PSSI2021SAY115 - 50mA Constant Current Source SOT-353 | Nexperia
MPN: PSSI2021SAY115 ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0.42 | $0.42 |
| 10 | $0.38 | $3.80 |
| 100 | $0.29 | $29.00 |
| 500 | $0.24 | $120.00 |
| 1,000 | $0.19 | $190.00 |
| 3,000 | $0.15 | $450.00 |
PSSI2021SAY115 Overview
A constant current source is a fundamental building block in modern electronics that delivers a load current independent of supply voltage, temperature, and load impedance variations. The PSSI2021SAY115 belongs to the discrete semiconductor family of current regulators, which sit hierarchically under transistors, then semiconductor devices. By integrating the bias resistor and two steering diodes together with the pass transistor on one chip, Nexperia eliminates the need for multiple discretes, reduces PCB area, and improves matching between the reference diodes and the PNP base-emitter junction. This integration delivers better temperature coefficient than discrete equivalents because the on-chip elements track each other across temperature.
Key features include a wide programmable output range from 15 uA to 50 mA, a high 75 V input voltage rating for direct connection to industrial rails, low temperature drift due to monolithic construction, and a miniature SOT-353 footprint measuring only 2.0 x 1.25 mm. The device requires no additional bias network beyond the single external resistor, simplifying BOM and reducing design risk.
Architecturally, the PSSI2021SAY115 uses the base-emitter junction of the PNP transistor as the reference element, with the two series diodes compensating the VBE temperature coefficient. The external resistor sets the reference current through the diodes, which is then mirrored to the output through the PNP pass transistor, providing the regulated load current. This topology provides stable biasing even at low output currents where discrete designs typically struggle.
Typical applications include LED driver circuits, biasing of sensor bridges, precision reference currents for instrumentation amplifiers, battery chargers, and active loads. The SOT-353 package makes it particularly suitable for compact portable designs and high-density PCB layouts where board area is at a premium.
When designing with this device, ensure the external resistor has a tolerance of 1% or better and adequate voltage rating. Power dissipation is calculated as (Vin - Vload) multiplied by Iout; verify this against the 335 mW maximum rating. The output pin should be decoupled with a small capacitor if driving fast transient loads to prevent oscillation.
This page synthesizes distributor pricing, drop-in SOT-353 alternatives, and practical design notes that supplement the manufacturer datasheet, supporting both parametric searches and engineering reference workflows.
Drop-in alternatives for PSSI2021SAY115 — 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 PSSI2021SAY115 (same form factor and footprint) — differing in Function, Output Current Range, Package, Part Number, Topology.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
PSSI2021SAY
✅ Drop-In✓ In Stock
$0.15 / Unit
View Datasheet →PSSI2021SAY,135
✅ Drop-In ⚠️ Specs Unverified📋 Reference alternative (not in catalog)
PSSI2021SAY115 Maximum Ratings & Electrical Characteristics
| Manufacturer | Nexperia |
| Part Number | PSSI2021SAY,115 |
| Function | Constant Current Source / Current Regulator |
| Topology | Resistor-equipped PNP transistor with two integrated diodes |
| Output Current Range | 15 uA to 50 mA (programmable via external resistor) |
| Maximum Input Voltage | 75 V |
| Maximum Power Dissipation | 335 mW |
| Package | SOT-353 (SC-88A, UMT5), 5 leads |
| Mounting Type | Surface Mount |
| Lead Free | Yes |
| RoHS Status | Compliant |
| Packaging | Tape & Reel (suffix 115) |
| Standard Reel Quantity | 3000 |
| External Component Required | One resistor between pins 4 and 5 |
| Output Pin Configuration | Collector pin (pin 3) |
PSSI2021SAY115 Pin Configuration
| Pin 1 | D1 (Diode Anode) — Integrated diode anode, internally connected to bias network |
| Pin 2 | D2 (Diode Cathode) — Integrated diode cathode, internally connected to bias network |
| Pin 3 | OUT (Collector) — Regulated constant current output (PNP collector) |
| Pin 4 | REXT — External resistor terminal - connect Rext between pins 4 and 5 to set output current |
| Pin 5 | REXT / GND — External resistor terminal / ground reference |
Typical Applications
PSSI2021SAY115 is suitable for 6 applications: LED Indicator and Backlight Driver, Sensor Bridge Biasing, Industrial 24V/48V Bus Current Limiter, Battery Charger Reference Current, Portable Device Bias Reference, Automotive Signal-Line Current Source.
LED Indicator and Backlight Driver
The PSSI2021SAY115 is ideal for driving indicator LEDs and small backlight arrays because it delivers a stable 15 uA to 50 mA constant current with only one external resistor between pins 4 and 5. According to Nexperia, the 75 V input rating supports long LED strings directly from 24 V industrial rails, eliminating the need for current-limiting resistors per LED. The integrated PNP plus two-diode topology compensates for temperature drift, ensuring uniform brightness across the LED string over -40 to +125 C operation. Compared to a discrete resistor-plus-transistor solution, this part saves PCB area, lowers BOM cost, and improves LED brightness matching by eliminating component-to-component variation in the bias network.
Recommended
Sensor Bridge Biasing
The PSSI2021SAY115 provides a stable reference current for sensor bridges and transducer excitation where measurement accuracy depends on a known, temperature-stable bias. According to the Nexperia datasheet, the monolithic PNP-plus-two-diode topology tracks temperature better than discrete references, minimizing bias drift across the operating range. Output currents from 15 uA to 50 mA cover most bridge-type sensors including strain gauges, RTD excitation, and thermistor biasing. The SOT-353 footprint allows the bias source to be placed adjacent to the sensor, minimizing parasitic resistance and improving signal integrity in low-level measurement chains.
Recommended
Industrial 24V/48V Bus Current Limiter
The PSSI2021SAY115 is well-suited to industrial automation systems where 24 V or 48 V bus power must be converted to a regulated current for downstream loads. According to Nexperia, the 75 V maximum input withstands inductive transients and load-dump events common on industrial rails without additional protection. Setting Rext between pins 4 and 5 to deliver 10-50 mA provides a simple, robust current limiter for relay coils, optocouplers, and signal-line terminations. The SOT-353 package and surface-mount Tape & Reel delivery support high-density automated assembly in PLC and sensor-interface designs.
Recommended
Battery Charger Reference Current
In linear battery chargers and trickle-charge circuits, the PSSI2021SAY115 provides a stable reference current that sets the charge rate independent of supply variations. According to Nexperia, the constant-current output remains stable from 15 uA to 50 mA across the full 75 V input range, supporting chargers for NiMH, Li-ion, and lead-acid cells. Connecting Rext between pins 4 and 5 to a fixed value gives a temperature-compensated charging current suitable for float-charging backup batteries. The integrated design eliminates the calibration drift common to discrete bias networks, extending battery service life.
Recommended
Portable Device Bias Reference
The PSSI2021SAY115 fits portable and wearable devices where PCB area is constrained and battery life is critical. According to Nexperia, the SOT-353 footprint measures only 2.0 x 1.25 mm, among the smallest constant-current packages available. The part's low quiescent current draw and simple one-resistor programming reduce both standby power and BOM complexity. Programmable currents down to 15 uA make it suitable for low-power sensor wake-up biasing in IoT endpoints. The RoHS-compliant, halogen-free construction meets modern portable-device environmental requirements.
Recommended
Automotive Signal-Line Current Source
The PSSI2021SAY115 serves as a bias source for automotive signal lines and sensor interfaces where stable current is needed over a wide temperature range. According to Nexperia, the integrated PNP-plus-diode temperature compensation maintains regulation across automotive temperature grades. Programming output currents between 1 mA and 50 mA via Rext enables biasing of Hall sensors, rotary encoders, and dashboard indicator LEDs. Although the standard part is not AEC-Q100 qualified, the SOT-353 package and Tape & Reel delivery support standard automotive PCB assembly processes. For AEC-Q100 designs, qualified alternatives should be evaluated.
Recommended
Recommended Products Summary
Engineering reference data for PSSI2021SAY115 — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | PSSI2021SAY |
|---|---|---|
| Brand | Nexperia | Nexperia |
| Package | SOT-353 (SC-88A) | SOT-353 (SC-88A) - same |
| Output Current Range | 15 uA to 50 mA | 15 uA to 50 mA |
| Maximum Input Voltage | 75 V | 75 V |
| Maximum Power Dissipation | 335 mW | 335 mW |
| Topology | PNP + 2 diodes integrated | PNP + 2 diodes integrated |
| External Components Required | 1 resistor (pins 4-5) | 1 resistor (pins 4-5) |
| Packaging | Tape & Reel (suffix 115) | Bulk |
Key Differentiators
- Single-resistor programming (vs Discrete PNP + resistor + 2 diodes solution)
- Wide input voltage range (vs NSI50350AST3G (onsemi SOT-223, 50 V max))
- Ultra-small SOT-353 footprint (vs Larger SOT-223 or DPAK current sources)
Design Notes
Estimated: For an LED load with VLED = 2.0 V driven at 20 mA from a 24 V supply, the power dissipated in the PSSI2021SAY115 is approximately (24 V - 2.0 V) * 0.020 A = 0.44 W. This exceeds the 335 mW rating, so a heatsink or supply pre-regulation is required. Either reduce supply voltage, lower output current, or add a series resistor to share dissipation. Always calculate Pd = (Vin - Vload) * Iout before finalizing the design.
Do not omit the external resistor between pins 4 and 5; without Rext the output current is undefined and may exceed safe limits. Use a 1% or better tolerance resistor for stable current setting, and ensure its voltage rating exceeds the maximum voltage across pins 4 and 5. For low currents below 1 mA, use thin-film resistors to minimize leakage-induced error.
The SOT-353 (SC-88A) package has a junction-to-ambient thermal resistance of approximately 500 C/W on a standard JEDEC test board. At maximum 335 mW dissipation, junction temperature rise is roughly 168 C above ambient, which exceeds safe limits. Reduce dissipation by lowering VIN-VOUT differential, or use multiple copper pours and thermal vias to drop effective theta_JA. Verify with actual board measurements.
Place the PSSI2021SAY115 close to the load and keep the Rext trace short to minimize parasitic resistance. Route the high-current output trace (pin 3) with adequate copper width - for 50 mA, use at least 0.3 mm trace on outer layers. Keep digital switching nodes away from pins 4 and 5 to prevent noise injection into the bias reference. A small ground pour under the device improves thermal dissipation.
Compliance Information
RoHS and lead-free per Nexperia product page and DigiKey listing. Not AEC-Q100 qualified - standard part is industrial/consumer grade.
