Basics of IC Design
1. Regulated power supply: LDO and DC-DC
The regulated power supply can be imagined as the following situation: When trying to take a continuous and small water flow from a large-diameter tap water pipe, two strategies can be used: one is to use a transfer valve
, And open the valve in a small position, this is how a linear power supply works (you can think of a valve as a transistor). The voltage adjustment transistor of the linear power supply is under a lot of "pressure" (specifically, it is converted into heat energy and dissipated);
or, it can be improved to let the water in the large water pipe flow into a relatively large "bucket", The small water pipe is connected to the bucket to get water, and then, what needs to be done is to intermittently open/close the valve on the large water pipe to ensure that the water in the bucket will neither be completely empty nor overflow due to too much——switching power supply That's the basic principle.
Principle of LDO
LDO is a low dropout regulator, which is a low dropout linear regulator.
LDO can dynamically adjust the Vout voltage, as shown in the figure:
Vdd is the input voltage.
Vout = ( Vdd/(Rds+R1+R2) ) * (R1+R2)
Here, Vbg=Vcmp in the operational amplifier is used. When Vout increases, Vcmp>Vbg, Rds will increase, so that the current Id will become smaller , so that Vout decreases.
When Vout decreases, Vcmp<Vbg, Rds will decrease, so that the current Id will increase and Vout will increase.
DC-DC principle
The step-down converter is as follows (Vi > Vo):
The schematic diagram of the step-down converter is shown in Figure 1. When the switch is closed, the voltage applied to both ends of the inductor is (Vi-Vo), and the inductor is changed by the voltage (Vi-Vo ) excitation, the magnetic flux increased by the inductance is: (Vi-Vo)*Ton.
When the switch is turned off, due to the continuation of the output current, the diode VD becomes conductive, the inductance is demagnetized, and the magnetic flux reduced by the inductance is: (Vo)*Toff.
When the state of the switch on and off is balanced, (Vi-Vo)*Ton=(Vo)*Toff, since the duty cycle D<1, so Vi>Vo, the step-down function is realized.
