ISO7320CDR Dual-Channel Digital Isolator Selection Guide: C vs FC, Automotive Q1, and When to Upgrade

Bottom Line: The TCA9517 from Texas Instruments is a bidirectional I2C bus repeater and level translator that isolates bus capacitance and bridges voltage domains between 1.0 V and 5.5 V. For SOIC-8 production boards choose the TCA9517DR; for compact QFN footprints pick the TCA9517DGKR; and when operating below 2.5 V on either side, require the A-suffix TCA9517ADGKR with its extended low-voltage support. All three variants share the same functional core: active pull-up drive on both sides, automatic direction detection, and ±15 kV ESD protection on the bus pins.

Why the TCA9517 Solves Two Problems at Once

Most I2C designs eventually hit two bottlenecks: bus capacitance limits how many devices you can hang on a single segment, and mixed-voltage SoCs need a translator between a 1.8 V processor and a 3.3 V or 5 V peripheral. The TCA9517 addresses both in a single 8-pin device by acting as an active repeater that regenerates and re-drives signals, eliminating the passive RC pull-up limitations of standard I2C topology.

The I2C specification (NXP UM10204 Rev 7) caps total bus capacitance at 400 pF for standard-mode and 400 pF for fast-mode (100 kHz and 400 kHz respectively). With a TCA9517 in place, each side of the repeater has its own independent 400 pF budget, effectively doubling the number of devices a single master can address. The device supports Standard-mode (100 kHz), Fast-mode (400 kHz), and is compatible with SMBus 2.0 operation.

Variant Differences: DR vs DGKR vs A-Suffix

Understanding the three stocked variants prevents costly late-stage redesigns.

Parameter	TCA9517DR	TCA9517DGKR	TCA9517ADGKR
Package	SOIC-8 (D)	SOT-23-8 / VSSOP-8 (DGK)	SOT-23-8 / VSSOP-8 (DGK)
Supply voltage (VCCA)	1.8 V – 5.5 V	1.8 V – 5.5 V	1.0 V – 5.5 V
Supply voltage (VCCB)	1.8 V – 5.5 V	1.8 V – 5.5 V	1.0 V – 5.5 V
Max propagation delay	230 ns	230 ns	230 ns
Footprint area	~29 mm²	~9 mm²	~9 mm²

The SOIC-8 TCA9517DR is the easiest to hand-solder and prototype. The DGK variants (TCA9517DGKR and TCA9517ADGKR) are 70% smaller in board area, making them the preferred choice for space-constrained wearables and IoT nodes. The A-suffix variant extends the lower supply rail from 1.8 V down to 1.0 V, which is essential for sub-threshold ultra-low-power MCUs such as those in JEDEC LPDDR4 companion applications.

Design Considerations

Capacitance Budget and Pull-Up Resistor Selection

The TCA9517 uses active open-drain outputs, so pull-up resistors on both sides must be sized independently for their respective bus segments. The standard formula from UM10204 applies: R_pull-up ≥ (VCC – V_OL_max) / I_OL_max and R_pull-up ≤ t_rise / (0.8473 × C_bus). For a 400 kHz fast-mode segment at 3.3 V with 50 pF total capacitance, a 2.2 kΩ pull-up satisfies both constraints with margin.

A common mistake is sharing the same pull-up value across both sides of the repeater when the two bus segments have different lengths and node counts. Always calculate C_bus separately for the Side A (master/processor) segment and Side B (peripheral) segment. For automated BOM sourcing of pull-up resistors and associated decoupling capacitors, use FindMyChip's component search to locate stock across 200+ verified distributors.

Voltage Domain Compatibility and EN Pin Usage

The TCA9517's direction is controlled automatically by the voltage levels seen on SDA and SCL pins—there is no separate direction control pin. The supply rails VCCA and VCCB set each side's logic thresholds independently. Ensure VCCA is connected to the master-side supply and VCCB to the peripheral-side supply to guarantee correct VOL and VOH thresholds.

The EN (enable) pin is active-high and referenced to VCCA. Tie EN high through a 100 kΩ resistor to VCCA for always-on operation. In applications that gate power to the peripheral side (VCCB) while keeping the master awake, drive EN low to tri-state the outputs and avoid back-powering the disabled peripheral rail through the TCA9517's internal clamp diodes.

Timing and Propagation Delay in 400 kHz Systems

At 400 kHz Fast-mode, each I2C bit period is 2500 ns. The TCA9517's maximum propagation delay of 230 ns consumes 9.2% of a single bit period, well within the I2C specification's 1000 ns maximum allowed rise/fall skew. However, cascading two TCA9517 devices in series (for example, to bridge three voltage domains) introduces up to 460 ns of end-to-end delay. This remains within Fast-mode margin but leaves zero headroom for additional RC delay from long PCB traces.

For Fast-mode Plus (FM+, 1 MHz) designs, the TCA9517 is not specified; the TCA9517A datasheet (SLVSCL8) explicitly lists 100 kHz and 400 kHz only. Engineers requiring 1 MHz operation should evaluate Texas Instruments' TCA9615 or NLAST4053 as alternatives and request a quote for comparative pricing across supply sources.

ESD Protection and Board Layout

Each I2C pin on the TCA9517 is protected to ±15 kV (Human Body Model, HBM) per IEC 61000-4-2 Level 4. This level of protection is sufficient for most benchtop and industrial panel applications where operators may inadvertently contact connector pins during maintenance.

Despite the robust on-chip ESD protection, keep SCL and SDA trace lengths below 30 cm to avoid antenna effects at board edges. Route the two I2C signals as a parallel differential pair with 5-10 mil spacing and avoid parallel routing alongside high-frequency clocks or switching power supply traces. Place a 100 nF ceramic decoupling capacitor (C0G or X5R, 0402 footprint) between each VCC pin and ground, located within 0.5 mm of the IC.

	Solution 1	Solution 2	Solution 3
Variant	TCA9517DR	TCA9517DGKR	TCA9517ADGKR
VCCA range	1.8–5.5 V	1.8–5.5 V	1.0–5.5 V
Package	SOIC-8	DGK-8	DGK-8
Typical application	Industrial	IoT/Wearable	Ultra-low-power

Common Pitfalls and Troubleshooting

Pitfall 1: Floating EN Pin Causes Intermittent Lock-up

Error: EN pin left unconnected or pulled through a high-impedance path to a switching rail.
Consequence: The TCA9517 randomly tri-states its outputs mid-transaction, corrupting I2C frames and causing the master to lock in a clock-stretching wait.
Fix: Always drive EN high through a 100 kΩ resistor to VCCA. If using GPIO-controlled shutdown, ensure the GPIO de-asserts EN before removing VCCB.

Pitfall 2: Identical Pull-Up Values on Both Sides

Error: Using 4.7 kΩ on both Side A (1.8 V) and Side B (3.3 V) without accounting for the lower VCCA.
Consequence: VOH on Side A barely reaches 1.26 V when I_OL from the TCA9517's open-drain output sinks current, violating the I2C minimum VOH = 0.7×VCC = 1.26 V threshold—leading to ACK errors at high temperature.
Fix: Calculate pull-up values separately per side using the UM10204 formulas. For 1.8 V rails, prefer ≤ 1.5 kΩ.

Pitfall 3: Cascading Two TCA9517s Without Verifying Timing

Error: Bridging three voltage domains (1.0 V → 1.8 V → 3.3 V) using two TCA9517s in series.
Consequence: Total propagation delay reaches ~460 ns end-to-end. Combined with 300 ns of cumulative trace/pull-up RC delay, the total can exceed I2C Fast-mode's 1000 ns rise-time allowance.
Fix: Limit cascading to two devices maximum and measure SDA/SCL eye diagrams with an oscilloscope at 400 kHz. If timing is marginal, slow the master clock to 100 kHz.

Pitfall 4: Missing Decoupling Causes Glitches Under Load Switching

Error: No local bypass capacitor on VCCA or VCCB.
Consequence: VCC droop during simultaneous SCL/SDA edge transitions causes VOH to dip below logic threshold momentarily, appearing as spurious NACK.
Fix: Place one 100 nF X5R/C0G 0402 capacitor per supply pin, within 0.5 mm of the IC.

Pitfall 5: Back-Powering When VCCB Is Removed Before VCCA

Error: Powering down the Side B peripheral supply while VCCA and the master's I2C bus remain active.
Consequence: Logic-high signals on SDA_B and SCL_B back-power through ESD clamp diodes into the VCCB rail, potentially holding it at 0.7 V below VCCA and interfering with the power sequencer.
Fix: Assert EN low (GPIO or sequencer signal) before removing VCCB, or add an active load / discharge resistor on VCCB during power-down.

FAQ

Q: What is the difference between TCA9517DR and TCA9517DGKR?
A: They are functionally identical—same silicon, same 230 ns max propagation delay, same 1.8 V–5.5 V supply range—but differ only in package. The DR suffix indicates an SOIC-8 package (larger, hand-solderable), while DGKR uses the smaller DGK (SOT-23-8 / VSSOP-8) package. Choose DR for prototypes and low-volume production; choose DGKR for space-constrained, high-volume SMT assembly.

Q: Can I use the TCA9517 with SMBus 2.0?
A: Yes. The TCA9517 is SMBus 2.0 compatible at both Standard-mode (100 kHz) and Fast-mode (400 kHz). SMBus adds a 10 kΩ mandatory pull-up and a SMBCLK timeout requirement of 25–35 ms; the TCA9517 passes these transparently since it does not perform any address decoding or timeout logic of its own.

Q: Does the TCA9517 work with I2C clock stretching?
A: Yes. Clock stretching is transparent to the TCA9517 because it detects direction from the instantaneous voltage on SCL and SDA—there is no state machine to get confused by extended low periods. The master holds SCL low and the repeater simply maintains the driven state on both sides.

Q: What happens if VCCA and VCCB are the same voltage?
A: The TCA9517 still provides bus isolation and capacitance buffering even when both supply rails are equal. This is a valid use case for extending a heavily loaded I2C bus at the same voltage level. Both sides have independent 400 pF budgets regardless of whether a voltage translation is occurring.

Q: Is the TCA9517ADGKR pin-compatible with TCA9517DGKR?
A: Yes, both use the identical DGK-8 footprint and pinout. The A-suffix variant is a drop-in upgrade that extends VCCA and VCCB down to 1.0 V. If your design may migrate to a lower-voltage MCU in a future revision, designing with the ADGKR footprint from the start avoids a PCB re-spin.

Conclusion

The TCA9517 family—TCA9517DR in SOIC-8, TCA9517DGKR in compact DGK-8, and TCA9517ADGKR for sub-1.8 V rails—gives hardware engineers a versatile, well-characterized solution for I2C bus isolation and level translation from 1.0 V to 5.5 V. Correctly sizing pull-up resistors per side, managing EN pin sequencing, and respecting the 400 pF per-segment capacitance budget are the three design practices that prevent the majority of field failures.

To source any TCA9517 variant across 200+ verified distributors with real-time stock and competitive China-based pricing, search FindMyChip or submit a quote request for bulk procurement. FindMyChip's 5-point authentication process ensures every part is genuine Texas Instruments silicon—critical for I2C designs that pass automotive or industrial EMC qualification.