RT9161-33PX - 300mA 3.3V LDO Regulator | Richtek | SOT-89
MPN: RT9161-33PX ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0.42 | $0.42 |
| 10 | $0.37 | $3.70 |
| 100 | $0.31 | $31.00 |
| 500 | $0.25 | $125.00 |
| 1,000 | $0.2 | $200.00 |
| 3,000 | $0.16 | $480.00 |
RT9161-33PX Overview
An LDO (low-dropout) linear regulator is a type of voltage regulator that maintains a stable output voltage even when the input voltage is very close to the output voltage. LDOs sit within the broader category hierarchy of linear regulators -> voltage regulators -> power management ICs -> semiconductors. Unlike switching regulators, an LDO introduces no switching noise onto the output rail but dissipates the difference between input and output as heat.
Key features of the RT9161-33PX include a fixed 3.3V output with ±2% tolerance, 300mA continuous output current, built-in current-limit protection, thermal shutdown, and a low quiescent ground current of approximately 110 µA across the entire load range. The SOT-89 package provides an exposed die attach pad (lead frame tab) that enables reasonable power dissipation for the 300mA load class with modest copper area on the PCB.
Architecturally, the RT9161-33PX is built around a CMOS pass-element topology with an internal bandgap reference and error amplifier, providing fast transient response and stable operation with low-ESR ceramic capacitors. The low dropout voltage - typically around 250 mV at full 300 mA load - allows the regulator to maintain regulation directly from a single-cell Li-ion source as it discharges.
Typical applications include single-cell Li-ion powered systems, USB peripherals, microcontroller core rails, real-time-clock (RTC) backup supplies, and post-regulation stages following switching converters. Its low quiescent current makes it especially attractive for always-on power domains in IoT endpoints and wearable devices where standby drain directly governs battery life.
When designing with the RT9161-33PX, observe the input/output voltage headroom for dropout, ensure the input capacitor is placed within a few millimetres of the IN pin to control input impedance, and follow SOT-89 thermal layout guidance - a small copper pour tied to the tab acts as the heatsink.
This page synthesizes distributor pricing, verified drop-in alternatives in the same SOT-89 footprint, and practical design notes not consolidated in the manufacturer datasheet - useful information gain for engineers shortlisting 3.3V LDO candidates.
Drop-in alternatives for RT9161-33PX — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
RT9161-33GX
✅ Drop-In📋 Reference alternative (not in catalog)
RT9161-18PX
✅ Drop-In📋 Reference alternative (not in catalog)
RT9161-30PX
✅ Drop-In📋 Reference alternative (not in catalog)
RT9161-20PX
✅ Drop-In📋 Reference alternative (not in catalog)
RT9161-31PX
✅ Drop-In📋 Reference alternative (not in catalog)
RT9161-33PX Maximum Ratings & Electrical Characteristics
| Manufacturer | Richtek Technology Corporation |
| Series | RT9161/A |
| Regulator Type | LDO (Low-Dropout) Linear Regulator |
| Output Type | Fixed |
| Output Voltage | 3.3 V |
| Output Voltage Tolerance | ±2% |
| Maximum Output Current | 300 mA |
| Quiescent Current (Ground) | 110 µA typical (zero to full load) |
| Current Limit | Built-in (internal current limit) |
| Thermal Protection | Built-in thermal shutdown |
| Package | SOT-89 |
| Mounting Type | Surface Mount |
| Marking Code | A3- |
RT9161-33PX Pin Configuration
| Pin 1 | VIN — Input voltage supply pin; place input capacitor within a few millimetres of this pin |
| Pin 2 | GND — Common ground return for input and output |
| Pin 3 | VOUT — Regulated 3.3V output; also tied to the SOT-89 tab for thermal dissipation |
Typical Applications
RT9161-33PX is suitable for 6 applications: Single-Cell Li-Ion 3.3V Rail, Microcontroller Core Supply (3.3V), RTC and Backup Supply Domains, Post-Regulation After Switching Converter, USB Peripheral 3.3V Rail, IoT Sensor Node Power.
Single-Cell Li-Ion 3.3V Rail
The RT9161-33PX is purpose-built for single-cell Li-ion battery-powered systems where a stable 3.3V rail is needed for microcontrollers, BLE SoCs, and sensors. Its 300 mA continuous rating covers typical MCU + radio peak loads, and the 110 µA quiescent current extends standby runtime in always-on domains. Placed directly after the protection FET and input filter, with a 1 µF ceramic on VIN and 1 µF on VOUT within a few millimtres of the pins, the regulator maintains regulation across the Li-ion discharge curve down to ~3.6V at full 300 mA load thanks to its low dropout voltage. Unlike a switching converter, the LDO adds no EMI onto the 3.3V rail, which is critical when supplying noise-sensitive radios such as BLE or Wi-Fi companion chips.
Recommended
Microcontroller Core Supply (3.3V)
The RT9161-33PX is well matched as a 3.3V core supply for low-power microcontrollers such as ARM Cortex-M0/M4 parts and 8-bit/16-bit MCUs from NXP, Microchip, and Renesas. The 300 mA continuous rating covers peak MCU load transients plus attached peripherals, while the ±2% output tolerance stays within most MCU VDD specifications. Engineers typically place the regulator between a 5V upstream rail and the MCU VDD pin with 1 µF ceramic input/output capacitors; the LDO's low quiescent current does not tax sleep-mode budgets, and the absence of switching noise keeps ADC readings clean when the analog and digital domains share the 3.3V rail.
Recommended
RTC and Backup Supply Domains
Real-time-clock and backup-supply domains benefit from the RT9161-33PX's combination of low quiescent current (110 µA) and tight 3.3V regulation. The part can supply RTC ICs, SRAM backup rails, and low-power supervisory reset devices without materially draining coin-cell or Li-ion reserves. Place the regulator at the head of the always-on domain with a 1 µF ceramic output capacitor for noise filtering; the absence of switching ripple is important for RTC accuracy since supply noise couples into the 32.768 kHz oscillator amplitude. Built-in current-limit and thermal-shutdown add fault tolerance in unattended always-on systems.
Recommended
Post-Regulation After Switching Converter
Many mixed-signal boards use a switching converter for bulk 5V or 3.6V rails and follow it with a low-noise LDO for noise-sensitive analog or RF blocks. The RT9161-33PX fits cleanly as the post-regulator in this topology: it strips switching ripple from the 3.3V rail feeding audio CODECs, sensors, or RF front-end modules. With 300 mA available, it can power small analog signal chains including op-amps, references, and sensor bias networks. Place the LDO close to the analog load with a ferrite bead or small LC filter on its input to attenuate upstream switching noise, and add a 1 µF ceramic output capacitor for ripple rejection.
Recommended
USB Peripheral 3.3V Rail
USB peripherals - hubs, controllers, and embedded device endpoints - need a clean 3.3V rail derived from the 5V VBUS. The RT9161-33PX drops the 5V USB rail to 3.3V with low dropout and tight regulation, providing the 300 mA needed for USB controller ICs and associated logic. Built-in current-limit protects the regulator from USB inrush or short-circuit faults, and thermal shutdown guards against sustained overloads when the device is plugged into non-compliant hosts. Place the LDO downstream of USB ESD/TVS protection with a 1 µF ceramic on both sides; the low-Iq design does not waste USB suspend current budget.
Recommended
IoT Sensor Node Power
IoT endpoints - wireless sensor nodes, smart-home devices, and wearable health monitors - need compact, low-quiescent 3.3V regulation from a small Li-ion or Li-polymer cell. The RT9161-33PX in SOT-89 delivers the required 300 mA peak for radio TX bursts while drawing only 110 µA in quiescent state, maximizing battery runtime. Place the regulator at the battery output, feeding the sensor MCU and radio module; add a 1 µF ceramic on input and output for stability. The fixed 3.3V output matches the supply requirement of sub-GHz radios, BLE SoCs, and many low-power Wi-Fi modules without external resistor dividers.
Recommended
Recommended Products Summary
Engineering reference data for RT9161-33PX — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | RT9161-33GX | RT9161-18PX | RT9161-30PX | RT9161-20PX | RT9161-31PX |
|---|---|---|---|---|---|---|
| Brand | Richtek | Richtek | Richtek | Richtek | Richtek | Richtek |
| Package | SOT-89 | SOT-89 - same | SOT-89 - same | SOT-89 - same | SOT-89 - same | SOT-89 - same |
| Output Voltage | 3.3 V | 3.3 V | 1.8 V | 3.0 V | 2.0 V | 3.1 V |
| Output Voltage Tolerance | ±2% | ±2% | ±2% | ±2% | ±2% | ±2% |
| Maximum Output Current | 300 mA | 300 mA | 300 mA | 300 mA | 300 mA | 300 mA |
| Quiescent Current | 110 µA typical | 110 µA typical | 110 µA typical | 110 µA typical | 110 µA typical | 110 µA typical |
| Current Limit | Built-in | Built-in | Built-in | Built-in | Built-in | Built-in |
| Thermal Shutdown | Built-in | Built-in | Built-in | Built-in | Built-in | Built-in |
Key Differentiators
- Low quiescent current for always-on domains (vs RT9161-33GX)
- Fixed 3.3V output versus other RT9161 family voltages (vs RT9161-30PX)
- Higher-current rating than typical SOT-23 LDOs (vs RT9161-33PV)
Design Notes
Estimated: at VIN=5V, VOUT=3.3V, and 300 mA load, the RT9161-33PX dissipates approximately (5V - 3.3V) × 0.3A = 0.51 W. The SOT-89 package has a typical theta_JA around 100-150 C/W on a 1-square-inch copper pour, producing a junction temperature rise of 50-75 C above ambient. Provide at least 100 mm² of copper connected to the tab for reliable operation; smaller copper pours will push the part toward thermal-shutdown under heavy loads.
Place the input and output capacitors within 2-3 millimetres of the RT9161-33PX pins. Use a 1 µF X5R/X7R ceramic on VIN to suppress input impedance spikes and a 1 µF X5R/X7R ceramic on VOUT for output stability. For higher load transients (>100 mA steps), increase the output capacitor to 2.2 µF or 4.7 µF to improve transient response. Keep ground return paths short and direct to minimize parasitic inductance.
Do not operate the RT9161-33PX below its minimum input voltage for the dropout specification - at 300 mA load the input must be at least VOUT + dropout. Avoid placing the regulator near high-current switching traces or inductors to minimize conducted and radiated noise coupling into the 3.3V rail. Do not omit the output capacitor: the part requires output capacitance for loop compensation. Verify the lot marking code (A3-) during incoming inspection to prevent counterfeit substitution.
Compliance Information
RoHS, REACH, lead-free, and halogen-free compliance status was not explicitly declared in the verified web data sources. Richtek product family policy typically provides RoHS-compliant options, but engineers should request lot certificates from distributors for production orders. AEC-Q100 not applicable - part is not automotive grade.
