Overvoltage Protection for 5V 1-Wire Slaves

Most 1-Wire devices operate from a 2.8V to 5.25V VPUP for read and write operations. EPROM devices, including the DS2406, DS2502, DS1982, DS2505, and DS1985, require a 12V programming pulse for write operations. The programming pulse poses an overvoltage threat to devices that cannot withstand voltages above 5.5V. Therefore, if the application requires that EPROM devices be written after the system is deployed, the 5V devices must be protected (Figure 1). The circuit in this article has a forward overvoltage protection of up to 40V, which provides system protection against voltages above the 12V EPROM programming pulse.


Figure 1. 1-Wire bus with 5V and 12V devices

Protection circuit requirements

A suitable protection circuit needs to meet the following requirements:

• Very low load on the 1-Wire bus
• Does not interfere with 1-Wire EPROM programming
• Proper Protection for 5V 1-Wire Devices
• Maintain full communication signal amplitude

In addition, it is best to use commonly available low-cost components to build the protection circuit.

Rationale

Figure 2 shows a very simple protection circuit. Zener diode U1 limits the gate voltage of Q1, and R1 limits the current through U1. Q1 is an n-channel MOSFET configured as a source follower, with the gate voltage minus a small offset voltage to reach the IO voltage of the 1-Wire slave device. To maintain the full amplitude of the communication signal, the offset voltage should be as low as possible. Depletion-mode MOSFETs with negative bias are well suited for this application. The Supertex® DN3135 was tested and its bias voltage was measured to be -1.84V (data sheet parameter VGS(OFF)). Therefore, the required gate voltage VG is 3.16V, which determines the threshold voltage of U1.


Figure 2. Protection Circuit Schematic

Unfortunately, the offset voltage of transistors varies from device to device and temperature to temperature. The "-1.84V" voltage can vary from -3.5V to -1.5V at room temperature. This variation makes it difficult to find a suitable Zener diode. In addition, low-voltage Zener diodes are usually specified at 5mA, which will affect the programming voltage of the 1-Wire EPROM. For example, if it is operating at 100µA, the voltage drop is well below the specified threshold. In this case, a shunt reference (much like a Zener diode) may be more appropriate, as it can reach the threshold voltage at very low currents. For example, the Maxim LM4040, powered by a 3.3V supply, only needs 67µA to reliably reach the reverse breakdown voltage. Based on the 1-Wire bus requirement of 67µA at 5V, we can calculate: R1 = (5V - 3.3V)/67µA = 25.4kΩ. About 10 slaves on the 1-Wire bus consume 67µA, which is acceptable for a 1-Wire master (such as the DS2480B). Now, let's check the current through R1 for the 12V programming pulse device:

I(R1) = (12V - 3.3V)/25.4kΩ = 343µA

(Formula 1)

The programming current of a 1-Wire EPROM is specified to be 10mA. An additional 1/3mA load will not cause any problems. Therefore, the circuit shown in Figure 2 will work when the MOSFET offset voltage is close to -1.8V, but this is not guaranteed. In practical applications, it is better to provide a protection circuit with an adjustable threshold.
Using a current source to implement an adjustable threshold

The circuit in Figure 3 uses a current source (U1) to set the maximum gate voltage of Q1. An ideal current source provides a current that is not affected by the voltage across it. For a given current, IOUT, the gate voltage can be adjusted by selecting different R1.


Figure 3. Improving the protection circuit using a current source

The NXP® PSSI2021SAY is a versatile single-chip current source (Figure 4). The device has four terminals, called VS, IOUT, GND, and REXT. If REXT is installed, it is connected in parallel with the internal 48kΩ nominal resistor.


Figure 4. Improved protection circuit

According to the product data sheet, IOUT is calculated as follows:

IOUT = 0.617/REXT(Ω) + 15µA

(Formula 2)

Here, REXT = 10kΩ, REXT is connected in parallel with the internal 48kΩ resistor, and the typical current is (61.7 + 15)µA = 76.7µA according to the PSSI2021SAY data sheet. The output current depends to some extent on the supply voltage VS, especially when the supply voltage is less than 5V. In the test, the current reached 76.7µA at 3.75V. At 12V, the current was 94µA. This result is also within the acceptable range due to the simple chip design.

The circuit shown in Figure 4 was tested with REXT = 10kΩ and R1 = 39kΩ. The 1-Wire adapter is Maxim's DS9097U-E25. Figures 5 and 6 show the 1-Wire adapter signal (top curve) and the signal of the protected slave device (lower curve). The programming pulse (Figure 6) causes a ±3V spike on the protected slave device, which lasts about 10µs. During the programming pulse, the voltage of the protected slave device rises to 6V, which may be potentially dangerous.


Figure 5. Communication waveforms: adapter (top), protected slave (bottom). The circuit shown in Figure 4 does not distort the 1-Wire signal.


Figure 6. Programming pulses: adapter (top), protected slave (bottom).

The disadvantage of the PSSI2021SAY is that it consumes a fairly high supply current. At 12V, including the 15µA at IOUT, the current is up to 370µA. Aside from the adjustable function, the circuit using the PSSI2021SAY is not much better than the solution in Figure 2.
Implementing an adjustable threshold based on a bandgap reference and discrete current sources

The PSSI2021SAY data sheet describes the basic principle of the circuit. The main disadvantage is its internal reference voltage, which is provided by the forward conduction voltage of two series diodes. Better performance can be obtained if a bandgap reference is used instead of the forward-biased diodes. The circuit shown in Figure 7 is equivalent to the PSSI2021SAY, consuming less current. Once the bandgap reference reaches its normal operating current, the current is almost independent of the voltage.


Figure 7. Protection circuit with bandgap reference

Transistor Q2, bandgap reference U1, and resistors R2 and R3 replace the PSSI2021SAY. R3 is selected to be 100kΩ, and the bandgap reference reaches its minimum operating current when IO is 2.2V. When IO is 5V, the current flowing through U1 is 38µA; when the IO voltage is 12V, the current is 108µA.

According to Kirchhoff's law, the following relationship can be obtained:

VBG = IE × R2 + VEB

(Formula 3)

For a general-purpose pnp transistor, such as 2N3906, VEB is typically 0.6V at room temperature and low collector current. Given that VBG is 1.235V, the equation can be broken down into:

R2 = (VBG - VEB)/IE = (1.235V - 0.6V)/IE = 0.635V/IE

(Formula 4)

To achieve the same nominal current (76.7µA) as the PSSI2021SAY circuit, R2 is calculated to be 8.2kΩ. When Q1 is the same as in Figure 2, VG ​​must be 3.2V. Ignoring the base current of Q2, IC equals IE. R1 can be calculated:

R1 = VG/IC = 3.2V/76.7µA = 41.7kΩ

(Formula 5)

To reduce the overall load on the 1-Wire master, the output current of the current source is reduced by increasing R1 and R2 by a factor of four (R2 = 33kΩ, R1 = 160kΩ), which reduces the current to 19µA, resulting in a maximum gate voltage of 3.08V. In practice, R1 needs to be adjusted to compensate for the VGS(OFF) tolerance of the MOSFET. If the voltage of the 1-Wire slave closely matches V(IO), then the appropriate value is found.

The circuit of Figure 7 was tested by replacing the Linear Technology® LT1004 (uncommon on the market) with a National Semiconductor® LM385. The 1-Wire adapter is a Maxim DS9097U-E25. Figures 8 and 9 show the 1-Wire adapter signal (upper trace) and the signal of the protected slave (lower trace). The programming pulse (Figure 9) produces a spike of about 10µs on the slave (2V rising, 1.5V falling). This circuit achieves better performance than Figure 4. During the programming pulse, the voltage of the protected slave device only rises to the 5V level.


Figure 8. Communication waveform without C1: adapter signal (upper), protected slave device (lower).


Figure 9. Programming pulse without C1: adapter signal (upper), protected slave device (lower).

To reduce the spike caused by the programming pulse, 100pF C1 is installed. Figures 10 and 11 show the test results. The communication waveform is slightly distorted. The peak amplitude is reduced (1.4V rising, 1.2V falling). Relative to Figure 9, the voltage will not drop below 3V. A 5.1V low-power Zener diode such as BZX84 from Q1 source to GND can clamp the rising spike, but does not affect the falling spike.


Figure 10. Communication waveform with C1 installed: adapter signal (upper), protected slave device (lower).


Figure 11. C1 installed. Programming pulse: adapter signal (lower), protected slave device (upper).

Protection threshold

The maximum voltage between IO and GND that the circuit in Figure 7 can withstand is determined by the following factors:

• Maximum safe current of U1
• VCE breakdown voltage of Q2
• VGD and VDS breakdown voltage of Q1

The maximum current of the LT1004 (U1) is 20mA, the breakdown voltage of the 2N3906 (Q2) is 40V, and the breakdown voltage of Q1 is 350V. The limited component is Q2. At 40V, the current through U1 is 143µA, which is well below the 20mA limit.

Summarize

If the 5V device can be protected from the programming pulse, it is possible to use 1-Wire EPROMs and 5V 1-Wire devices on the same bus. The simple protection circuit shown in Figure 2 can provide protection under certain conditions, but the gate-to-source turn-off voltage of the MOSFET varies widely, so it is not the best choice and requires a "matched" transistor and shunt reference. The circuit shown in Figure 4 can adjust the tolerance of the compensation MOSFET, but it puts a large load on the 1-Wire master device. Since the PSSI2021SAY can withstand voltages up to 75V, this circuit has protection capabilities up to 75V. The circuit shown in Figure 7 has similar functions to Figure 4, but can achieve better performance and put a much lower load on the 1-Wire master device. Its protection voltage is 40V, which is limited by Q2. The level of protection can be improved by choosing a transistor with a higher VCE breakdown voltage.