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參數(shù)資料
| 型號(hào)： | MC33394 |
| 廠商： | 飛思卡爾半導(dǎo)體(中國(guó))有限公司 |
| 元件分類： | CAN |
| 英文描述： | Switch Mode Power Supply with Multiple Linear Regulators and High Speed CAN Transceiver(帶有多個(gè)線性穩(wěn)壓器和高速CAN收發(fā)器的開關(guān)方式電源) |
| 中文描述： | 開關(guān)電源的多線性穩(wěn)壓器和高速CAN收發(fā)器（帶有多個(gè)線性穩(wěn)壓器和高速的CAN收發(fā)器的開關(guān)方式電源） |
| 文件頁數(shù)： | 25/44頁 |
| 文件大?。?/td> | 641K |
| 代理商： | MC33394 |
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25
MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
5. APPLICATION INFORMATION
This section provides information on external components
that are required by the 33394. The IC is designed to operate
in an automotive environment. Conducted immunity and
radiated emissions requirements have been addressed
during the design. However, the IC requires some external
protection.
Protection is required for all pins connected directly to
battery. The module designer should use an MOV or another
transient voltage suppressor in all cases, when the load
dump transition exceeds + 45 volts with respect to ground.
Protection should also include a reverse battery protection
diode (or relay) and input filter. This is required to protect the
33394 from ESD and +/– 300V ignition transients. Typical
configurations are shown in Figure 1. Outputs and inputs
connected directly to connector pins require module level
ESD protection.
5.1. Selecting Components for Linear Regulators
The output capacitor of the linear regulator serves two
different purposes. It maintains the linear regulator loop
stability, and it provides an energy reservoir to supply current
during very fast load transients. This is especially true when
supplying highly modulated loads like microcontrollers and
other high–speed digital circuits. Due to the limited
bandwidth of the linear regulators, the output capacitor is
selected to limit the ripple voltage caused by these abrupt
changes in the load current. During the fast load current
transients, the linear regulator output capacitor alone
controls the initial output voltage deviation. Hence, the output
capacitor’s equivalent series resistance (ESR) is the most
critical parameter.
The outputs, which do not experience such severe
conditions (the VREF e.g.), use the output capacitor mainly
for stability purpose, and therefore its capacitance value can
be significantly smaller. The typical output capacitor
parameters are: C = 1.0
μ
F; ESR = 2.0 ohms. When a
ceramic 1
μ
F capacitor is used, the ESR can be provided by
a discrete serial resistor (see Figure 20).
The following example shows how to determine the output
capacitance for a heavily loaded output supplying digital
circuits.
5.1.1. Selecting the Output Capacitor Example:
The output capacitance must be selected to provide
sufficiently low ESR. The selected capacitor must have an
adequate voltage, temperature and ripple current rating for
the particular application.
In order to calculate the proper output capacitor
parameters, several assumptions will be made.
1) During the very fast load current transients, the linear
regulator can not supply the required current fast enough,
and therefore for a certain time the entire load current is
supplied by the output capacitor. 2) The capacitor’s
equivalent series inductance (ESL) is neglected. These
assumptions can greatly simplify the calculations, and are
reasonable for most of practical applications. Then the ESR
of the output capacitor has to satisfy the following condition:
ESR
Vo
Io
Where:
Vo is the maximum allowed linear regulator voltage drop
caused by the load current transient.
Io is the maximum current transient, which can occur due to
the abrupt step in the linear regulator load current.
In this example the VDDH output with the 400 mA load step
is considered with the maximum voltage drop of 100mV. This
gives the output capacitor’s maximum ESR value of:
100 mV
400 mA
This level of ESR requires a relatively large capacitance. In
order to maintain the linear regulator stability and to satisfy
large load current steps requirements the solid tantalum
capacitor 100
μ
F/10V with ESR = 200 m
. One device that
meets these requirements is the TPSC107K010S020
tantalum capacitor from the AVX Corporation.
VESR
ESR
Io
200 m
In the next step, the voltage drop associated with the
capacitance can be calculated:
Io
t
C
ESR
250 m
400 mA
80 mV
VC
0.4 A
5
F
s
100
20 mV
Where:
C is the output capacitance.
t is the linear regulator response time.
Io is the maximum current transient, which can occur due to
the abrupt step in the linear regulator load current.
Assuming that the capacitor ESL is negligible, the total
voltage drop in the voltage regulator output caused by the
current fast transient can be calculated as:
Vtotal
VESR
VC
A ceramic capacitor with capacitance value 10nF should
be placed in parallel to provide filtering for the high frequency
transients caused by the switching regulator.
Properly sized decoupling ceramic capacitor close to the
microprocessor supply pin should be used as well. Table 1
shows the suggested output capacitors for the 33394 IC
linear regulator outputs.
Other factors to consider when selecting output capacitors
include key off timing for memory retention. Though the
VKAM is not a heavily loaded output, the VKAM output
capacitor has to have a sufficiently large capacitance value to
supply current to the microcontroller for a certain time after
battery voltage is disconnected.
80 mV
20 mV
100 mV
F
Freescale Semiconductor, Inc.
For More Information On This Product,
n
.
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