Texas Instruments | Special Features of the SN65HVS88x Family of Digital Input Serializers | Application notes | Texas Instruments Special Features of the SN65HVS88x Family of Digital Input Serializers Application notes

Texas Instruments Special Features of the SN65HVS88x Family of Digital Input Serializers Application notes
Application Report
SLAA401 – October 2008
Special Features of the SN65HVS88x Family of Digital
Input Serializers
Thomas Kugelstadt .......................................................................................... ICP - Industrial Interface
ABSTRACT
The SN65HVS880 and SN65HVS882 provide additional performance features not
mentioned in the product data sheets. These features include an extended
negative-input voltage range, representing a sensor switch off-condition, and the
capability of a 5V back-supply in case the 24-V field supply collapses.
Negative Input Voltage Range
While the data sheet specifies the range of field input voltage between 0V and 30V for the HVS880 and
0V to 35V for the HVS882, it is possible to extend the lower limit to –5 V, provided that input resistors, RIN,
are switched in series with the field inputs IP 0:7 as demonstrated in Figure 1.
SN65HVS 88 x
Vcc
24 V
SIP
IIN 0:7
- 5 V to 34 V
DB 0:1
RLIM
GND
Serializer
RE 0:7
Signal Conditioning
R IN 0:7
= 1.2 kW
5VOP
VREG
LD
CLK
CE
SOP
TEMP
TOK
0V
Figure 1. Applying Negative Input Voltages Requires the Insertion of Series Input Resistors
For positive input voltages, each one of the inputs sinks current whose maximum value is determined by
an internal current limiter. In the case of negative input voltages however, an input starts sourcing current
and the only current limiting provided is through the input resistors RIN. To assure that the total power
consumption stays within the thermal capacity of the device, the recommended minimum resistor value for
RIN is 1.2 kΩ.
Figure 2 shows the V–I characteristic for an input with RIN = 1.2kΩ represented by the red slope in the
third quadrant.
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Back-Supply
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35
30
25
VIN - V
20
15
10
5
0
-5
-4
-3
-2
-1
0
1
IIN - mA
2
3
4
Figure 2. V–I Input Characteristic for RIN = 1.2kΩ
Back-Supply
Some rare applications require back-supply in the event of a field supply collapse. To ensure continuous
operation it is possible to apply a 5V supply to the regulator output (5VOP) of the HVS88x device. Care
must be taken to ensure that the V24 input is floating before applying the back supply, so as to avoid the
internal regulator from being driven from both input and output sides. Figure 3 suggests a simple
back-supply circuit with field supply detection.
5V
R2
VGate
Q2
R1
SN65HVS800
VField
V24
VOut
5VOP
Q1
R1 = 100 kW, R2 = 270 kW,
Q1 = Q2 = ZVP3306,
COut
RL
COut = 3.3 mF
Figure 3. Back-Supply Circuit
The circuit basically senses the loss of the field supply via the transistor Q1 and the voltage divider R1–R2
and applies an external 5V supply via Q2 to the regulator output. The blue arrows indicate the current flow
during normal operation, the red arrows the flow during back-supply.
Choosing two matched P-channel MOSFETs and making the voltage divider nonsymmetrical ensures that
the switch-off threshold of Q1 is reached before the switch-on threshold of Q2 and vice versa. Also, the
resistor values selected ensure that for a maximum field supply of VField = 30V, the VGS of Q2 stays well
below its specified maximum of ±20V.
Figure 4 shows the timing diagram for the loss and the recurrence of VField.
2
Special Features of the SN65HVS88x Family of Digital Input Serializers
SLAA401 – October 2008
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Back-Supply
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30
VField
0
20
VGate
0
30
V24 Input Floating
V24
0
5
Q1 = Off
VOut
Q2 = On
Q2 = Off
Q1 = On
0
0.5
1.5
t - Time - ms
2.5
Figure 4. Back-Supply Timing
During collapse of the field supply, the VGate voltage of Q1 and Q2 tracks VField. When the VGate drops
below the gate-source threshold voltage of Q1, Q1 becomes high impedance, thus disconnecting the V24
input from the field supply. With falling VGate, VGS of Q2 increases. When it exceeds the MOSFET’s
on-threshold voltage, Q2 conducts and provides the external 5V supply to 5VOP.
When VField returns, the gate-source thresholds of Q1 and Q2 are passed in reverse sequence. This
ensures that the 5V supply is disconnected from VOut prior to the application of VField.
Note that the orange colored curves in Figure 4 present the individual VOut responses of Q1 and Q2.
Applying a bulk capacitor of COut ≥ 3.3µF smooths the output voltage to a stable 5V level.
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Special Features of the SN65HVS88x Family of Digital Input Serializers
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