AN-1380: Generating Secondary Fault Supplies for Fault Protected Switches PDF

AN-1380
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Generating Secondary Fault Supplies for Fault Protected Switches
by Paul O’Sullivan
INTRODUCTION
FAULT PROTECTION OVERVIEW
The new range of fault protected switches from Analog Devices,
Inc., (ADG5462F, ADG5243F, ADG5248F, and ADG5249F)
allow for user defined fault protection levels. Two secondary
power supplies on the device, POSFV and NEGFV, are the
required secondary power supplies that set the level at which
the overvoltage protection is engaged. POSFV can be supplied
from 4.5 V up to VDD, and NEGFV can be supplied from VSS to
0 V. If a secondary supply is not available, these pins (POSFV
and NEGFV) must be connected to VDD (POSFV) and VSS
(NEGFV). In that case, the overvoltage protection then engages
at the primary supply voltages. When the voltages at the source
inputs exceed POSFV or NEGFV by a threshold voltage, VT, the
channel turns off or, if the device is unpowered, the channel
remains off. The source input remains high impedance while
the channel is off.
Internal circuitry enables the switch to detect overvoltage inputs
by comparing the voltage on the source pins with POSFV and
NEGFV. A signal is considered overvoltage if it exceeds the
secondary supply voltages by the voltage threshold (VT). The
threshold voltage is typically 0.7 V, but it ranges from 0.8 V at
−40°C down to 0.6 V at +125°C.
The secondary supplies (POSFV and NEGFV) provide the
current required to operate the fault protection and, therefore,
must be low impedance supplies. For that reason, they cannot
be generated from a resistor divider off the main supply rails.
This application note describes some of the options for generating
the secondary supply rails depending on the system requirements, and the advantages and disadvantages of each of the
supply configuration options.
When an overvoltage condition is detected on a source pin (Sx),
the switch automatically opens and the source pin becomes
high impedance and ensures that no current flows through the
switch. The drain pin is then either pulled to the supply that was
exceeded or goes open circuit, depending on the device and the
configuration of the DR pin, when available.
POSFV
ESD
PROTECTION
ESD
Dx
Sx
ESD
DR
SWITCH
DRIVER
LOGIC
BLOCK
Figure 1. Switch Channel and Control Function
VIN
VPOSFV + VT
VOUT
VOUT
VPOSFV + VT
VPOSFV + VT
OUTPUT SHOWN FOR
DR = GND
OUTPUT SHOWN FOR
DR = FLOATING/HIGH
Figure 2. ADG5462F Drain Output Response During Fault Condition
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13636-002
OUTPUT
DRAINS
THROUGH
LOAD
OUTPUT
CLAMPED
AT VPOSFV
NEGFV
13636-001
FAULT
DETECTOR
AN-1380
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1 Fault Protection Overview .............................................................. 1 Table of Contents .............................................................................. 2 Revision History ........................................................................... 2 Data Acquisition Signal Chain Using the ADG5462F Channel
Protector ............................................................................................ 3 POSFV/NEGFV Configuration Options ...................................... 4 Using Separate Low Impedance Power Supplies for
POSFV/NEGFV.............................................................................4 Adding a Diode from the Primary Supply Rail to the
Secondary Supply Rail ..................................................................5 Using Reverse Breakdown Voltage of Zener Diode to
Configure POSFV/NEGFV..........................................................5 Summary ............................................................................................6 Tying POSFV to VDD and NEGFV to VSS (or GND)................ 4 REVISION HISTORY
12/15—Revision 0: Initial Version
Rev. 0 | Page 2 of 6
Application Note
AN-1380
DATA ACQUISITION SIGNAL CHAIN USING THE
ADG5462F CHANNEL PROTECTOR
Figure 3 shows an example of a portion of a data acquisition
signal chain where the ADG5462F channel protector is used.
The PGA uses ±15 V supply rails to achieve optimal analog
performance and the ADC downstream has an input signal
range of 0 V to 5 V.
VDD
VSS
+15V
–15V
POSFV
NEGFV
+5V
GND
VDD VSS
POSFV
NEGFV
VIN
PGA
ADC
20
18
DATA
5.5V SINGLE SUPPLY
16
ON RESISTANCE (Ω)
The channel protector sits between the programmable gain
amplifier (PGA) and the analog-to-digital converter (ADC),
allowing the signal to pass through in normal operation, but
protecting the ADC by clamping any overvoltage outputs from
the PGA to between 0 V and 5 V.
Figure 4 highlights the advantage of using separate primary and
secondary supply voltages on the ADG5462F channel protector.
The ADC signal range in this example is 0 V to 5 V. Using a
switch with a 5.5 V single supply results in a large variation in
the on resistance (RON) of the switch across the signal range,
causing a detrimental impact on system performance
specifications such as THD + N. Utilizing the flat RON region of
the switch with a ±15 V primary supply optimizes the system
performance. The ADC is then protected by the thresholds set
by the secondary supply rails.
14
12
10
8
±15V DUAL SUPPLY
6
4
ADDITIONAL PROTECTED RANGE
FLAT OPERATION REGION, MINIMIZED THD + N
0
–16 –14 –12 –10 –8 –6 –4 –2 0 2 4 6 8 10 12 14 16
+5V
13636-003
+5V
SIGNAL VOLTAGE (V)
Figure 4. Flat RON Range of Operation
Figure 3. ADG5462F Channel Protector Application Example
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13636-004
2
AN-1380
Application Note
POSFV/NEGFV CONFIGURATION OPTIONS
There are a number of ways in which the POSFV and NEGFV
fault supplies can be configured. The main considerations are as
follows:
•
•
•
Analog switch performance required: sets the requirements
for primary supply rails
Fault protection levels required by downstream components:
sets voltage requirement for secondary rails
Availability of other system supply rails: determines the
requirement for generating POSFV/NEGFV supplies
drain pin clamps to ~0.7 V above VDD/POSFV during a fault
condition before the switch turns off; therefore, there is a small
overshoot above VDD for a short duration (~500 ns) that is seen
by downstream devices. This amount of energy, however, is more
benign than a 1 kV ESD pulse and should not cause any concerns
in a system, as shown in the scope plot in Figure 7.
+29V SPIKE
T
+28V OV
+22.7V CLAMP
tRESPONSE ~500ns
A number of different options are detailed in the following
sections.
+22V VDD
Tying POSFV to VDD and NEGFV to VSS (or GND) is the
simplest configuration and sets the fault thresholds at the same
voltage as the primary supply rails. In the case of a fault, the
drain pin clamps to VDD + 0.7 V or VSS − 0.7 V.
CH1 5.00V
CH3 5.00V
VDD VSS
0.1µF
0.1µF
CH2 5.00V
M1.00µs
A CH1
18.0V
13636-007
TYING POSFV TO VDD AND NEGFV TO VSS (OR GND)
USING SEPARATE LOW IMPEDANCE POWER
SUPPLIES FOR POSFV/NEGFV
Using separate low impedance power supplies for POSFV and
NEGFV is the default mode of operation in many applications.
In the example described previously in the Data Acquisition
Signal Chain Using the ADG5462F Channel Protector, there
were already suitable supply rails available for the user, for
example, ±15 V supplies for the PGA and +5 V/GND supplies
for the ADC. In such a case, the wider supply range is used for
optimal analog performance and the secondary supplies protect
downstream components from overvoltage faults above the
expected signal range.
13636-005
VDD VSS
NEGFV
POSFV
Figure 7. Overshoot During Fault Condition
Figure 5. Primary Supplies Shorted to Secondary Supplies
T
VDD/POSFV
VDD VSS
0.1µF
0.1µF
POSFV
0.1µF
NEGFV
0.1µF
DRAIN
SOURCE
A CH1
11.0V
Figure 6. Drain Output Response to Positive Overvoltage (POSFV Tied to VDD)
There are both advantages and disadvantages to consider when
using this configuration.
Advantages to Tying the Supplies
This is the simplest configuration; no additional supply rails or
discrete components are required.
Disadvantages to Tying the Supplies
If the VDD/VSS range is reduced to meet the protection voltage
requirements, then RON performance is not optimized
compared to setting a wider VDD/VSS range. In addition, the
Rev. 0 | Page 4 of 6
VDD VSS
13636-008
M2.00µs
NEGFV
CH2 5.00V
CH4 5.00V
POSFV
CH1 5.00V
13636-006
4
Figure 8. Separate Low Impedance Secondary Supply Rails
Application Note
AN-1380
T
T
VDD
VDD
POSFV
POSFV
DRAIN
SOURCE
DRAIN
SOURCE
CH1 5.00V
CH3 5.00V
CH2 5.00V
CH4 5.00V
M2.00µs
A CH1
11.0V
CH1 5.00V
CH3 5.00V
Figure 9. Drain Output Response to Positive Overvoltage
(Dedicated POSFV Supply Rail)
Advantages to Separate Supplies
Using separate supplies provides optimum RON performance. In
addition, the user sets fault thresholds according to specific
protection requirements of downstream components.
Disadvantages to Separate Supplies
An important consideration is that separate low impedance
power supply rails are required for this configuration. If these
are not available in the system, they must be generated from a
dc-to-dc converter or from a buffered resistor divider from the
primary supplies.
ADDING A DIODE FROM THE PRIMARY SUPPLY
RAIL TO THE SECONDARY SUPPLY RAIL
There may be cases where downstream components are very
sensitive to overvoltage stresses above the primary supply rails.
In these cases, the drain pin clamping to VDD + 0.7 V for 500 ns
following a fault may not be acceptable. One option to reduce
the clamp voltage to the approximate VDD voltage level is to add
a diode between VDD and POSFV. With POSFV at a diode drop
below VDD, the fault threshold and the clamp voltage are
approximately equal to the VDD voltage.
VDD VSS
0.1µF
13636-010
NEGFV
VDD VSS
A CH1
11.0V
Because the internal drain clamp diodes are referenced to the
secondary supplies and because the secondary supply is not
driven by a low impedance source, this solution is only suitable
for situations where the source pin goes into fault rapidly. If the
source pin is brought into fault at a slow ramp rate, the POSFV
or NEGFV pin can pull with the fault and remain a diode drop
below the fault voltage. This can cause a scenario where the
overvoltage event is not detected. If the ramp rate into a fault
condition is slow, then a larger POSFV/NEGFV stabilization
capacitor is helpful.
There are both advantages and disadvantages to consider when
using this configuration.
Advantages to Adding a Diode
Adding a diode from the primary supply rail to the secondary
supply rail limits overshoot above VDD to sensitive downstream
circuitry. In addition, this configuration sets a custom fault
threshold without generating additional system rails.
Disadvantages to Adding a Diode
Additional discrete components (that is, two diodes) are
required to generate the POSFV/NEGFV rails. There is also
the possibility of a slightly reduced signal range (if internal
and external diode drops do not match, the fault detector may
trip inside the primary supply rails). This configuration is also
not suitable for fault conditions with a slow ramp rate.
USING REVERSE BREAKDOWN VOLTAGE OF
ZENER DIODE TO CONFIGURE POSFV/NEGFV
0.1µF
POSFV
0.1µF
M2.00µs
Figure 11. Drain Output Response to Positive Overvoltage
(Diode from VDD to POSFV)
There are both advantages and disadvantages to consider when
using this configuration.
0.1µF
CH2 5.00V
CH4 5.00V
13636-011
4
13636-009
4
Figure 10. Diodes Configured Secondary Supply Rails
In cases where additional custom supply rails are not available,
the POSFV and NEGFV supplies need to be generated by the
system designer. One option to achieve this is to use a Zener
diode between the primary supply rail and the secondary
supply rail, and then utilize the reverse breakdown voltage due
to the POSFV/NEGFV supply current (Zener voltage) to
configure the secondary rails.
Zener diodes are readily available with breakdown voltages of
2 V and above, so any POSFV/NEGFV voltage can be generated
using this method.
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AN-1380
Application Note
The drain response is similar to having a dedicated secondary
supply rail, as shown in Figure 9. There may be some variation
in Zener voltage across different devices, and the Zener voltage
also varies over temperature. Therefore, the POSFV/NEGFV
voltage (and, hence, the fault threshold voltage) is not as well
regulated as a dedicated supply rail. However, it is a cheap and
simple way to configure a secondary supply rail if the fault
threshold tolerance is sufficient for the application.
VDD VSS
0.1µF
0.1µF
Using a Zener diode configuration sets the custom fault
threshold without generating additional system rails.
Disadvantages for Using a Zener Diode
The Zener diode configuration requires additional discrete
components to generate POSFV/NEGFV rails. Consequently,
variations in Zener voltage across devices and temperature
directly impact fault threshold accuracy. The Zener diode
configuration is not suitable for fault conditions with a slow
ramp rate (similar to the previous diode configuration).
SUMMARY
13636-012
VDD VSS
NEGFV
0.1µF
POSFV
0.1µF
Advantages for Using a Zener Diode
Figure 12. Zener Diode Configured Secondary Supply Rails
There are both advantages and disadvantages to consider when
using this configuration.
The fault protected switches allow the user to set a specific fault
threshold at which the switch turns off. The ability to set a wide
primary supply voltage enables the switch to achieve optimum
analog performance (for example, flatter, lower RON). If a lower
fault threshold voltage is required, then POSFV and NEGFV
require separate low impedance supplies to generate those
thresholds.
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AN13636-0-12/15(0)
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