High harmonic generation is a versatile experimental technique for probing ultrafast electron dynamics. While in the past it has been employed typically in dielectrics and semiconductors, recently high harmonic generation was also observed from a topological surface [Schmid et al., Nature 593, 385 (2021)]. It has been found that harmonic orders in the intermediate range of 13-18 continuously shift when the carrier envelope phase (CEP) is varied. In this work, we adopt a minimal model of the topological surface state and calculate analytically the high-harmonic spectrum. We derive formulae describing the parametric dependencies of CEP shifts in high harmonics; in particular, we have a transparent result for the shift of the (peak) frequency ω when changing the CEP φ: dω/dφ=−2f¯′ω/ω0, where ω0 describes the fundamental driving frequency and f¯′ characterizes the chirp of the driving laser pulse. We compare the analytical formula to full-fledged numerical simulations finding only 17% average absolute deviation in dω/dφ. Our analytical result is fully consistent with experimental observations. It therefore provides the first understanding of the phenomenon of CEP shifts in solids based on analytically derived parametric dependencies
