MAS9171AS MAS [Micro Analog systems], MAS9171AS Datasheet - Page 5

• Manufacturer Part Number: MAS9171AS
• Description: OCTAL PORT PROTECTON
• Manufacturer: MAS [Micro Analog systems]

APPLICATION INFORMATION
In the late 70ties microcontrollers have been introduced into most of the low cost applications in the field of
consumer, industrial and automotive. Lowest cost 8-Bit microcontrollers are used in modern computer
monitors for user programmable adjustment of the picture geometrie. A modern car can carry up to 40
microcontrollers that control motor managment, air condition, wipers, brakes, instruments,......
The increased density of complex electronic systems results in high electromagnetic fields with a wide
frequency spectrum in the environment. Examples for typical noise sources are DC-Motors, ignitors,
discharge lamps, switch-mode supplies and all different kinds of transmitters like mobile phones and radio
stations. The requirements on EMI immunity are defined in several DIN, ISO and IEC standards.
Design-Engineers have to think of how to protect sensitive components like microcontrollers against
overvoltage pulses. Especially pulses with short rise times might cause problems because most of the
standard components are not designed to work properly at frequencies above the cut-off frequency.
In addition the layout of the PCB is very critical at high frequencies. Filters components have to be placed
very closely to the connectors leading to external wires that may work as antennas. It is quite common that a
lot of filter and protection components have to be designed in at a very late phase of the project when starting
EMI measurements with pre-production parts. This can cause delays to the product introduction phase and
therefore it is very important to think about the limits of sensitive components already with the first Prototypes.
A typical protection circuit for a digital input of a
HCMOS microcontrollers consists of one series
resistor, one or two block capacitors to Vss and
two clamping diodes to Vss/Vdd.
The series resistor shall limit the maximum current
into the internal protection diodes to a value below
the latch-up current of the input structure (typ.
25mA). Together with the capacitor it builds up a
1th order RC-filter.The purpose of the filter is to get
a stable signal within the sampling period of the
microcontroller without need of software filtering.
For resistor values > 100 kOhms however, the
leakage current of the port may cause unwanted
dropout
voltage which is not any longer neglectible to the input voltage. Over the full temperature and supply voltage
range this can cause severe tolerance problems. Those inputs need extra clamping diodes to the Vss/Vdd
and a lower value series resistor.
Most of 0805/1204 SMD resistors are specified for voltages < 200V. For higher voltages two resistors in
series are recommended. Especially for applications with a lot of input ports the protection circuitry around
the microcontroller consumes a big amount of board space and therefore makes the board layout very
difficult even when using SMD components. Each component is a potential source of an error in production
and must be tested with In-circuit testers.
GENERAL
INPUT PROTECTION FOR MICROCONTROLLERS
V in
Fig.2 : Discrete protection circuit
V dd
P 1
DA9171A.000
July 31, 1997
uC
5