A 0.45-V input, 0.4-V output on-chip gate boosted (OGB) buck converter with clock gated digital PWM controller in 40-nm CMOS achieved the highest efficiency to date with the output power less than 40μW. A linear delay trimming by a logarithmic stress voltage (LSV) scheme to compensate for the die-to-die delay variations of a delay line in the PWM controller with good controllability is also proposed.