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Interface (Data Transmission)
Texas Instruments Incorporated
Interfacing high-voltage applications to
low-power controllers
By Thomas Kugelstadt
Senior Applications Engineer
A common requirement of industrial applications is to
interface high-voltage potentials, such as signal outputs of
sensor switches and AC rectifiers, to the peripheral input
ports of low-voltage microcontrollers (MCUs) and digital
signal processors. A new generation of interface circuits
providing this function are digital-input serializer (DIS)
devices. They can sense digital input voltages ranging
from as low as 6 VDC up to 300 VDC and convert them
into 5-V serial data streams while consuming almost 80%
less power than a discrete design. This capability makes
DIS devices the most power- and cost-efficient solution in
industrial interface applications.
This article explains the functional principle of a DIS and
its configuration in a typical industrial interface design.
Functional principle
Understanding the operational principle of a DIS is faster
accomplished by seeing the device in the context of an
entire interface design as shown in Figure 1. A high-voltage
supply in the range of 10 to 34 V supplies the sensor
switches, S0 to S7, and the DIS. The ON/OFF status of
each sensor switch is detected by the eight parallel field
inputs of the device, then internally processed and made
available to the low-voltage inputs of a parallel-in, serial-
out shift register. An MCU provides the necessary control
signal to the serial interfac
e of
the DIS via a digital isolator.
Firstly, a load pulse at the LD input latches the switch’s
status information into the shift. Then a clock signal
applied to the CLK input serially shifts the register content
out of the DIS into a controller register via the isolator.
S0 to S7 comprise a wide range of sensor switches, such
as proximity switches, relay contacts, limit switches, push
buttons, and many more. While the input resistors, R
IN0
to
R
IN7
, are optional, they can serve two purposes when
implemented. One is that in high-voltage applications,
some industrial standards might require input resistors as
a safety precaution to prevent fire hazards in the event of
an input short circuit. The other purpose is to raise the
ON/OFF threshold voltage of a sensor switch.
Internally, each input signal is checked for signal
strength and stability. A current comparator detects
whether the input current is higher than a predefined
leakage threshold, and a voltage comparator checks
whether the input voltage is higher than an internally
Figure 1. Stand-alone digital-input system
10 to 34 V
0 V
SN65HVS882
Voltage
Regulator
Current-
Limit
Setting
V
CC
5VOP
TOK
DB0
DB1
3.3 V
R
LIM
5 V
Temperature
Sensor
Debounce
Filter
V
DD
ISO7242
MSP430™
MCU
V
CC1
V
CC
2
S0
R
IN0
IP0
SIP
LD
Current
Detect
I/P
Debounce
Filter
O/P
CL
K
SCK
MISO
Current
Limiter
Voltage
Detect
CE
GND1
GND2
DGND
RE0
ON
OFF
Channel 0
GND
S7
R
IN7
IP7
Channel 7
SOP
RE7
GND
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High-Performance Analog Products
www.ti.com/aaj
4Q 2010
Analog Applications Journal
Texas Instruments Incorporated
Interface (Data Transmission)
fixed reference voltage. If both comparator outputs are
logic high, a programmable debounce filter checks whether
the new input status is caused by a short but strong noise
transient, or whether the signal presence outlasts the
debounce time and thus presents a true input signal.
For a true input signal, the filter output presents the
corresponding logic level to the parallel inputs of the shift
register and also switches the output of the internal cur-
rent limiter accordingly. For an OFF condition (when the
switch is open), the filter output is low, and the output of
the current limiter is switched to ground. For an ON
condition, the filter output is high, and the output of the
current limiter is connected to a signal-return output
(RE). Connecting a light-emitting diode (LED) to an RE
output allows for the visible indication of a switch’s status.
Input configuration
To configure a DIS for various applications, the current
and voltage capability of its input, IPx, must be known, as
well as its switching thresholds. For that purpose, Figure 2
shows a more detailed block diagram of a channel’s input
stage. During a sensor switch’s OFF-to-ON transition, the
two parameters of interest are the positive-going voltage
threshold at a device input, V
IP-ON
, and its selected current
limit, I
IN-LIM
.
While V
IP-ON
is internally fixed at 5.2 V, I
IN-LIM
can be
adjusted via an external precision resistor, R
LIM
. Note that
setting the current limit affects all device inputs equally.
I
IN-LIM
is derived from a reference current, I
REF
, via a cur-
rent mirror, making I
IN-LIM
= 72 × I
REF
. I
REF
is determined
by the ratio of an internal bandgap reference to the resis-
tor value, R
LIM
(I
REF
= V
REF
/R
LIM
). The current limit can
therefore be expressed as a function of R
LIM
:
1.25 V
90 V
I
=×
72
=
(1)
IN-LIM
R
R
LIM
LIM
Solving for R
LIM
then provides the required resistor value
for a desired current limit:
90 V
R
=
(2)
LIM
I
IN-LIM
For low-voltage applications using a 12-V supply, setting
the current limit via R
LIM
might be the only calculation
required. Because the device inputs can tolerate voltages
of up to 34 V, switching the 12-V supply directly to a digital
Figure 2. Simplified block diagram of a single-channel input stage
1.25 V
REF
5 V
Mirror 1
n = 72
I
IN-LIM
I
REF
R
LIM
Mirror 2
n = 0.5
I
IN-LIM
I
LEAK
Is I> I
?
IN
LEAK
I
IN
Is t
> t?
IPx
SIG
DB
Debounce
(DB) Filter
To
Serializer
I
IN
Limiter
V
REF2
Is V> 5.2 V?
IP
REx
ON
OFF
Valid ON Signal
GND
21
Analog Applications Journal
4Q 2010
www.ti.com/aaj
High-Performance Analog Products
Interface (Data Transmission)
Texas Instruments Incorporated
input causes no damage to the device. With V
IP-ON
= 5.2 V,
the ON threshold lies almost in the middle of the 12-V
input-voltage range. Figure 3 shows the schematic of this
simple circuit design. With the low-current LED indicator
requiring a forward current of I
IN-LIM
= 2 mA, R
LIM
is
determined via Equation 2 to be 45 kΩ, with the closest
1% value being 44.8 kΩ.
However, for high-voltage designs using a supply of 24 V
or more, an input resistor is needed to raise the ON thresh-
old into the middle of the input-voltage range. Figure 4
presents this case, with the input-current limit assumed to
be 2 mA. The input resistor now separates the device’s
input voltage, V
IP
, from the field input voltage, V
IN
, thus
raising the actual ON threshold to V
IN-ON
= V
IP-ON
+ R
IN
× I
IN-LIM
. Inserting the specified 5.2-V threshold for V
IP-ON
and expressing I
IN-LIM
through Equation 1 yields V
IN-ON
=
5.2 V + R
IN
× 90 V/R
LIM
. Solving for R
IN
then provides the
required input-resistor value for a desired ON threshold:
R
LIM
(3)
R
=
(V
−
5.2 V)
×
IN
IN-ON
90 V
In order to set the ON threshold in the circuit in Figure 4
to V
IN-ON
= 12 V, the input resistor is determined via
Equation 3:
44.8 k
Ω
R
=
(12 V
−
5.2 V)
×
=
3.385 k
Ω
,
IN
90 V
with the closest 1% value being 3.4 kΩ.
This simple design methodology can be applied to input
voltages of up to 60 V. Higher voltages, however, will
increase V
IP
above its specified maximum of 34 V, so a
Figure 3. Switch ON condition: V
IP-ON
= 5.2 V, I
IN-LIM
= 90 V/R
LIM
0 V 12 V
V
CC
Sx
2.0
SN65HVS882
IPx
1.5
I
IN-LIM
R
LIM
1.0
R
44.8 k
V
IP-ON
REx
LIM
5.2 V
0.5
GND
0
0
2
4
6810 12
Input Voltage
(
V
)
Figure 4. Switch ON condition: V
IN-ON
= 12 V, I
IN-LIM
= 2 mA
0 V 24 V
V
CC
R
3.4
IN
Sx
2.0
SN65HVS882
IPx
1.5
I
IN-LIM
R
LIM
1.0
R
44.8 k
V
IN-ON
V
IP-ON
REx
LIM
0.5
GND
0
0
4
8
12 16 20 24
Input Voltage (V)
22
High-Performance Analog Products
www.ti.com/aaj
4Q 2010
Analog Applications Journal
Texas Instruments Incorporated
Interface (Data Transmission)
clamping element in the form of a Zener diode is required
to prevent the device input from overvoltage stress.
Figure 5 gives an example of a mains voltage detector,
often used in building automation systems. Here the AC
mains voltage of 240 V
rms
is rectified, thus yielding a peak
input of 340 VDC. At such high voltages it is necessary to
minimize the I
2
R losses within the input resistor. Therefore,
the current limit is simply set to 0.5 mA by making R
LIM
=
90 V/0.5 mA = 180 kΩ.
The ON threshold is set to 150 V by making R
IN
=
(150 V – 5.2 V) × 180 kΩ/90 V = 289.6 kΩ, with 291 kΩ as
the closest 1% value. At V
IN-ON
= 150 V, V
IP-ON
= 5.2 V, and
current limiting sets in. Beyond the ON threshold, V
IP
increases linearly until the Zener voltage of approximately
30 V is reached. At that moment, the Zener diode starts
clamping; and the Zener current, I
Z
, adds to the current
limit (I
IN-LIM
) to make up the total input current, I
IN
.
Serial interface
Reading the status information of the digital field inputs is
easy and can be performed by using either shift register
timing or serial peripheral interface timing.
When shift register timing is u
sed
, a short low-active
pulse applied to the load input (LD) latches the status
information of the digital inputs into the shift register. A
subsequent clock signal at CLK, consisting of eight con-
secutive clock cycles, serially shifts the data out of the DIS
register into the input register of an MCU. Each data shift
occurs at the rising edge of the clock signal (Figure 6).
Figure 5. Switch ON condition: V
IN-ON
= 150 V, I
IN-LIM
= 0.5 mA
5 V
V
CC
R
291 k
IN
SN65HVS885
1.0
I
IN
240 V
rms
IPx
0.75
I
IN
I
IN-LIM
I
Z
R
LIM
I
IN-LIM
REx
0.5
R
180 k
LIM
0.25
GND
I
Z
0
0
50
100 150 200 250 300 350
Input Voltage (V)
Figure 6. Serial-interface operation using shift register timing
CLK
1 2 3 4 5 6 7 8
LD
IP0:IP7
SOP
IP7 IP6 IP5 IP4 IP3 IP2 IP1 IP0 SIP
23
Analog Applications Journal
4Q 2010
www.ti.com/aaj
High-Performance Analog Products
Interface (Data Transmission)
Texas Instruments Incorporated
Designing input modules with a high channel count is
possible by daisy-chaining multiple DIS devices. In this
case the serial output of a leading device is connected
with the serial input of a following device. Figure 7 shows
the simplicity of a daisy-chained, 64-channel digital-input
module requiring only three interface lines.
Powering the interface
DIS devices allow for a variety of power-supply configura-
tions. When powered from an industrial 24-V bus, the DIS
can supply 5-V regulated output to digital isolators and
MCUs. For 5-V controllers (Figure 8a), the direct connec-
tion of supply and serial interface (SIF) lines is straight-
forward. However, 3.3-V controllers require a low-dropout
regulator (LDO) for the supply line and a voltage divider
in the serial output (SOP) line (Figure 8b). Control signals
from a 3.3-V controller towards the DIS are correctly
interpreted.
In applications without a bus supply, it is possible to
back-supply a DIS by driving the 5-V output as a supply
Figure 7. Daisy-chained, 64-channel digital-input module
IN31
IN24
IN23
IN16
IN15
IN8
IN7
IN0
SN65HVS882
SN65HVS882
SN65HVS882
SN65HVS882
FPGA Serial
Interface (SIF)
STE
SCK
MISO
SN65HVS882
SN65HVS882
SN65HVS882
SN65HVS882
IN32
IN39
IN40
IN47
IN48
IN55
IN56
IN63
Figure 8. Bus-powered digital-input system
SN65HVS882
SN65HVS882
TPS76333
5V
5V
3.3 V
V
CC
10 to 34 V
V
CC
10 to 34 V
5VOP
5VOP
LDO
V
DD
V
DD
MSP430
MCU
MSP430™
MCU
LD
LD
10 to 34 V
10 to 34 V
CLK
CLK
SOP
SOP
1 k
GND
GND
GND
GND
2 k
0 V
0 V
(a) With 5-V controller
(b) With 3.3-V controller
24
High-Performance Analog Products
www.ti.com/aaj
4Q 2010
Analog Applications Journal
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