This paper presents a 10-μs transient recovery time and a low electromagnetic interference dc-dc buck converter with a second-order delta-sigma (Δ-Σ) modulator. The proposed buck converter employs the techniques of oversampling, noise shaping, fast-transient path, mode selector, and second-order Δ-Σ modulator to achieve spur reduction and to improve transient recovery time. As a result, the noise tones of the output voltage are less than the other conventional pulsewidth modulation converters. In addition, the fast-transient path could correct output voltage immediately, and the mode selector controls the duty cycle of the converter to limit overshoot voltage and undershoot voltage. The chip was implemented using a 0.35-μm Taiwan Semiconductor Manufacturing Company CMOS process. The measured results show that the transient recovery time is 10 μs from heavy load to light load and from light load to heavy load, and the noise power spectrum demonstrates a -64-dBm peak noise when the output voltage is 2 V and the output current is 300 mA. A maximum efficiency of 89% is measured at 3 V output voltage and at 3.6 V supply voltage.