NCP3170ADR2G ON Semiconductor, NCP3170ADR2G Datasheet - Page 15

• Manufacturer Part Number: NCP3170ADR2G
• Description: IC BUCK SYNC/ASYNC ADJ 3A 8SOIC
• Manufacturer: ON Semiconductor
• Series: -r
• Type: Step-Down (Buck), PWM - Current Moder

Specifications of NCP3170ADR2G

• Internal Switch(s): Yes
• Synchronous Rectifier: Both
• Number Of Outputs: 1
• Voltage - Output: 0.8 V ~ 12.8 V
• Current - Output: 3A
• Frequency - Switching: 500kHz
• Voltage - Input: 4.5 V ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

be rounded to 4.7 mH. The inductor should support an RMS
current of 3.01 A and a peak current of 3.51 A. A good
design practice is to select an inductor that has a saturation
current that exceeds the maximum current limit with some
margin.
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 NCP3170 is
given by the following equation:
D
F
I
L
V
increases as L
between dynamic response and ripple current.
categories: copper and core losses. 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:
PP
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
From Equation 10, it is clear that the ripple current
The power dissipation of an inductor falls into two
SlewRate
1.02 A +
= Output inductance
= Input voltage
= Output voltage
= Duty ratio
= Switching frequency
= Peak−to−peak current of the inductor
= Output inductance
= Output voltage
I
1.85
PP
OUT
LOUT
+
ms
A
3.3 V
decreases, emphasizing the trade−off
V
4.7 mH
OUT
L
+
+
OUT
V
12 V * 3.3 V
IN
L
4.7 mH
( 1 * 27.5% )
( 1 * D )
* V
OUT
F
500 kHz
SW
OUT
³
³
(eq. 10)
(eq. 9)
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15
DCR
I
LP
geometry of the selected core, core material, and wire used.
Most vendors will provide the appropriate information to
make accurate calculations of the power dissipation at which
point the total inductor losses can be captured by the
equation below:
LP
LP
LP
LP
Output Capacitor Selection
output capacitor are DC voltage rating, ripple current rating,
output ripple voltage requirements, and transient response
requirements.
the life time of a product. When selecting a capacitor it is
important to select a voltage rating that is de−rated to the
guaranteed operating life time of a product. Further, it is
important to note that when using ceramic capacitors, the
capacitance decreases as the voltage applied increases; thus
a ceramic capacitor rated at 100 mF 6.3 V may measure
100 mF at 0 V but measure 20 mF with an applied voltage of
3.3 V depending on the type of capacitor selected.
current at full load with proper derating. The capacitor RMS
ratings given in datasheets are generally for lower switching
frequencies than used in switch mode power supplies, but a
multiplier is given for higher frequency operation. The RMS
current for the output capacitor can be calculated below:
Co
I
ra
combination of the ripple current selected, the output
capacitance selected, the Equivalent Series Inductance
(ESL), and Equivalent Series Resistance (ESR).
RMS
OUT
The core losses and AC copper losses will depend on the
The important factors to consider when selecting an
The output capacitor must be able to operate properly for
The output capacitor must be rated to handle the ripple
The maximum allowable output voltage ripple is a
CU_DC
Core
CU_AC
CU_DC
tot
RMS
67 mW + 61 mW ) 5 mW ) 1 mW
0.294 A + 3.0 A
LP
CO
tot
RMS
+ LP
= Inductor DC resistance
= Inductor RMS current
= Inductor DC power dissipation
= Inductor core power dissipation
= Inductor AC power dissipation
= Inductor DC power dissipation
= Total inductor losses
= Output capacitor RMS current
= Output current
= Ripple current ratio
+ I
LP
61 mW + 3.01
CU_DC
CU_DC
OUT
ra
12
34%
) LP
+ I
12
³
RMS
CU_AC
2
2
6.73 mW
) LP
DCR ³
Core
³
(eq. 12)
(eq. 13)
(eq. 11)