When can we write Heisenberg uncertainty Principle as a equality?
It is important to define the time and frequency widths $\Delta_t$ and $\Delta_{\omega}$ of a signal before discussing any special forms of the uncertainty principle. There is no unique definition of these quantities. With appropriate definitions it can be shown that only the Gaussian signal satisfies the uncertainty principle with equality.

Consider a signal $f(t)$ with Fourier transform $F(\omega)$ satisfying

$$\int_{-\infty}^{\infty}f^2(t)dt=1\quad\textrm{(unit energy)}\\
\int_{-\infty}^{\infty}t|f(t)|^2dt=0\quad\textrm{(centered around }t=0)\\
\int_{-\infty}^{\infty}\omega|F(\omega)|^2d\omega=0\quad\textrm{(centered around }\omega=0)$$

None of these conditions is actually a restriction. They can all be satisfied (for signals with finite energy) by appropriate scaling, translation and modulation.

If we now define time and frequency widths as follows

$$\Delta_t^2=\int_{-\infty}^{\infty}t^2|f(t)|^2dt\\
\Delta_{\omega}^2=\int_{-\infty}^{\infty}\omega^2|F(\omega)|^2d\omega$$

then the uncertainty principle states that

$$\Delta_t^2\Delta_{\omega}^2\ge\frac{\pi}{2}\tag{2.6.2}$$

(if $f(t)$ vanishes faster than $1/\sqrt{t}$ for $t\rightarrow\pm\infty$)

where the inequality is satisfied with equality for the Gaussian signal

$$f(t)=\sqrt{\frac{\alpha}{\pi}}e^{-\alpha t^2}\tag{2.6.3}$$

The equation numbers above correspond to the proof below which is from Wavelets and Subband Coding by Vetterli and Kovacevic (p.80):