AMIS-492X0 Datasheet, PDF (11/22 Pages) ON Semiconductor – Fieldbus MAU

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. A shunt regulator - for establishing a supply voltage of V_CC (typ. = 5V) used by the analog circuitry
2. A series regulator - for establishing a supply voltage of V_DD (typ. = 3V) used for digital circuitry
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
The shunt regulator controls its sink current to the SHUNT pin so that the voltage applied to the SHSETIN pin is equal to V_REF. The V_CC
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
together, and V_CC and SHUNT pins are connected to a power source of high impedance (e.g., current mirror circuit of signal driver), the
shunt regulator provides 5V power to itself and external circuits. A capacitor of 5μF or larger capacity is necessary to stabilize this
regulator. Figure 13 shows C10 (22μF) connected to Pin 8 to accomplish stabilization.
It is possible to increase the V_CC voltage up to 6.2V by dividing V_CC with an external network to supply the appropriate voltage to
SHSETIN pin. In this case, SHSET pin must be kept open. The output voltage is determined by the following equation:
V_CC = V_REF × (1 + R₁ / R₂)
Shunt Regulator
(Internal Configuration)
System
VCC
VCC
18
3.25Rsh
VREF
Rsh
Cfb
16Meg
50pF
SHUNT
8
A6
25mA
(Max)
Shunt Regulator
(External Configuration)
System
VCC
VCC
18
Cfb
3.25Rsh
R1
16Meg
SHUNT
VREF
50pF
8
Rsh
A6
25mA
(Max)
R2
7 ^SHSET
6 ^SHSETIN
9 ^SGND
7 ^SHSET
N/C
6 ^SHSETIN
Figure 4: Shunt Regulator
Rev. 6 | Page 11 of 22 | www.onsemi.com
9 ^SGND