Texas Instruments | Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector | Application notes | Texas Instruments Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector Application notes

Texas Instruments Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector Application notes
Application Report
SLVA600 – July 2013
Using the TPS3700 as a Negative Rail Over- and
Undervoltage Detector
Darwin Fernandez ................................................................... Linear Power - Supply Voltage Supervisors
ABSTRACT
The TPS3700 is a wide voltage window comparator that can be used in overvoltage (OV) and
undervoltage (UV) detection. This application note describes a simple approach to use the TPS3700 for
negative voltage monitoring applications such as the negative rail on op amps, DACs, ADCs, and other
high-precision analog circuitry that may need UV or OV protection.
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Contents
Negative UV, OV Detection Solution .....................................................................................
Setting the OV, UV Thresholds using R1, R2, and R3 ................................................................
Setting the Appropriate Output Logic Levels ............................................................................
3.1
Wired-OR Configuration ...........................................................................................
Design Example .............................................................................................................
Conclusion ...................................................................................................................
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5
List of Figures
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2
3
4
5
6
..............................................................
Output Circuit Configuration ...............................................................................................
Wired-OR Output Configuration ...........................................................................................
TPS3700 Negative Monitoring Design Example ........................................................................
OUTA and OUTB Response for VDD Voltage Rising ..................................................................
OUTA and OUTB Response for VDD Voltage Falling .................................................................
Monitoring a Negative Voltage Rail Using the TPS3700
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1
Negative UV, OV Detection Solution
1
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Negative UV, OV Detection Solution
Figure 1 shows a typical circuit for monitoring a negative rail. Resistors R1, R2, and R3 are calculated to
set the overvoltage and undervoltage threshold. Rp1a, Rp1b, Rp2a, and Rp2b are used with D1 and D2 to
clamp the logic level LOW to approximately 0 V and provide a proper logic level HIGH to the VPU voltage.
Figure 1. Monitoring a Negative Voltage Rail Using the TPS3700
2
Setting the OV, UV Thresholds using R1, R2, and R3
This section discusses the design calculations for setting the OV and UV thresholds. The TPS3700
Resistor Divider Calculator (SLUC425) in the TPS3700 product page is also a helpful tool in calculating
R1, R2, and R3.
R3 (W ) =
Vthp
´
VMon
| IDivider | | VOV |
(1)
Where:
Vthp = Positive-going input threshold voltage in volts (0.4 V typical).
VMon = Nominal-sensed rail voltage in volts.
IDivider = Current through the resistor divider in amperes.
VOV = Overvoltage threshold for OUTB triggering LOW in volts.
æ Vthn
VMon ö
´
R2 (W ) = ç
÷÷ - R3(actual)
ç|I
è Divider | | VUV | ø
(2)
Where:
Vthn = Negative-going input threshold voltage in volts (0.3945 V typical).
VUV = Undervoltage threshold for OUTA triggering LOW in volts.
æ VMon
R1(W ) = ç
ç|I
è Divider
2
ö
÷ - R2(actual) - R3(actual)
| ÷ø
Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector
Copyright © 2013, Texas Instruments Incorporated
(3)
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Setting the Appropriate Output Logic Levels
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3
Setting the Appropriate Output Logic Levels
Figure 2. Output Circuit Configuration
Figure 2 shows the output circuit used to set the appropriate output logic levels during an overvoltage and
undervoltage condition. During an undervoltage or overvoltage condition, OUTA/OUTB is asserted to
–VSS and D1/D2 turns ON to its forward voltage Vf1/Vf2. D1/D2 and Rp1b/Rp2b should be chosen such
that their forward voltage and sinking current is small enough to ensure a logic level LOW to the load the
signal it is driving. Note the datasheet characterizes this device with IOL = 5 mA (max) of sinking current.
IOL can be higher at the expense of a higher VOL voltage.
- VSS - | Vƒx |
Rpxb (W ) =
IOL
(4)
When –VSS is at its nominal voltage, OUTA and OUTB are released, Dx is OFF, and OV, UV is pulled up
to VPU voltage. VPU must be chosen as shown in Equation 5 such that the +20-V absolute maximum on
VOUTA and VOUTB is not violated.
VPU (Max ) = + 20 V + ( - VSS )
(5)
The pullup resistor Rpxa is chosen depending on the desired minimum VOH and rated output leakage
current. Leakage current for the TPS3700 is rated for 300 nA (max). Equation 6 does not include possible
current paths to and from the load.
V - VOH
Rpxa (W ) = PU
ILeakage
(6)
D1 and D2 should be chosen such that the operating conditions do not exceed what it is rated for. That is:
Vf ≤ VOL (maximum)
Vr > VPU
Io > IOL
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Design Example
3.1
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Wired-OR Configuration
Because the TPS3700 utilizes open-drain outputs, it can be arranged in a wired-OR configuration to
trigger an output LOW when there is either an OV or UV condition. Figure 3 shows this configuration. With
this circuit, Equation 4 and Equation 5 still hold.
Figure 3. Wired-OR Output Configuration
4
Design Example
The following design example uses the TPS3700 under the following conditions for monitoring a –12-V rail
and triggers a reset when the rail falls below 10% or rises above 10% of –12 V.
Figure 4. TPS3700 Negative Monitoring Design Example
4
Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector
Copyright © 2013, Texas Instruments Incorporated
SLVA600 – July 2013
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Conclusion
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As shown in Figure 5 and Figure 6, OUTA will trigger LOW during a –10% drop (–10.8 V) while OUTB will
trigger LOW during a +10% rise (–13.2 V). In between this window, the outputs are pulled up to VPU.
Yellow: VSS-5V/div
Pink: UV-5V/div
Blue: OV-5V/div
Figure 5. OUTA and OUTB Response for VDD Voltage Rising
Figure 6. OUTA and OUTB Response for VDD Voltage Falling
5
Conclusion
This application report has demonstrated simple circuit and design considerations for using the TPS3700
to monitor negative voltage rails and output the appropriate logic levels.
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Using the TPS3700 as a Negative Rail Over- and Undervoltage Detector
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5
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