1 Introduction of Smart Watch Fast Transient Performance Requirements

In the smart watch application, the LED shines at a repetitive rate on the wrist, which reflected from the blood in the artery and veins gets modulated according to the oxygen content in the blood. According to the sampling theory, LED needs to flash at the frequency of 50 Hz at least to sample the signal range from 0.5 to 10 Hz relatively accurately. Since the current in LED ramps from 0 to 200 mA during hundreds of nanoseconds, the undershoot of the Output voltage is extremely large, as is shown in Figure 1-1.

If the sampling begin before output voltage return to normal value, the data accuracy is degraded extraordinarily. For example, the PSRR and SNR decreases dramatically. Hence the sampling is expected to trigger until the output voltage settles to target value, which waiting period is denoted as t_tran and the sampling time is t_sample( which is definitely customizable). This settled value is the same across all the sampling periods and the LED can be turned off after the t_sample. Thus the minimum t_PWM time can be calculated by t_tran plus t_sample. All through the t_PWM period, the LED current source needs to be always on, which dominates the power consumption.

Assuming the sample times being equal, with fast transient performance shown in Figure Figure 1-2, the t_tran can be reduced from 100 us to 10 us, thus the period of Led_PWM can be reduced as well, denoted as t_PWM. With reducing LED light time, the system reduces energy consumption and improves its efficiency.