Fast Transient Performance Advantages in Smart Watch Applications with TPS61299
Abstract
Today smart watches are often designed with the ability to measure blood oxygen and monitor heartbeat. The application works by means of an LED that emits light onto the wrist, while the photodiode on the other side receives the reflected light, comparing the difference between the emitted light and the received light to calculate the oxygen level. Low-frequency blinking of LEDs, is challenging low-power supply designs. Especially in the moment when the LEDs light up, the load current suddenly ramps up, the output voltage to fluctuate as a result. The shorter settling time is, the shorter LED illumination time is, as a result, the more energy consumption of a portable device such as the smart bracelet is saved. In this way, the settling time of the output voltage is of the importance. Therefore, the transient performance is often an important indicator in examining power devices in smart watch.
This application note proposes a power supply solution for the TPS61299 boost converter with fast transient performance. Compared to common power solutions, smart watch with TPS61299 can extend the standby time about one day, about 6.67%.
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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 ttran and the sampling time is tsample( which is definitely customizable). This settled value is the same across all the sampling periods and the LED can be turned off after the tsample. Thus the minimum tPWM time can be calculated by ttran plus tsample. All through the tPWM period, the LED current source needs to be always on, which dominates the power consumption.
Figure 1-1 Typical Transient
Response
Figure 1-2 Fast Transient
ResponseAssuming the sample times being equal, with fast transient performance shown in Figure Figure 1-2, the ttran can be reduced from 100 us to 10 us, thus the period of Led_PWM can be reduced as well, denoted as tPWM. With reducing LED light time, the system reduces energy consumption and improves its efficiency.
