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參數資料
| 型號： | NCV8881PWR2G |
| 廠商： | ON Semiconductor |
| 文件頁數： | 30/33頁 |
| 文件大小： | 321K |
| 描述： | IC REG TRPL BUCK/LINEAR 16SOIC |
| 標準包裝： | 1 |
| 拓撲： | 降壓（降壓）（1），線性（LDO）（2） |
| 功能： | 車載 |
| 輸出數： | 3 |
| 頻率 - 開關： | 170kHz |
| 電壓/電流 - 輸出 1： | 3.3 V ~ 8 V，1.5A |
| 電壓/電流 - 輸出 2： | 5V，100mA |
| 電壓/電流 - 輸出 3： | 8.5V，40mA |
| 帶 LED 驅動器： | 無 |
| 帶監控器： | 無 |
| 帶序列發生器： | 無 |
| 電源電壓： | 5 V ~ 19 V |
| 工作溫度： | -40°C ~ 150°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-SOIC（0.295"，7.50mm 寬）裸露焊盤 |
| 供應商設備封裝： | 16-SOIC W |
| 包裝： | 標準包裝 |
| 其它名稱： | NCV8881PWR2GOSDKR |
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NCV8881
http://onsemi.com
30
of input offset current error, it is customary to calculate R1
with R2 set at 1 kW.
SMPS Snubber
A resistor and ceramic capacitor must be connected in
series between the SW pin and ground. Typical values are
10 W and 1 nF.
SMPS Freewheeling Diode Selection
The freewheeling diode in the SMPS provides the
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
OUT
, the input voltage V
IN
and the output
voltage V
SWOUT
by:
I
D(avg)
+I
OUT
1*
V
SWOUT
V
IN
(eq. 2)
The freewheeling diode should have a current rating equal
to the maximum NCV8881 current limit, such as the
MBRA340T3.
Inductor Selection
Mechanical and electrical considerations, as well as cost
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
spaceconstrained applications. From an electrical
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:
InductorSlewRate+
dI
L
dt
+
V
L
L
(eq. 3)
Where I
L
is the inductor current, L is the output inductance,
and V
L
is the voltage drop across the inductor.
This equation indicates that larger inductor values limit
the regulators ability to slew current through the output
inductor in response to output load transients.
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 regulators maximum achievable slew rate and decrease
the necessary capacitance, at the expense of higher ripple
current.
In continuous conduction mode, the peaktopeak ripple
current is calculated using the following equation:
I
PP
+T
SW
V
SWOUT
L
1*
V
SWOUT
V
IN
(eq. 4)
Where T
SW
is the switching period. From this equation it
is clear that the ripple current increases as L decreases,
emphasizing the tradeoff 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
The output capacitor is a basic component for the fast
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.
DV
SWOUT(ESR)
+DI
SWOUT
ESR
(eq. 5)
A lower ESR produces a lower DV during load transient.
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:
DV
SWOUT(CHARGE)
(eq. 6)
+
DI
SWOUT
2
L
2 C
SWOUT
V
IN(MIN)
D
MAX
*V
SWOUT
Where, D
MAX
is the maximum duty cycle value, which is
90%. Although the ESR effect is not in phase with the
discharging of the output voltage, DV
SWOUT(ESR)
can be
added to DV
SWOUT(CHARGE)
to give a rough indication of
the maximum DV
SWOUT
during a transient condition.
Simulation can also help determine the maximum
DV
SWOUT
; however, it will ultimately have to be verified
with the actual load since the ESL effect is dependent on
layout and the actual loads di/dt.
SMPS Input Capacitor Selection
Besides voltage rating, a primary consideration for
selecting the input capacitor is input RMS current rating.
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