This paper presents a buck type dc-dc converter for uA/mA-scale solar cells under low light intensity conditions. The maximum power point tracking and voltage regulation are achieved with single stage DC-DC conversion. The proposed system is designed and simulated in a 0.18um CMOS technology. The proposed circuits conduct voltage tracking and regulation based on input&output voltage. It utilizes a PID closed loop and hysteresis voltage band to control the PWM on power MOSFETs. In this design, the dc-dc converter dynamically controls the solar cell to operate at the maximum power point with a 0.64% error and obtains an output voltage of 1.8 V with a 0.2% ripple. A conversion efficiency of 92.5% on capacitor C1 at 300uAinput current and an end-to-end efficiency of 68.4% are achieved in simulation through using the input and output bi-directional closed-loop control method.