NCV8881PWR2G ON Semiconductor, NCV8881PWR2G Datasheet - Page 30

• Manufacturer Part Number: NCV8881PWR2G
• Description: IC SMPS WITH WATCHDOG 16SOIC
• Manufacturer: ON Semiconductor
• Series: -r

Specifications of NCV8881PWR2G

• Topology: Step-Down (Buck) (1), Linear (LDO) (2)
• Function: Automotive
• Number Of Outputs: 3
• Frequency - Switching: 170kHz
• Voltage/current - Output 1: 3.3 V ~ 8 V, 1.5A
• Voltage/current - Output 2: 5V, 100mA
• Voltage/current - Output 3: 8.5V, 40mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 5 V ~ 19 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.295", 7.50mm Width) Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

of input offset current error, it is customary to calculate R1
with R2 set at 1 kW.
SMPS Snubber
series between the SW pin and ground. Typical values are
10 W and 1 nF.
SMPS Freewheeling Diode Selection
inductor current path when the power switch turns off, and
is sometimes referred to as the commutation diode. The
diode peak inverse voltage must exceed the maximum
operating input voltage in order to accommodate any higher
peak voltage produced by switchnode ringing. The peak
conducting current is determined by the internal current
limit. The average diode current can be calculated from the
output current I
voltage V
to the maximum NCV8881 current limit, such as the
MBRA340T3.
Inductor Selection
influence the selection of an output inductor. From a
mechanical perspective, smaller inductor values generally
correspond to smaller physical size. Since the inductor is
often one of the largest components in SMPS system, a
minimum inductor value is particularly important in
space−constrained applications.
perspective, smaller inductor values correspond to faster
transient response. The maximum current slew rate through
the output inductor for a buck regulator is given by:
Where I
and V
the regulator’s ability to slew current through the output
Consequently, output capacitors must supply sufficient
charge to maintain regulation while the inductor current
“catches up” to the load. This 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
A resistor and ceramic capacitor must be connected in
The freewheeling diode in the SMPS provides the
The freewheeling diode should have a current rating equal
Mechanical and electrical considerations, as well as cost
This equation indicates that larger inductor values limit
inductor in response to output load transients.
L
is the voltage drop across the inductor.
L
SWOUT
is the inductor current, L is the output inductance,
I
D(avg)
Inductor Slew Rate +
OUT
by:
+ I
, the input voltage V
OUT
1 *
V
dI
From
dt
SWOUT
L
V
+
IN
IN
V
L
and the output
L
an
electrical
(eq. 2)
(eq. 3)
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30
the necessary capacitance, at the expense of higher ripple
current.
current is calculated using the following equation:
is clear that the ripple current increases as L decreases,
emphasizing the trade−off between dynamic response and
ripple current. For most applications, the inductor value falls
in the range between 10 mH and 22 mH. There are many
magnetic component suppliers providing energy storage
inductor product lines suitable such as the Wurth TPC series
or TOKO DSH104C series inductors, which are
recommended for the automotive radio applications.
SMPS Output Capacitor Selection
response of the power supply. In fact, during load transient,
it supplies the current to the load for first few microseconds,
where after the controller recognizes the load transient and
proceeds to increase the duty cycle. Neglecting the effect of
the ESL, the output voltage has a first drop due to the ESR
of the capacitor.
In addition, a lower ESR produces a lower output voltage
ripple. The voltage drop due to the output capacitance
discharge can be approximated using the following
equation:
Where, D
90%. Although the ESR effect is not in phase with the
added to DV
the maximum DV
Simulation can also help determine the maximum
DV
with the actual load since the ESL effect is dependent on
layout and the actual load’s di/dt.
SMPS Input Capacitor Selection
selecting the input capacitor is input RMS current rating.
DV
In continuous conduction mode, the peak−to−peak ripple
Where T
The output capacitor is a basic component for the fast
A lower ESR produces a lower DV during load transient.
discharging of the output voltage, DV
Besides voltage rating, a primary consideration for
+
SWOUT
SWOUT(CHARGE)
2
I
PP
MAX
C
; however, it will ultimately have to be verified
SW
DV
SWOUT
+ T
SWOUT(CHARGE)
is the switching period. From this equation it
SWOUT(ESR)
is the maximum duty cycle value, which is
SW
SWOUT
DI
V
V
SWOUT
SWOUT
IN(MIN)
L
+ DI
during a transient condition.
to give a rough indication of
2
SWOUT
D
L
1 *
MAX
* V
SWOUT(ESR)
V
SWOUT
ESR
V
IN
SWOUT
(eq. 4)
(eq. 5)
(eq. 6)
can be