ncp3170 ON Semiconductor, ncp3170 Datasheet - Page 14

• Manufacturer Part Number: ncp3170
• Description: Ncp3170 Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

that are LC filtered to produce a lower DC output voltage
(V
the on time relative to the switching period (T) or switching
frequency. The ratio of high side switch on time to the
switching period is called duty ratio (D). Duty ratio can also
be calculated using V
Drop (V
(V
D
FSW
T
TOFF
TON
V
VHSD
VLSD
VOUT
Inductor Selection
rule of thumb for the design where the percentage of ripple
current in the inductor should be between 10% and 40%.
When using ceramic output capacitors, the ripple current can
be greater because the ESR of the output capacitor is smaller,
thus a user might select a higher ripple current. 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.
OUT
IN
4.7 mH +
The buck converter produces input voltage (V
When selecting an inductor, the designer may employ a
Using the ripple current rule of thumb, the user can
OUT
HSD
L
OUT
).
). The output voltage can be changed by modifying
LSD
+
I
3.0 A
= Duty ratio
= Switching frequency
= Switching period
= High side switch off time
= High side switch on time
= Input voltage
= High side switch voltage drop
= Low side switch voltage drop
= Output voltage
OUT
), and the High Side Switch Voltage Drop
D +
D +
D +
V
= Ripple current
= Output current
= Ripple current ratio
OUT
ra
V
V
34%
OUT
V
T
IN
OUT
12 V
IN
T
ON
V
* V
ra +
F
, V
OUT
F
³ 27.5% +
( 1 * D ) +
SW
SW
IN
500 kHz
HSD
) V
+
, the Low Side Switch Voltage
I
OUT
DI
) V
( 1 * D ) ³
1
T
LSD
LSD
T
OFF
3.3 V
T
12 V
( 1 * 27.5% )
[
IN
) pulses
(eq. 2)
(eq. 3)
(eq. 4)
(eq. 5)
(eq. 6)
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14
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 current
and peak current are required.
I
I
ra
I
I
ra
OUT
OUT
RMS
OUT
PK
SW
OUT
When selecting an inductor, the designer must not exceed
19
17
15
13
11
9
7
5
3
1
10
3.01 A + 3 A
I
3.51 A + 3 A
Figure 43. Inductance vs. Current Ripple Ratio
RMS
13
4.7 mH
I
4.4 V
PK
+ I
+ I
16
OUT
7 V
RIPPLE CURRENT RATIO (%)
= Ripple current ratio
= Duty ratio
= Switching frequency
= Output current
= Output inductance
= Ripple current ratio
= Output current
= Inductor RMS current
= Ripple current ratio
OUT
= Output current
= Inductor peak current
19
18 V
22
1 )
1 )
1 )
1 )
34%
34%
25
ra
12
ra
12
2
2
2
³
2
³
28
³
31
34 37
(eq. 7)
(eq. 8)
40