratio of ripple current to
maximum output current is given in Equation 5.
DI
I
ra
establish acceptable values of inductance for a design using
Equation 6.
D
F
I
L
ra
the current rating of the part. To keep within the bounds of
the part’s maximum rating, a calculation of the RMS and
peak inductor current is required.
I
I
ra
OUT
OUT
OUT
RMS
I
SW
OUT
PK
L
Using the ripple current rule of thumb, the user can
35
33
31
29
27
25
23
21
19
17
15
13
When selecting an inductor, the designer must not exceed
11
OUT
9
7
5
3
Figure 21. Inductance vs. Current Ripple Ratio
+ I
10
I
12.21 mH +
+
RMS
OUT
13
I
OUT
+ I
@ 1 )
2.01 A + 2.01 A *
16
V
@ ra @ F
OUT
OUT
CURRENT RIPPLE RATIO (%)
= Ripple current
= Output current
= Ripple current ratio
= Duty ratio
= Switching frequency
= Output current
= Output inductance
= Ripple current ratio
= Output current
= Inductor RMS current
= Ripple current ratio
ra
2.0 A
@
2
19
SW
³ 2.28 A + 2.0 A @ 1 )
1 ) ra
ra + DI
22
@ (1 * D) ³
28%
12
12 V
Iout
25
2
³
28
350 kHz
1 )
32%
31
2
@ (1 * 27.5%)
2
34
28%
37
2
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(eq. 5)
(eq. 6)
(eq. 7)
(eq. 8)
40
10
I
I
ra
be rounded to 12 mH. The inductor should also support an
RMS current of 2.01 A and a peak current of 2.28 A.
mechanical and electrical considerations. From a
mechanical perspective, smaller inductor values generally
correspond to smaller physical size. Since the inductor is
often one of the largest components in the regulation system,
a minimum inductor value is particularly important in space
constrained applications. From an electrical perspective, the
maximum current slew rate through the output inductor for
a buck regulator is given by Equation 9.
L
V
V
regulator’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level.
Reduced inductance to increase slew rates results in larger
values of output capacitance to maintain tight output voltage
regulation. In contrast, smaller values of inductance
increase the regulator’s maximum achievable slew rate and
decrease the necessary capacitance, at the expense of higher
ripple current. The peak−to−peak ripple current for
NCP3127 is given by the following equation:
D
F
Ipp
L
V
From Equation 10 it is clear that the ripple current increases
as L
dynamic response and ripple current.
categories: copper and core losses. The copper losses can be
further categorized into DC losses and AC losses. A good
first order approximation of the inductor losses can be made
using the DC resistance as shown below:
OUT
PK
SlewRate
SW
OUT
OUT
IN
OUT
OUT
A standard inductor should be found so the inductor will
The final selection of an output inductor has both
Equation 9 implies that larger inductor values limit the
The power dissipation of an inductor falls into two
Ipp +
OUT
V
decreases, emphasizing the trade−off between
LOUT
0.57 A +
OUT
L
OUT
= Output inductance
= Input voltage
= Maximum output voltage
= Duty ratio
= Switching frequency
= Peak−to−peak current of the inductor
= Output inductance
= Output voltage
+
@ F
= Output current
= Inductor peak current
= Ripple current ratio
(1 * D)
V
0.72
IN
SW
3.3 V
L
* V
12 mH @ 350 kHz
OUT
ms
A
³
OUT
+
12 V * 3.3 V
(1 * 27.5%)
³
12 mH
(eq. 10)
(eq. 9)