A folding voltage-to-time converter (VTC) is proposed for low-power time-based (TB) flash ADCs performing voltage-to-time-to-digital conversion. Conventional VTCs in TB flash ADCs generate multiple time outputs or have nonlinear conversion gain, resulting in a large power consumption in time-to-digital converters (TDCs) due to power-inefficient architectures using a lot of comparators. The proposed VTC generates a single time output with a large-and-linear conversion gain because the proposed VTC folds the whole voltage input range several times and each voltage-to-time conversion is defined within the voltage range reduced by folding. This allows the TDC power consumption to be reduced by using power-efficient architectures. Moreover, the VTC generates digital outputs as a result of the folding operation, thereby relaxing the resolution requirement of the TDC. A 7-bit TB ADC is implemented with a 4x folding VTC having a 2-bit digital output and a 5-bit pipelined TDC for high-speed low-power operation. A TB ADC fabricated in a 1-V 65-nm CMOS process achieves a 4-GS/s sampling frequency, 11.3-mW power consumption, a 34.58-dB SNDR, and a 64.5-fJ/conv.-step figure of merit (FoM).