S

5

DA9171A.000

July 31, 1997

APPLICATION INFORMATION

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GENERAL

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.

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INPUT PROTECTION FOR MICROCONTROLLERS

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

Vdd

uC

P1

Vin

Fig.2 : Discrete protection circuit

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.