TI例程SPLL_3ph_DDSRF(解耦双同步坐标系软件锁相环)中LPF(低通滤波器)的离散方程不知道如何推导出来的，求解答?

程序中对应的离散方程

// Low pass filter //
//-------------------------//

spll_obj->y[1] = (spll_obj->d_p_decoupl*spll_obj->k1) - (spll_obj->y[0]*spll_obj->k2);
spll_obj->d_p_decoupl_lpf = spll_obj->y[1] + spll_obj->y[0];
spll_obj->y[0] = spll_obj->y[1];