High PSRR 1.2V and 3.3V LDO Regulators for 5V Input Rails

Designing power management rails for mixed-voltage architectures often requires stepping down a 5V bus to clean, low-voltage rails such as 3.3V and 1.2V. When powering sensitive analog blocks, RF transceivers, high-speed ADCs, or modern digital cores like FPGAs, power supply rejection ratio (PSRR) and output noise floor are critical parameters that directly impact system performance.

Selecting the right low-dropout (LDO) regulator involves balancing PSRR, quiescent current, output current capability, and package size. Below is an engineering evaluation of three prominent LDO parts well-suited for systems requiring 5V input rails with 1.2V and 3.3V outputs.

ON Semiconductor NCP163ASN330T1G (3.3V Rail)

The NCP163ASN330T1G is a fixed 3.3V output LDO housed in a compact SOT-23-5 package. It features an impressive 92dB PSRR and ultra-low output noise, making it an exceptional choice for noise-sensitive 3.3V rails. Operating from an input voltage range of 2.2V to 5.5V, it easily accommodates a 5V input rail while providing a very low dropout voltage of typically 105mV at maximum load.

With a quiescent current of just 12μA, it is well-suited for battery-powered or always-on architectures where standby efficiency matters. Its primary limitation is its fixed 3.3V output, meaning a separate part code is required for other voltage rails. Output current typically scales up to 250mA depending on thermal dissipation.

ON Semiconductor NCV8163ASN120T1G (1.2V Rail)

For the 1.2V rail—often utilized for core voltages in microcontrollers, FPGAs, or PHY chips—the NCV8163ASN120T1G is a robust counterpart to the NCP163 family. It delivers the same ultra-low noise profile and 92dB PSRR in a TSOP-5 package while supporting a 2.2V to 5.5V input range.

Crucially, the NCV prefix indicates AEC-Q100 automotive qualification, ensuring operation across an extended temperature range of -40°C to +125°C. It delivers up to 250mA of output current with a 12μA quiescent current, making it reliable for harsh industrial and automotive environments requiring exceptionally clean 1.2V supplies.

Diodes Incorporated AP7343D-33FS4-7B (Alternative 3.3V Rail)

If your 3.3V rail demands an ultra-compact footprint or higher output current rather than maximum PSRR, the AP7343D-33FS4-7B provides an alternative in a tiny X2-DFN1010-4 package measuring just 1x1mm. It supports an input voltage range of 1.7V to 5.25V and delivers up to 300mA of output current with tighter initial voltage regulation accuracy of 1%.

However, compromises include a lower PSRR of 75dB, higher output noise, and a quiescent current of 35μA. It includes an active enable pin for power sequencing, making it ideal for space-constrained digital boards where absolute peak noise rejection is secondary to board density and current delivery.

Design Best Practices for High-Performance LDOs

• Input and Output Capacitance: Always place low-ESR ceramic capacitors (typically 1μF to 4.7μF) directly adjacent to the input and output pins. Insufficient capacitance or high-ESR capacitors can induce regulator instability and degrade published PSRR figures.
• Thermal Management: SOT-23 and DFN packages rely heavily on PCB copper planes for heat sinking. Maximize copper trace area connected to the ground and output pins to handle continuous thermal dissipation under maximum dropout conditions.
• Layout Routing: Keep high-current digital switching traces away from the LDO output node to prevent electromagnetic coupling into sensitive low-voltage rails.