In this paper, a compact low-noise digital pixel with 15-bit pixel-level two-step analog-to-digital converter (ADC) is proposed. The coarse quantization adopts a 10-bit pulse frequency modulation conversion in which a three-phase charge-reset circuit is proposed to reduce reset noise charge which is the major noise of this ADC. The fine quantization is implemented by a 5-bit ramp conversion and reuses the coarse quantization circuit. A 640×512 readout integrated circuit for infrared focal plane array with the proposed 15μm pitch digital pixel is designed in the 0.18μm 1P5M CMOS process for verification. Post-layout simulation results show that each pixel only consumes 31nW at 120fps. The RMS noise at maximum signal is 0.85LSB.