“Naturally, we often deal with various chips. Some engineers may not know the inside of the chip very well. Many students directly turn to the application page of the Datasheet when applying a new chip, and build the peripheral according to the recommended design. As a result, even if there is no problem with the application, it ignores more technical details, and does not accumulate better experience for its own technical growth. Today, taking a DC/DC step-down Power supply chip LM2675 as an example, I will try to explain the internal design principle and structure of the next chip as much as possible. Students in the IC industry can take a look at it. Welcome to advise!

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Naturally, we often deal with various chips. Some engineers may not know the inside of the chip very well. Many students directly turn to the application page of the Datasheet when applying a new chip, and build the peripheral according to the recommended design. As a result, even if there is no problem with the application, it ignores more technical details, and does not accumulate better experience for its own technical growth. Today, taking a DC/DC step-down power supply chip LM2675 as an example, I will try to explain the internal design principle and structure of the next chip as much as possible. Students in the IC industry can take a look at it. Welcome to advise!

Typical application circuit of LM2675-5.0

Open the DataSheet of LM2675, first look at the block diagram

This picture contains all the internal unit modules of the power chip. We have already understood the BUCK structure. The main function of this chip is to drive the MOS tube, and to form a loop control PWM drive power MOS tube by detecting the output state through the FB pin. , to achieve voltage regulation or constant current output. This is an asynchronous mode power supply, that is, the freewheeling device is an external diode instead of an internal MOS tube.

Let’s take a look at how each function is implemented

1. Reference voltage

Similar to the reference power supply for board-level circuit design, the reference voltage inside the chip provides a stable reference voltage for other circuits on the chip. This reference voltage requires high precision, good stability and small temperature drift. The reference voltage inside the chip is also called the bandgap reference voltage, because this voltage value is similar to the bandgap voltage of silicon, so it is called the bandgap reference. This value is about 1.2V, as shown in the following figure:

Here to go back to the textbook to talk about the formula, the current and voltage formulas of the PN junction:

It can be seen that it is an exponential relationship, and Is is the reverse saturation leakage current (that is, the leakage current of the PN junction due to minority carrier drift). This current is proportional to the area of ​​the PN junction! That is, Is->S.

In this way, it can be deduced that Vbe=VT*ln(Ic/Is) !

Returning to the above figure, the operational amplifier analyzes VX=VY, then it is I1*R1+Vbe1=Vbe2, so we can get: I1=△Vbe/R1, and because the gate voltages of M3 and M4 are the same, so the current I1=I2 , so the formula is derived: I1=I2=VT*ln(N/R1) N is the ratio of the PN junction area of ​​Q1 and Q2!

Returning to the above figure, the operational amplifier analyzes VX=VY, then it is I1*R1+Vbe1=Vbe2, so we can get: I1=△Vbe/R1, and because the gate voltages of M3 and M4 are the same, so the current I1=I2 , so the formula is derived: I1=I2=VT*ln(N/R1) N is the ratio of the PN junction area of ​​Q1 and Q2!

In this way, we finally get the reference Vref=I2*R2+Vbe2, the key point: I1 has a positive temperature coefficient, and Vbe has a negative temperature coefficient, and then adjust it through the N value, but achieve a good temperature compensation! Get a stable reference voltage . N is generally designed according to 8 in the industry. To achieve zero temperature coefficient, Vref=Vbe2+17.2*VT is calculated according to the formula, so it is about 1.2V. At present, a reference of less than 1V can be achieved in the low-voltage field, and in addition to the temperature coefficient, There are problems such as power supply ripple suppression PSRR, which cannot be deepened due to limited levels. The final diagram is like this, and the design of the op amp is of course very particular:

Simulation of temperature characteristics as shown in the figure:

2. Oscillator OSC and PWM

We know that the basic principle of the switching power supply is to use the PWM square wave to drive the power MOS tube, so naturally we need a module that generates oscillation. square wave.

The final detailed circuit design diagram is as follows:

A technical difficulty here is the slope compensation in current mode. In order to stabilize the slope when the duty cycle is greater than 50%, an additional compensation slope is added. I also have a superficial understanding, and interested students can learn in detail.

3. Error amplifier

The function of the error amplifier is to sample the feedback voltage in order to ensure the output constant current or constant voltage. So as to adjust the PWM driving the MOS tube, as shown in the diagram:

4. Drive circuit

The structure of the final driving part is very simple, that is, a large area MOS tube with strong current capability.

5. Other module circuits

The other module circuits here are to ensure that the chip can work normally and reliably. Although it is not the core of the principle, it really occupies an important position in the design of the chip.

Specifically, there are several functions:

1. Start the module

The role of the startup module is naturally to start the chip to work, because at the moment of power-on, it is possible that the current of all transistors is 0 and remains unchanged, so it cannot work. The role of the start-up circuit is equivalent to “lighting a fire” and then shutting it off. As shown in the figure:

At the moment of power-on, S3 is naturally turned on, and then S2 is turned on to turn on M4 Q1, etc., M1 M2 is turned on, the constant current source circuit on the right works normally, and S1 is also turned on, so turn off S2 to complete the startup. If there is no S1 S2 S3, all transistor currents are 0 instantaneously.

2. Overvoltage protection module OVP

It is well understood that when the input voltage is too high, the output is turned off through the switch to avoid damage, and a protection point can be set through the comparator.

3. Over temperature protection module OTP

The temperature protection is to prevent the chip from being damaged by abnormally high temperature. The principle is relatively simple. It uses the temperature characteristics of the transistor and then sets the protection point through the comparator to turn off the output.

4. Overcurrent protection module OCP

In the case of output short circuit, for example, by detecting the output current to feedback control the state of the output tube, it can be turned off or current limited. As shown in the current sampling, the current of the transistor is proportional to the area. Generally, the area of ​​the sampling tube Q2 will be one thousandth of the area of ​​the output tube, and then the drive of the MOS tube is controlled by the voltage comparator.

There are also some other auxiliary module designs.

6. Constant current source and current mirror

Inside the IC, how to set the working state of each transistor is through the bias current. The constant current source circuit can be said to be the cornerstone of all circuits, and the bandgap reference is also generated because of this, and then the current mirror is used for each functional module. To provide current, the current mirror is to set the required current size through the area of ​​the transistor, similar to a mirror.