Even Cheapskates Need Reliable I2C Isolation!

Modern interface isolators integrate interface transceivers with an internal isolation barrier. Older designs, prior to this integration, use separate, standalone isolators in combination with standard interface transceivers to accomplish the same interface isolation. Surprisingly, these legacy designs still offer a significant cost advantage over their integrated counterparts.

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The discrete I²C isolator is among the most often considered isolators. This most stubborn, hard-to-die design continues to fascinate many engineers due to its simplicity. Circuit performance, however, changes with variations in temperature and supply, making re-designs inevitable. Still, this effort can be avoided when using an integrated version.

I²C Bus Operation

The I²C or inter-integrated circuit bus is a multi-master bus with two bidirectional signal lines: one for clock (SCL), the other for data (SDA). Bus access is via open-drain outputs (Fig. 1). Both signal lines connect to the bus supply via pull-up resistors. When a node accesses the bus, its open-drain output pulls the signal line to ground potential. This is considered a logic zero. When the same output is high-impedance, the signal line is pulled up to the potential of the bus supply. This is considered a logic one.

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1. The I2C bus uses bidirectional, open-drain signal lines for clock and data. The signal lines connect to VBUS via pull-up resistors. Pulling a line to ground potential is considered a logic zero. Otherwise, it’s a logic one.

Four different speed modes exist: standard mode (100 kbits/s), fast mode (400 kbits/s), fast mode plus (1 Mbit/s), and high-speed mode (3.4 Mbits/s).

I²C specifies two roles for bus nodes. A master node always provides the clock signal and the slave addresses. It also initiates and ends data transfers. A slave node always receives the clock and the addresses from the master and responds to master requests (Fig. 2). If a slave is not ready to respond, it pulls down the SCL line to force the master to wait until the line is released (high) again.

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2. The master initiates a start condition, sends the slave address, and indicates whether it wants to read from or to write to a slave. The slave acknowledges the master request and either receives or transmits data.

Signal transitions from one logic state to another during a transfer occur when SCL is low. Transitions at the start or end of a transfer occur while SCL is high. While I²C can be used over a wide voltage range, the most commonly used supply levels are 3.3 V and 5 V.

Isolated I²C Bus

I²C buses using separate power supplies for master and slave nodes commonly apply signal isolators between the master node and the slave nodes to prevent the creation of unintentional ground loops (Fig. 3). Using a single ground connection could prove fatal, as this shortens different ground potentials. The resulting ground loop currents can deteriorate the signal integrity of the clock and data signals and even damage bus node circuits.

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3. In an isolated I2C bus, the master node is isolated from a group of slave nodes. While the lower bus capacitance on the master side requires higher pull-up resistor values, the higher bus capacitance on the slave side needs lower resistor values to ensure equal transition times on both sides.

Integrated I²C isolator

To isolate a bidirectional signal path with unidirectional isolators, an integrated I²C isolator internally splits a bidirectional line into two unidirectional signal lines, each of which is isolated via a single isolation barrier (Fig. 4). Each channel has an open-drain output. Side one of the isolator connects to a low-capacitance I²C master node, typically the bus controller. Side two is designed for connecting to a fully loaded I²C bus with up to 400 pF of bus capacitance.

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4. The integrated I2C isolator generates a higher low-level output of VOL* = 0.8 V at output B than the output low-level of VOL = 0.4 V created by a signal source at SDA1. This difference enables comparator C to distinguish between the low levels initiated by SDA2 or SDA1.

To prevent the internal signal loop from latching up, the loop is broken at the output of buffer B, whose output low-level (V_OL*) of 0.8 V is higher than the 0.4-V output low-level (V_OL) of the master node at SDA1. Comparator C has upper and lower input voltage thresholds that distinguish between the 0.4-V low-potential driven by SDA1 and the raised 0.8-V output low-level from B.

Thus, comparator C allows low-level voltages from SDA1 to pass through to SDA2. In the opposite direction, however, it blocks low-level signals created at SDA2 from returning to SDA2, which prevents device latch-up.

Discrete Or Cheapskate Solution

In a discrete I²C isolator, a single transistor performs the comparator function (Fig. 5). While the base terminal assumes the function of the comparator’s reference input, the emitter is the comparator’s signal input SDA1 and the collector is the comparator output.

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5. When the MCU pulls SDA1 low, two equal currents, IPUa and IE, flow into the MCU’s output stage creating a VOL1 of 0.4 V.

Take an I²C isolator operating from a 5-V supply and supporting data rates of up to 200 kbits/s. In transmit direction from SDA1 to SDA2, the MCU starts a communication by pulling SDA1 low. Two currents, I_PUa through pull-up resistor R_PU1 and emitter current I_E from transistor Q1, flow together into the open drain of the MCU’s SDA output, creating the low-level output voltage, V_OL1.

The current through the pull-up resistor is calculated with:

I_PUa = V_CC1 – V_OL1/R_PU1

Limiting the current in the SDA1 line to 1 mA ensures low V_OL levels of approximately 100 mV in the output stages of the MCU’s I/O and the isolator IC. Assuming a V_OL1 of 0.1 V, a supply of V_CC1 = 5 V, and a pull-up resistor of R_PU1 = 10 kΩ, I_PUa = 490 μA. Letting an equal current flow through Q1 requires a collector resistor of:

R_C = V_CC1 – V_CE–sat– V_OL1/I_C

With I_C ~ I_E ~ I_PUa, and a saturation voltage of V_CE-sat = 0.1 V, the collector resistor R_C = 10 kΩ.

A switching transistor chooses a transistor with a minimum forward current gain of ≥ 100. To ensure transistor saturation, the base current should be I_B = 50 μA for a collector current of I_C = 500 μA. The bias current, I_B2, should be approximately 10 times the base current, making I_B2 = 500 μA and I_B1 = I_B2 + I_B = 550 μA.

To turn T1 fully on, its base potential must be larger than the maximum base-emitter voltage, plus the low-level output voltage at SDA1:

V_B = V_BE–MAX+ V_OL

For T1, V_BE-max is given with 850 mV, yielding a base potential of:

V_B = 0.85 V + 0.1 V = 0.95 V

At low currents, the forward voltage of a Schottky diode is assumed with V_F = 0.3 V. The value of bias resistor R_B2 is given with:

R_B2 = (V_B – V_F)/I_B2 = (0.95 V – 0.3 V)/500 µA = 1.3 kΩ

Then the upper bias resistor, R_B1, can be calculated via:

R_B1 = (V_CC1 – V_B)/I_B1 = (5 V – 0.95 V)/550 µA = 7.36 kΩ

Choose the closest resistor from the E-96 series with R_B1 = 7.32 kΩ.

In the receive direction, from SDA2 to SDA1, communication starts when the I²C bus is pulled low at SDA2 and the isolator output, OUT_C, turns low.

First, the isolator’s push-pull output must be converted into an open-drain output via diode D1. The output low-level of the isolator is assumed with V_OL = 0.1 V, the forward voltage of D1 with V_F = 0.3 V. To turn Q1 fully off, a base-emitter voltage of V_BE-OFF = 0.4 V is chosen. This requires an SDA1 potential or V_OL2 of:

V_OL2 ≥ V_B – V_BE–OFF= 0.95 V – 0.4 V = 0.55 V

To provide a sufficient guard band to V_OL1, V_OL2 is raised to 0.7 V. The emitter resistor, R_E, is calculated via:

R_E = (V_OL2 – V_F – V_OL)/I_PUb

The current through the pull-up resistor is determined by:

I_PUb = (V_CC1 – V_OL2)/R_PU1

Inserting numerical values yields a charge current of:

I_PUB = (5 V – 0.7 V)/10 kΩ = 430 µA

and an emitter resistor:

R_E = (0.7 V – 0.3 V – 0.1 V)/430 µA = 698 Ω

Figure 7 shows the final circuit. Note that the value for the pull-up resistor on side 2, R_PU2, is not given as it depends on the actual bus capacitance and the required time constant R_PU2 • C_B2.

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6. When SDA2 is pulled low, the isolator output (OUT C) in combination with diode D1 and emitter resistor RE create an output low-level of VOL2 = 0.7 V. This level allows designers to clearly distinguish between VOL1 = 0.4 V and VOL2 = 0.7 V, which turn Q1 on or off.

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7. The final discrete isolator is just a few more cents than the triple isolator IC. But the circuit requires redesign should the supply voltages and/or the VOL levels of the I2C output stages change.

References

Download ISO1540 and ISO1541 data sheets at www.ti.com/iso154-ca and www.ti.com/iso1541-ca.

Thomas Kugelstadt is an applications manager with Texas Instruments. He is responsible for defining new, high-performance analog products and developing complete system solutions for industrial interfaces with robust transient protection. He is a graduate engineer from the Frankfurt University of Applied Science. He can be reached at [email protected].