iC-VX
3-CHANNEL DIFFERENTIAL LINE DRIVER
Rev C1, Page 7/11
EXAMPLE 1: Short lines
Short lines of 5m, for example, are approximations of capacitive load for the iC; no adjustment of characteristic
impedance is required. With each switching slope changeover losses of Pc= 1/2 VB × I(A) per channel occur in
the iC. The load capacity is reloaded with the guaranteed driver current I(A)$ 30mA. These changeover losses
determine the possible cut-off frequency, since the high chip power loss without cooling results in shutdown of
the iC. At high capacitive load the transmission rate can also be limited by the fall and rise times wich reduce the
signal strength.
24V
5V
C2
1µF
C1
1µF
3
6
16
VCC
VT
VB1
PLC
A1
15
14
L=5m, CL=500pF
1
2
7
E1
E2
E3
A
2k
NA1
CHAN1
CHAN2
CHAN3
A2
13
11
B
2k
NA2
A3
10
9
Z
2k
NA3
THERMAL SHUTDOWN
BIAS
iC-VX
VEE
PROG
5
VSUB
12
VB2
4
8
Fig. 1: Balanced data transmission at low capacitive load, PROG pin open: I(A)$ 30mA
As a typical application, Fig. 1 shows the transmission of the output signals of an incremental rotary encoder
(track A, track B, index pulse Z) to a programmable control (PLC). The maximum signal frequency which is
limited by the power loss can be estimated by standardizing the limiting values of the example for short lines:
2
413K&Ta
70K
500pF
24V
75K/W
2
fmax . 200kHz ×
×
×
×
×
(1.1)
CL
VB
Rthja
channels
If the slew-rate is the limiting factor, the following applies for the maximum signal frequency (saturation voltages
neglected):
30mA
fmax
.
(1.2)
4×VB×(CL%1nF)
CL
= Capacitive load at output A to output NA
VB = Supply voltage
Ta
= Ambient temperature
Rthja = Thermal resistance chip/board/ambient (Rthja = Rthjb + Rthba
)
ICHAUS [ IC-HAUS GMBH ]
