PCA9508D,118 - Hot-Swap I2C Level Translating Repeater | NXP
MPN: PCA9508D,118 ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $1.28 | $1.28 |
| 10 | $1.15 | $11.50 |
| 100 | $0.98 | $98.00 |
| 500 | $0.84 | $420.00 |
| 1,000 | $0.72 | $720.00 |
PCA9508D,118 Overview
An I2C-bus repeater or bus buffer is an interface IC inserted into an I2C or SMBus segment to extend bus capacitance beyond the standard 400 pF limit, isolate bus segments, and translate between different logic voltage domains. In the power management hierarchy, it sits between the bus controller and peripherals as a signal-conditioning element, distinct from GPIO expanders or I2C switches, because it neither generates addresses nor gates channels - it simply repeats and strengthens bus signals.
The PCA9508's headline feature is hot-swap support: the A-side drivers are disabled until the bus is idle and VCC(A) exceeds 0.8 V and VCC(B) exceeds 2.5 V, allowing boards to be inserted into or removed from a live backplane without corrupting bus transactions. Its zero static voltage offset preserves the I2C logic levels, unlike offset-based buffers, so full multi-master arbitration and clock stretching behave transparently across the repeater.
Internally the CMOS device uses open-drain driver stages on both sides with rise-time acceleration to drive larger bus capacitance, retaining all I2C operating modes including standard-mode 100 kHz and fast-mode 400 kHz. The EN pin permits system-controlled enable and disable, and should only be toggled when the bus is idle, per the NXP datasheet.
Typical applications include live-insertion server and telecom backplanes, mixed-voltage I2C systems bridging 1.2 V or 1.8 V controllers to 3.3 V or 5 V peripherals, and long or heavily loaded bus segments that need capacitive-drive boost.
Design consideration: keep EN transitions to idle periods only, and provide pull-up resistors sized to each segment's capacitance, since the repeater does not replace segment pull-ups.
This page adds distributor-style alternatives, cross-reference analysis, and design notes beyond what the NXP datasheet alone provides.
Drop-in alternatives for PCA9508D,118 — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Variants in this series
Same-series models that are drop-in compatible with PCA9508D,118 (same form factor and footprint) — differing in Function, Package.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
TCA9517ADR
✅ Drop-In📋 Reference alternative (not in catalog)
TCA9517DR
✅ Drop-In✓ In Stock
$0.53 / Unit
View Datasheet →TCA9801D
✅ Drop-In📋 Reference alternative (not in catalog)
TCA9803D
✅ Drop-In📋 Reference alternative (not in catalog)
PCA9508D,118 Specifications
| Function | Hot-swappable level translating I2C-bus repeater / buffer |
| Channels | 1 channel (SDA/SCL pair, bidirectional) |
| B-side (High) Voltage Range | 2.7 V to 5.5 V |
| Maximum Bus Frequency | 400 kHz (I2C fast mode) |
| Supported Buses | I2C-bus, SMBus |
| Voltage Offset | Zero static voltage offset |
| Hot-Swap Support | Yes (drivers enabled only when bus idle, VCC(A) > 0.8 V, VCC(B) > 2.5 V) |
| Enable Pin | Yes (EN, system controlled) |
| Technology | CMOS |
| Package | SO-8 (SOP, 3.90 mm, SOT96-1, MS-012) |
| Pins | 8 |
| Mounting Type | Surface Mount |
| Packaging Suffix | 118 = Tape & Reel |
PCA9508D,118 Pin Configuration
| Pin 1 | A1 — A-side serial data (SDA), low-voltage domain, 0.9 V minimum |
| Pin 2 | A2 — A-side serial clock (SCL), low-voltage domain |
| Pin 3 | GND — Ground reference |
| Pin 4 | EN — Enable input; change state only when bus is idle |
| Pin 5 | B1 — B-side serial data (SDA), 2.7 V to 5.5 V domain |
| Pin 6 | B2 — B-side serial clock (SCL), 2.7 V to 5.5 V domain |
| Pin 7 | VCC(B) — High-voltage side supply, 2.7 V to 5.5 V |
| Pin 8 | VCC(A) — Low-voltage side supply, down to 0.9 V |
Typical Applications
PCA9508D,118 is suitable for 6 applications: Live-Insertion Server and Telecom Backplanes, Mixed-Voltage I2C System Integration, I2C Bus Capacitance Extension for Long Buses, SMBus Hot-Plug in Rack-Mount Systems, Automotive and Industrial Sensor Aggregation, Optical Modules and Consumer Electronics Cards.
Live-Insertion Server and Telecom Backplanes
The PCA9508's defining fit for backplane systems is its hot-swap gating: the A-side (card-side) drivers remain disabled until the bus is idle, VCC(A) exceeds 0.8 V, and VCC(B) exceeds 2.5 V, so a card sliding into a live shelf cannot glitch the shared SDA/SCL lines mid-transaction. Placed on each removable card between the card's local I2C domain (often 1.8 V or 3.3 V) and the backplane bus (3.3 V or 5 V), it translates voltage levels while adding zero static offset, preserving multi-master arbitration among shelf managers and other cards. The repeater also isolates the card's capacitance from the backplane, keeping segment loading within the 400 pF I2C limit at 400 kHz. The EN pin lets the shelf controller disable a misbehaving card's bus connection - toggle it only when the bus is idle, per the NXP datasheet.
Recommended
Mixed-Voltage I2C System Integration
Modern systems routinely mix 1.2 V, 1.8 V, or 2.5 V sensor and controller domains with legacy 3.3 V or 5 V I2C buses. The PCA9508 bridges these directly: its A-side operates down to 0.9 V while the B-side spans 2.7 V to 5.5 V, covering virtually all common logic families without discrete FET-resistor level shifter stages. Because it introduces zero static voltage offset, the logic-low level seen by any master is not degraded cumulatively as it would be with offset-based buffers - critical when several peripherals must simultaneously pull the bus low during clock stretching or arbitration. At 400 kHz fast mode, the repeater retains full timing margin, and rise-time acceleration helps drive the heavier capacitance typical of board-level buses. Compared to a discrete two-MOSFET translator, the PCA9508 adds directional intelligence and supply-valid gating, avoiding bus lock-up conditions common with simple FET translators.
Recommended
I2C Bus Capacitance Extension for Long Buses
The standard I2C specification limits bus capacitance to 400 pF; long cables, backplanes, and many-device buses routinely exceed this, causing excessive rise times and failed 400 kHz timing. Inserting a PCA9508 splits the bus into segments, each with its own pull-ups and capacitance budget, while the repeater's rise-time acceleration actively drives the loaded segment edges. Because the device has zero static voltage offset, cascading or long chains do not accumulate the low-level offset that plagues offset-based repeaters, so multi-master arbitration and slave clock stretching still work correctly. Typical uses include industrial chassis interconnects, LED display strings, and battery-management daisy chains where cell boards sit meters from the controller. Design the pull-up values per segment for a 300 ns rise time at 400 kHz and verify with an oscilloscope under maximum load, since the repeater amplifies drive only, it cannot fix a badly under-pulled segment.
Recommended
SMBus Hot-Plug in Rack-Mount Systems
SMBus power-supply and fan-tray management in servers and rack systems frequently involves physically replacing modules while the shelf remains powered. The PCA9508 supports both I2C-bus and SMBus at up to 400 kHz, and its supply-valid, bus-idle gating makes it well suited to these hot-plug points: during module insertion, contacts make and break repeatedly while rail voltages ramp, conditions that would corrupt an unprotected bus or falsely clock a slave. With the PCA9508, the module's bus interface stays tristate until VCC(A) exceeds 0.8 V, VCC(B) exceeds 2.5 V, and the bus is idle - per the NXP product information - after which the repeater joins the segments transparently with zero offset. Place it at the module connector, keep the EN pin driven by the module's good-power signal or strapped high, and never toggle EN during bus activity.
Recommended
Automotive and Industrial Sensor Aggregation
Automotive body controllers and industrial sensor clusters gather I2C data from multiple nodes with different supply domains - 1.8 V sensor hubs, 3.3 V microcontrollers, and 5 V actuator-side peripherals. The PCA9508 bridges these domains at 400 kHz while its supply-valid gating protects the bus during brownouts and load-dump-related voltage dips, when a segment's rail may collapse below the 0.8 V / 2.5 V thresholds and the repeater gracefully disconnects rather than transmitting corrupted levels. The zero-offset architecture keeps arbitration robust when several ECU-local masters share the bus. For genuine AEC-Q100-qualified deployments, verify the specific orderable grade against the NXP qualification documentation, as this must be confirmed per orderable part number rather than assumed. Segment the bus so each side's capacitance stays within specification, and use pull-ups tolerant of the crank/stop-cycle rail range.
Recommended
Optical Modules and Consumer Electronics Cards
Optical transceiver management (SFP-style I2C EEPROM polling), smart-card readers, and consumer docking systems all present hot-removable I2C devices. The PCA9508's hot-swap behavior shields the host bus from connector bounce and partial-power states during module swap, while level translation accommodates the low-voltage (0.9 V to 1.8 V) memory or controller domain inside the module against a 3.3 V host bus. Its single-channel 400 kHz operation matches the 100 kHz to 400 kHz management-interface speeds typical of these modules, and zero offset ensures the host's ACK and clock-stretch polling of the module's EEPROM behaves identically to a directly connected device. The tiny 8-pin SOIC footprint fits inside SFP and M.2 form-factor constraints, and the D,118 tape-and-reel orderable supports automated assembly for volume module production.
Recommended
Recommended Products Summary
Engineering reference data for PCA9508D,118 — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | TCA9517ADR | TCA9517DR | TCA9517DGKR | TCA9801D | TCA9803D |
|---|---|---|---|---|---|---|
| Package | SOIC-8 (SOT96-1) | SOIC-8 - same | SOIC-8 - same | VSSOP-8 (different footprint) | SOIC-8 - same | SOIC-8 - same |
| Brand | NXP Semiconductors | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments | Texas Instruments |
| Static Voltage Offset | Zero | Zero | Zero | Zero | Integrated 0.34 mA pull-ups | Integrated pull-ups |
| Hot-Swap Support | Yes (supply-valid, bus-idle gating) | No | No | No | No | No |
Key Differentiators
- True hot-swap insertion with supply-valid gating (vs TCA9517DR)
- Zero static voltage offset preserves arbitration (vs TCA9801D)
- Wide low-side range down to 0.9 V (vs TCA9517ADR)
Design Notes
Power both rails in the correct sequence relative to the application: the hot-swap logic requires VCC(A) > 0.8 V and VCC(B) > 2.5 V before the drivers engage, so a B-side rail that comes up late simply delays bus connection rather than causing damage. Decouple both VCC(A) and VCC(B) with 100 nF ceramic capacitors placed within 5 mm of pins 8 and 7 respectively. Estimated: at typical interface-IC supply currents (a few mA), dissipation in the SOIC-8 is negligible and no thermal copper pour is needed.
Keep the EN trace short and, if it is driven by a GPIO, ensure it defaults to the disabled state during power-up so the part never joins the bus in a partial-power condition. Provide independent pull-up resistor networks on the A-side and B-side SDA/SCL - the repeater does not transfer pull-ups across the barrier. For 400 kHz fast-mode segments, size pull-ups so the measured rise time stays under the I2C fast-mode spec with the actual cable and device capacitance; the rise-time acceleration helps but is not a substitute for correct pull-up selection.
Per the NXP datasheet, only change the EN pin state when the bus is idle - toggling EN during an active transaction can corrupt the transfer in progress and hang a slave. A second common pitfall is substituting TCA9517-class pin-compatible parts in a hot-swap socket: those devices lack supply-valid gating and will pass glitches during card insertion. Finally, if bus lock-up is observed after brownout events, verify that the supply thresholds (0.8 V / 2.5 V) match your rail sequence; a B-side rail held below 2.5 V keeps the repeater permanently disabled.
Compliance Information
The 118 orderable suffix on NXP parts conventionally indicates RoHS-compliant, lead-free tape-and-reel packaging, but explicit compliance statements were not present in the provided web data; verify against the NXP product page for PCA9508D before final qualification.
