Dry information | Dynamically change the DC-DC feedback resistance and adjust the output voltage
DC-DC refers to DC-to-DC power supply, and DC stands for Direct Current.
DC-DC is generally composed of a control chip, an inductor coil, a filter capacitor, etc. The picture below is a classic DC-DC reference circuit:
The DC-DC control chip itself generally does not have the function of dynamically adjusting the output voltage.
I have a circuit board on hand that has the function of dynamically changing the feedback resistor external to the DC-DC control chip to adjust the output voltage :
This circuit board uses a PMU chip. PMU is the Power Management Unit, which is the power management unit:
The corresponding schematic diagram is as follows:
It looks messy. After all, it is a PMU that manages several power supplies, which is a bit complicated.
This article only focuses on the DC-DC part of the PMU. For the convenience of analysis, other irrelevant circuits are deleted:
Correspondence between DC-DC schematic diagram and physical circuit board:
For the DC-DC circuit in the PMU, let's start to analyze the circuit principle of " dynamically adjusting the output voltage ".
1. When the SLEEP_CTL output by the main control chip is low level.
At this time, transistor Q1 is not conducting:
The circuit equivalent is:
Since the feedback voltage Vfb of this DC-DC is fixed at 0.6V, and the output voltage VDD12 is divided by the feedback resistors R15 and R17, it is the feedback voltage Vfb, which has the following formula:
Therefore, the values of feedback resistors R15 and R17 are determined, and the output voltage VDD12 is determined:
Calculate that VDD12≈1V at this time.
2. When the SLEEP_CTL output by the main control chip is high level.
At this time, transistor Q1 is turned on:
The circuit equivalent is:
In this way, resistor R18 is connected to become a feedback resistor, connected in parallel with resistor R17, and the corresponding formula becomes :
The output voltage VDD12 becomes:
The measured VDD12 output is 1.31V.
At this point, the analysis of the principle of adjusting the output voltage by dynamically changing the feedback resistor outside the DC-DC control chip is completed.
3. Fast Question & Ans wer
1. Why do we need to dynamically adjust the DC-DC output voltage?
Answer: Taking the circuit board in this article as an example, VDD12 is the core power supply voltage of the main control chip. The voltage requirement is 1.31V during normal operation and is reduced to 1V during sleep, so it needs to be adjusted dynamically.
2. What is the function of capacitor C37?
Answer: Capacitor C37 is a DC-DC feedforward capacitor. Its purpose is to adjust the response speed of the chip's output voltage. We will analyze it in detail later when we have the opportunity.
3. What are the functions of capacitors C38 and C39?
Answer: For power supply filtering, they are filter capacitors.
4. Can transistor Q1 be directly replaced with a MOS tube?
Answer: Yes, and the MOS tube model is often 2N7002. Replacing MOS tubes can further reduce power consumption, but the price of MOS tubes is much higher than that of triodes.
Recommended reading
· END ·
京公网安备 11010802033920号