AMIS-49200 Datasheet, PDF (16/22 Pages) AMI SEMICONDUCTOR – Fieldbus MAU Chip

AMIS-49200 Fieldbus MAU
Data Sheet
4.3.3. Slew Rate Controller
Amplifier (A3), shown in the above figure, controls the slew rate. The amplifier converts the current signals from the tri-level modulator
to a voltage signal, VDRV. It controls its slew rate with a capacitor (C_RT) connected to the CRT pin. The waveform at the VDRV pin is
symmetric and the fall/rise times are determined by the following equation:
t_F, t_R = 2.0[µs] + 0.12 [µs/pF] × C_RT
The constant part comes from the internal capacitor (not shown). It is recommended to make a guard pattern on your circuit board
around the CRT pin and the hot side of C_RT to avoid unnecessary interference.
4.3.4. Current Drive Amplifier
The drive amplifier is an operational amplifier optimized to drive current drivers for 31.25kbps voltage-mode medium. Its input and
output signals are exposed to allow flexible design of the external driver. Note that this amplifier cannot directly sink the necessary
current from the medium. In the following drive circuit the current (I_BUS) through the current-detect resister (R_F) is determined by the
following equation.
I_BUS = (1/R_F) × (V_DRV - V_MID) × (R_B/R_A).
A diode and/or a resister connected to the emitter are necessary to shift the DC level of CCOUT and to suppress the loop gain. The
resistance value depends on your design (overall gain and emitter current).
Vmid
VDRV
Current Control
Circuit
R_A
RA
CCINP
23
24
CCINM
RB
RB
VCC
CCOUT
A4
25
I_BUS
Rf
Figure 9: Current Control Circuit
4.4 Receive Block
The receive block contains three sub-blocks, which are internally connected:
1. A band pass filter – to filter the desired incoming communication signal.
2. Carrier detector – generates the RxA signal by detecting the signal amplitude.
3. Zero-cross detector generates the RxS signal by detecting the high/low transitions of the Manchester code.
AMI Semiconductor – Jan. 07, M-20532-003
16
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