AMIS-492X0 Datasheet, PDF (15/22 Pages) ON Semiconductor – Fieldbus MAU

AMIS-492x0
N_VL
N_Vs
Tri-Level Modulator
&
Slew Control
Active Low
4R
80K
Active Low
4R
80K
R
20K
VCC
20R
400K
A3
VMID
Figure 9: Tri-level Modulator
VDRV
1.2K
21
1.2K
CRT
19
1.2K
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 =
[ R₃ V_mid (R₁₂ + R₁₁ ) ] - [ V_drv (R₂ R₁₁ + R₃ R₁₁ ) ]
- [ R_F ( R₂ R₁₂ + R₃ R₁₂ ) ]
A diode and/or a resistor 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).
Rev. 6 | Page 15 of 22 | www.onsemi.com