Texas Instruments | Overvoltage protection with comparator circuit | Application notes | Texas Instruments Overvoltage protection with comparator circuit Application notes

Texas Instruments Overvoltage protection with comparator circuit Application notes
Analog Engineer's Circuit: Amplifiers
SNOAA20 – January 2019
Overvoltage protection with comparator circuit
Design Goals
Supply
Load
Operating Voltage Range
MAX Operating Voltage (VOVER)
SUPPLY < VOVER
Comparator Output Status (OUT)
SUPPLY ≥ VOVER
12V to 36V
30V
VOL < 0.4V
VOH = SUPPLY
Design Description
This overvoltage protection circuit uses a high-voltage comparator with a push-pull output stage to control
a P-Channel MOSFET that connects the SUPPLY to the LOAD. When the SUPPLY voltage exceeds the
overvoltage threshold (VOVER), the output of the comparator goes HIGH and disconnects the LOAD from
the SUPPLY by opening the P-Channel MOSFET. Likewise, when the SUPPLY voltage is below VOVER,
the output of the comparator is LOW and connects the LOAD to the SUPPLY.
T1
LOAD
Z2
R1 200k
R3 20k
U1 TLV1805
++
SUPPLY
--
R4 100k
OUT
SD
+IN
R2 100k
C1 100u
Z1
Design Notes
1. Select a high-voltage comparator with a push-pull output stage.
2. Select a reference voltage that is below the lowest operating voltage range for the SUPPLY.
3. Calculate values for the resistor divider so the critical overvoltage level occurs when the input to the
comparator (+IN) reaches the comparator's reference voltage.
4. Limit the source-gate voltage of the P-Channel MOSFET so that it remains below the device's
maximum allowable value.
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Overvoltage protection with comparator circuit
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Design Steps
1. Select a high-voltage comparator with a push-pull output stage that can operate at the highest possible
SUPPLY voltage. In this application, the highest SUPPLY voltage is 36V.
2. Determine an appropriate reference level for the overvoltage detection circuit. Since the lowest
operating voltage for the SUPPLY is 12V, a 10V zener diode (Z1) is selected for the reference (VREF).
3. Calculate value of R3 by considering the minimum bias current to keep the Z1 regulating at 10V. A
minimum bias current of 100uA is used along with the minimum SUPPLY voltage of 12V.
4. Calculate the resistor divider ratio needed so the input to the comparator (+IN) crosses the reference
voltage (10V) when the SUPPLY rises to the target overvoltage level (VOVER) of 30V.
5. Select values for R1 and R2 that yield the resistor divider ratio of 0.333V by using the following equation
or using the online "Voltage Divider Calculator" at http://www.ti.com/download/kbase/volt/volt_div3.htm.
If using the following equation, choose a value for R2 in the 100k-ohm range and calculate for R1. In
this example, a value of 100k was chosen for R2.
6. Note that the TLV1805 which is used in application circuit has 15mV of hysteresis. This means that the
actualy switching threshold will be 7.5mV higher than the switching threshold (VREF) when the
SUPPLY is rising and 7.5mV lower when the SUPPLY is falling. The result of the hysteresis is most
easily seen in the DC Simulation curve. Since SUPPLY is resistor divided down by a factor of 3, the
net impact to the SUPPLY switching threshold is 3 times this amount.
7. Verify that the current through the resistor divider is at least 100 times higher than the input bias
current of the comparator. The resistors can have high values to minimize power consumption in the
circuit without adding significant error to the resistor divider.
8. Select a zener diode (Z2) to limit the source-gate voltage (VSG) of the P-Channel MOSFET so that it
remains below the device's maximum allowable value. It is common for P-Channel, power MOSFETs
to have a VSG max value of 20V, so a 16V zener is placed from source to gate.
9. Calculate a value for the current limiting resistor (R4). When SUPPLY rises above 16V and Z2 begins to
conduct, R4 limits the amount of current that the comparator output will sink when its output is LOW.
With a nominal SUPPLY voltage of 24V, the sink current is limited to 80µA.
2
Overvoltage protection with comparator circuit
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Design Simulations
DC Simulation Results
Voltage
OUT (V)(V)
T
35.98
18.00
0.00
10.00
20.00
30.00
40.00
(V)(V)
SUPPLY
Input
voltage
Transient Simulation Results
T
12.00
+IN
4.00
30.02
LOAD
12.00
35.98
OUT
0.00
36.00
SUPPLY
12.00
0.00
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500.00m
1.00
Time (s)
1.50
2.00
Overvoltage protection with comparator circuit
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References:
1. Analog Engineer's Circuit Cookbooks
2. SPICE Simulation File SNOAA20
3. TI Precision Labs
Design Featured Comparator
TLV1805-Q1 / TLV1805
VS
3.3 V to 40 V
VinCM
Rail-to-rail
VOUT
Push-Pull
VOS
500 µV
Hysteresis
15 mV
IQ
135 µA
tPD(HL)
250 ns
www.ti.com/product/tlv1805
Design Alternate Comparator
TLV3701 / TLV370x-Q1
2.5 V to 16 V
4V to 16 V
VinCM
Rail-to-rail
-1 V from VDD
VOUT
Push-Pull
Push-Pull
1.2 mV
VOS
250 µV
Hysteresis
n/a
n/a
IQ
0.56 µA
9.5 µA/Ch
tPD(HL)
4
TLC3702 / TLC3702-Q1
VS
36 µs
0.65 µs
www.ti.com/product/tlv3701
www.ti.com/product/tlc3702
Overvoltage protection with comparator circuit
Copyright © 2019, Texas Instruments Incorporated
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