AMIS-492x0

4.0 Theory of Operation

4.1 Overview

The AMIS-492x0 incorporates two different power supply circuits.

Both derive their power from the bus.

Using the internal

configuration, the shunt regulator is set for 5V and the series regulator is set for 3V. Users can modify either power supply by adding

external components. The AMIS-492x0 Fieldbus can also monitor these power supply voltages and generate power-fail signals if they

fall below a specified value. Please refer to the AMIS-492x0 Fieldbus MAU Reference Design Application Note for ways to adjust the

shunt and series voltage regulators.

The AMIS-492x0 Fieldbus MAU transmits a Manchester-encoded signal provided from a standard MDS-MAU interface. The output

driver makes it possible to design various signal circuits, which depend on the power requirements of your device. The slew rate of the

signal can be controlled to minimize unnecessary radiation as specified in IEC/ISA standards.

The AMIS-492x0 Fieldbus MAU has a built-in band pass filter which makes it easy to design your own receiver. The receive block

operates on a Manchester-encoded signal. It decodes the signal and verifies proper amplitude with a zero-cross and carrier detect

circuit, respectively. Detected signals are then passed on to a controller with the standard MDS-MAU interface.

4.2 Power Supply Block

The power supply block contains four sub-blocks:

1.

2.

3.

Two low voltage detectors - for monitoring the two supply voltages

4.

A bandgap voltage reference - which is used internally for generating a bias level for AC signals

4.2.1. Shunt Regulator

input is divided by an internal network to provide a voltage equal to Vref at the SHSET pin. If SHSET and SHSETIN pins are tied

shunt regulator provides 5V power to itself and external circuits. A capacitor of 5

μF or larger capacity is necessary to stabilize this

regulator. Figure 13 shows C10 (22

μF) connected to Pin 8 to accomplish stabilization.

SHSETIN pin. In this case, SHSET pin must be kept open. The output voltage is determined by the following equation:

Rsh

3.25Rsh

25mA

(Max)

VCC

SHUNT

SGND

SHSETIN

SHSET

Shunt Regulator

(Internal Configuration)

9

6

7

18

8

A6

System

VCC

Rsh

3.25Rsh

25mA

(Max)

VCC

SHUNT

SGND

SHSETIN

SHSET

Shunt Regulator

(External Configuration)

9

6

7

18

8

A6

System

VCC

R2

R1

N/C

VREF

VREF

Cfb

50pF

16Meg

Cfb

50pF

16Meg

Figure 4: Shunt Regulator

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