Negative rail, 500 mA, 2x2 mm — what the specs mean for the board
The ADP7185ACPZN2.5-R7 is a negative-output LDO from Analog Devices, delivering a fixed -2.5 V at up to 500 mA from a -5.5 V max input rail. This is the part to reach for when you need a clean, regulated negative supply for op-amp rails, ADC reference grounds, or split-supply sensor front-ends — the kind of rail that, if it sags or carries ripple, injects noise straight into the signal chain. The 8-LFCSP (2x2 mm) package with exposed pad means the thermal path runs through the PCB copper, not the air.
PSRR that holds up past 100 kHz
PSRR is specified at 80 dB at 1 kHz, rolling off to 40 dB at 1 MHz. That 40 dB at 1 MHz is the number that matters for a switching regulator feeding this LDO — a 2 MHz buck converter's fundamental ripple at 2 MHz sees about 35 dB of rejection, but its switching harmonics at 4, 6, 8 MHz see progressively less. If the upstream switcher runs at 400 kHz, the fundamental ripple at 400 kHz sees roughly 55 dB of rejection, which is enough to keep the output noise below 10 µV RMS with a typical 10 µF output cap. The dropout voltage is 0.36 V max at 500 mA. That means the input rail must stay below -2.86 V (absolute) to maintain regulation at full load — a -3.3 V rail gives 0.44 V of headroom, which is tight but workable if the -3.3 V rail itself is regulated. A -5 V rail gives 2.14 V of headroom, which is comfortable and keeps the LDO in its high-PSRR region.
Protection features include over-current limiting and a Power-On Reset that holds the output in a known state until the input rail is stable.