Conventional constant-on-time (COT) control for DC-DC buck converter is apt to be affected by the noise caused by parasitic effect including not properly specified and temperature dependent equivalent series resistance (ESR) and equivalent series inductance (ESL). As a result, the safety operation area (SOA) of the COT is limited by the selection of external components. In this paper, the calibrated anti-ESL (CAESL) technique and the calibrated gain and BW (CGB) technique for alleviating ESL and ESR variation, respectively, are proposed to ensure a robust COT control. Furthermore, degraded output regulation caused by enlarged ESL effect due to input battery voltage variation is also solved by the CAESL technique. The proposed COT converter fabricated in 28nm CMOS technology uses an output capacitor with an ESR smaller than 1m, output ripple of 20mV, and high efficiency higher than 95%. The CAESL circuit can tolerate ESL voltage variation from 0 to 50mV even when operation temperature varies from -40 to 120°C.