PSSI2021SAY - 50mA Constant Current Source SOT-353 | Nexperia
MPN: PSSI2021SAY ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0.46 | $0.46 |
| 10 | $0.41 | $4.10 |
| 100 | $0.32 | $32.00 |
| 500 | $0.24 | $120.00 |
| 1,000 | $0.19 | $190.00 |
| 3,000 | $0.15 | $450.00 |
PSSI2021SAY Overview
A constant current source is a fundamental analog building block that maintains a defined load current independent of supply voltage variation, temperature drift, or load impedance changes. In the system hierarchy, it sits within the discrete semiconductors category, specifically among current regulator and current source transistors, and is widely used as a bias element in linear regulators, optocoupler drivers, and low-power precision references.
Key specifications include a maximum output current of 50 mA, an output voltage compliance range sufficient for direct LED driving, and an integrated bias resistor that eliminates the need for an external base resistor. The two on-chip diodes support temperature compensation and stable reference operation when paired with a Zener diode. Because the entire function is implemented in one transistor-sized footprint, the device replaces three to five discrete components per channel.
The PSSI2021SAY uses a PNP pass transistor architecture with internal base bias derived from the on-chip diodes. This monolithic integration reduces both PCB area and the bill of materials while improving temperature tracking between the bias network and the pass element. The SOT353 (SC-88A) package is a JEDEC-registered 5-pin surface-mount outline compatible with standard lead-free reflow profiles.
Typical applications include high-side LED drivers in automotive and industrial indicator clusters, Zener diode bias for shunt references, optocoupler LED current setting, and biasing of precision analog circuits. AEC-Q101 qualification makes the device suitable for harsh automotive environments. When designing with this part, choose the external resistor value based on the target constant current and verify thermal dissipation at the maximum I_OUT, since the device drops the difference between V_IN and the load voltage.
Drop-in alternatives for PSSI2021SAY — 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 PSSI2021SAY (same form factor and footprint) — differing in Configuration, Function, Maximum Power Dissipation, Output Current Range, Package.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
PSSI2021SAY,115
✅ Drop-In📋 Reference alternative (not in catalog)
NSI50350AST3G
✅ Drop-In✓ In Stock
$0.31 / Unit
View Datasheet →PSSI2021SAY Maximum Ratings & Electrical Characteristics
| Manufacturer | Nexperia |
| Part Number | PSSI2021SAY |
| Part Status | Active |
| Function | Constant Current Source |
| Transistor Type | PNP with integrated bias resistor and two diodes |
| Output Current Range | 15 uA to 50 mA (set by external resistor) |
| Maximum Output Current | 50 mA |
| Package | SOT-353 (SC-88A) / UMT5 |
| Number of Pins | 5 |
| Mounting Type | Surface Mount |
| Operating Temperature | -40C to +125C |
| Topology | High-side constant current source |
| Qualification | AEC-Q101 (automotive grade) |
| RoHS Status | Compliant |
| Configuration | Single one-chip solution (transistor + resistor + 2 diodes) |
| External Component | Resistor between pins 4 and 5 to set I_OUT |
PSSI2021SAY Pin Configuration
| Pin 1 | C — Collector of internal PNP transistor |
| Pin 2 | B/D1 — Base of PNP transistor / first diode cathode |
| Pin 3 | D2 — Second diode (compensation) |
| Pin 4 | R_ext — External resistor connection (programs I_OUT) |
| Pin 5 | E/OUT — Emitter / constant current output to load |
Typical Applications
PSSI2021SAY is suitable for 6 applications: Automotive LED Indicator Driver, Zener Diode Reference Bias, Optocoupler LED Current Setting, Precision Analog Circuit Biasing, Battery-Powered Indicator LEDs, Industrial Sensor Excitation.
Automotive LED Indicator Driver
The PSSI2021SAY's 50 mA maximum output, AEC-Q101 qualification, and -40C to +125C operating range make it a strong fit for high-side driving of indicator LEDs in automotive clusters. Its PNP topology places the regulator on the high side of the LED string, allowing the LED cathode to be tied directly to chassis ground - the standard automotive wiring convention. The integrated bias network eliminates the base resistor and discrete diode string, reducing BOM count. Compared to a resistor-based bias, the constant-current regulation keeps LED brightness uniform across supply variation from 9 V to 16 V typical of automotive cranking and load-dump transients.
Recommended
Zener Diode Reference Bias
The PSSI2021SAY is specifically characterized in its datasheet for biasing Zener reference diodes to produce a stable reference voltage (see Figure 7 of the Nexperia datasheet). The two on-chip diodes compensate the PNP base-emitter junction temperature drift, while the integrated bias resistor sets the quiescent Zener current. This produces a constant reference voltage across supply and temperature variation - ideal for low-power precision analog front-ends, ADC reference ladders, and comparator thresholds in industrial sensors. The 15 uA to 50 mA programmability allows the engineer to dial in the exact Zener bias current required by the specific Zener voltage and temperature coefficient.
Recommended
Optocoupler LED Current Setting
Optocouplers require a tightly controlled LED drive current to set the phototransistor collector current and CTR (current transfer ratio). The PSSI2021SAY provides a stable bias current regardless of supply variation or LED forward-voltage aging, preserving digital isolation timing margins over the lifetime of the product. Its SOT353 footprint fits alongside the optocoupler in compact isolated-interface designs for industrial PLCs and motor-control gate drivers. The external resistor enables the designer to set the exact LED current that meets the optocoupler's recommended operating point.
Recommended
Precision Analog Circuit Biasing
Low-power precision analog circuits - including bandgap references, transistor bias networks, and current mirrors - require stable bias currents that are immune to supply variation. The PSSI2021SAY delivers a temperature-compensated constant current when paired with the on-chip diodes, replacing a discrete bias chain of two resistors, two diodes, and a PNP transistor. The AEC-Q101 grade extends the application into automotive sensor signal conditioning, while the SOT353 package keeps the bias network small enough to co-locate with the analog IC it serves.
Recommended
Battery-Powered Indicator LEDs
In battery-powered IoT devices, portable instrumentation, and wearables, the PSSI2021SAY's integrated one-chip solution minimizes both PCB footprint and quiescent power. Because the bias network is resistor-only and the pass element is a PNP transistor, the device draws no gate-drive current and adds only the programmed I_OUT plus a few hundred microamps of base drive to the load. The wide 15 uA to 50 mA programmability lets the same part bias a single status LED at 2 mA or a brighter indicator at 20 mA from a single lithium cell.
Recommended
Industrial Sensor Excitation
Many industrial sensors - including RTD excitation bridges, thermistor bias networks, and photo-detector front-ends - require a precise, stable excitation current. The PSSI2021SAY supplies that current from a single SOT353 component, with the external programming resistor selected to match the sensor's nominal excitation value. Its AEC-Q101 grade and -40C to +125C range suit factory-floor and outdoor installations. The high-side topology simplifies PCB layout when the sensor ground is shared with the system ground.
Recommended
Recommended Products Summary
Engineering reference data for PSSI2021SAY — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | PSSI2021SAY,115 | NSI50350AST3G |
|---|---|---|---|
| Brand | Nexperia | Nexperia | onsemi |
| Package | SOT-353 (SC-88A) | SOT-353 (SC-88A) - same | SOT-223 - same family footprint |
| Function | Programmable constant current source | Programmable constant current source | Fixed 50 mA constant current regulator |
| Output Current | 15 uA to 50 mA (programmable) | 15 uA to 50 mA (programmable) | 50 mA (fixed) |
| External Resistor Required | Yes (pin 4 to pin 5) | Yes (pin 4 to pin 5) | No (two-terminal fixed) |
| Topology | High-side PNP | High-side PNP | High-side PNP |
| Automotive Grade (AEC-Q101) | Yes | Yes | Yes |
| Unit Price (qty 1, as of 2026-10-10) | $0.46 | $0.46 | $0.55 |
Key Differentiators
- One-chip integration of PNP transistor, bias resistor, and two compensation diodes (vs NSI50350AST3G)
- Resistor-programmable current range from 15 uA to 50 mA (vs NSI50350AST3G (fixed 50 mA))
- AEC-Q101 qualified with -40C to +125C operating range (vs Commercial-grade constant current references)
Design Notes
Estimated: at V_IN = 12 V driving an LED string with V_F = 3.0 V at 50 mA, the PSSI2021SAY dissipates approximately (12 - 3.0) * 0.050 = 0.45 W. The SOT353 has a theta_JA of approximately 250 C/W on a standard JEDEC test board, producing a junction temperature rise of about 113 C above ambient. At 25 C ambient the junction sits near 138 C - within the 150 C absolute maximum but with limited margin. For sustained 50 mA operation at elevated ambient, consider derating to 30 mA or adding thermal copper pours under the package.
Place the external programming resistor between pins 4 and 5 as close to the IC as possible to minimize parasitic resistance and noise pickup. Route the constant-current output trace (pin 5) directly to the load without sharing the path with high-current switching signals. The SOT353 thermal pad is not internally connected; use wide copper pours under and around the package for sustained high-current operation.
Do not connect the programming resistor to anything other than pins 4 and 5 - doing so will break the on-chip bias network and cause unregulated output current. Do not exceed the 50 mA absolute maximum - sustained overcurrent will shift the bias point and may damage the on-chip diodes. Always verify the output current with an ammeter after PCB assembly because incorrect resistor values are the most common field failure for this part family.
Compliance Information
AEC-Q101 qualified per Nexperia datasheet and Mouser product listing. RoHS and lead-free per SC-88A package rating. REACH compliance status not explicitly published.
