Working principle of brushless motor

The photo-generated current of PIN photodiodes is weak, the minimum is only a few picoamperes (pA), and the maximum is only a few hundred microamperes (μA). PIN photodiodes have two working modes: photoconductive and photoinduced voltage [2]. In the photoconductive method, a reverse bias voltage is added across the photodiode, which increases the width of the depletion layer of the photodiode, thereby greatly reducing its parasitic capacitance and improving the working speed, but the linearity is poor and dark The current is large, so it is not suitable for precise photocurrent detection. There is no voltage drop on the photodiode working in the photovoltage mode, that is, zero bias, as shown in Figure 1. The light sensitivity and linearity are very good, suitable for precision applications, so the photodiode is used to introduce photodiodes in the general light detection circuit.

The higher the sensitivity of the light detection circuit, the smaller the bias current of the amplifier used and the larger the feedback resistance RF. For example, when the sensitivity is 1mV / pA, the current of 1pA needs to flow through the feedback resistor RF of 1000M Ω to reach 1mV. In the CMOS process, polysilicon is used to make an accurate value resistor [3], and its resistance value is calculated as follows:

R = R ڤ × (W / L) (2)

Among them, R ڤ—— polysilicon sheet resistance value, unit: Ω / μm2;

W——width of polysilicon resistance, unit: μm;

L——The length of polysilicon resistance, the unit is μm.

Generally, R R is relatively small, about 20Ω / □, even in high resistance process, R R only reaches a few kΩ / □. There is no doubt that a large resistance of 1000M Ω will occupy a considerable area of ​​the chip, and its distributed capacitance is also very large.

In practice, considering the factors such as engineering implementation and cost, the general infrared receiver head detects photocurrent in the range of tens of nanoamperes to hundreds of microamperes. The feedback resistor only needs 1M Ω to achieve a sensitivity of 1mV / nA, and a resistance of 1M Ω can also be easily realized in manufacturing. However, in an environment with strong background light, the incident photo-generated current will cause a sudden increase in the output voltage, and the output of the amplifier is prone to saturation distortion (the working voltage of the infrared receiver head is generally 5V or lower, this design is for 5V work Voltage). Obviously, in the case of a wide input current range, this method of implementing current-voltage conversion through a fixed resistance is not feasible.