Several questions in the chip manual

shaorc

[If you don't understand, ask] This is part of the datasheet of the power conversion chip MP2315 I have the following questions and hope to have them answered [1] In Figure 1, the explanation of the GND pin is "system ground" and "the reference ground for the output voltage". Does this mean that GND is the reference ground for the output voltage of the SW pin? Does the regulated output voltage mentioned in the GND pin refer to the SW pin (switch output)? [2] If this chip is used, is the system ground GND in Figure 1 the common ground on the PCB board? 【3】What does the purple box in Figure 2 mean? The original text seems to require connecting an external resistor for feedback voltage regulation, but the actual application diagram (Figure 3) has three resistors. I don’t quite understand it here Figure 1 Figure 2 Figure 3

maychang

[1] In Figure 1, the explanation of the GND pin is "system ground" and "reference ground for output voltage". Does it mean that GND is the reference ground for the output voltage of the SW pin? No. It is the "ground" of VOUT.

maychang

Does the regulated output voltage mentioned in the GND pin refer to the SW pin (switch output)? No. It refers to the "ground" of VOUT.

maychang

【2】Is the system ground GND in Figure 1 the common ground on the PCB if this chip is used? It refers to the common end of VIN and VOUT.

qwqwqw2088

SW is the switch output, which is a dual MOS tube push-pull drive circuit. The purple box at the output end is connected to an external resistor to feedback the voltage regulation. It is an external voltage division method. Many chips are like this on the outside. It is more common and not necessarily a resistor.

shaorc

maychang posted on 2019-3-26 16:13 【1】In Figure 1, the explanation of the GND pin is "system ground" and "the reference ground for the output voltage". Does it mean that GND is the reference ground for the output voltage of the SW pin...

So when the SW pin is output and has not yet been connected to C5 and L1, where is the ground?

shaorc

maychang published on 2019-3-26 16:16 [2] If this chip is used, is the system ground GND in Figure 1 the common ground on the PCB board? It refers to the common end of VIN and VOUT.

Oh, it is the common end of them. If you don’t know where the “ground” of the object under test is on the PCB board, but want to use an oscilloscope to measure the voltage, where should the ground clip be loaded? If it is a single discrete device, can you use an isolated oscilloscope and add it to both ends of the object for measurement? If it is an integrated device or module, that is, only one end has a pin extending out, do you have to find the corresponding ground?

maychang

shaorc posted on 2019-3-26 17:22 So where is the ground when the SW pin is output and not connected to C5 and L1?

"So where is the ground when the SW pin is output and not connected to C5 and L1?" The SW pin is the connection point between the two MOS tubes inside the chip. The source of one of the two MOS tubes is also connected to the GND pin.

maychang

[quote]shaorc posted on 2019-3-26 17:33 Oh, it is the common terminal of the two. If you don't know where the "ground" of the object under test is on the PCB board, but you want to use an oscilloscope to measure the voltage, where should the ground clip be loaded? The oscilloscope ground clip must be connected to the reference point of the voltage to be measured. The source of the upper tube of the half-bridge circuit is not grounded, so the oscilloscope cannot generally directly measure the voltage waveform between the drain and source of the upper tube of the half-bridge.

maychang

shaorc posted on 2019-3-26 17:33 Oh, it is the common terminal of the two. If you don't know where the "ground" of the object under test is on the PCB board, but you want to use an oscilloscope to measure the voltage ground clip...

If it is a single discrete device, can you use an isolated oscilloscope to measure it at both ends of the object? Of course you can use an isolated probe to measure it.

maychang

shaorc posted on 2019-3-26 17:33 Oh, it is the common terminal of the two. If you don't know where the "ground" of the object under test is on the PCB board, but you want to use an oscilloscope to measure the voltage ground clip...

If it is an integrated device or module, that is, only one end has a pin extending out, do you have to find the corresponding ground? Integrated circuits generally always have a GND pin, or Vss (negative power supply pin).

shaorc

maychang posted on 2019-3-26 17:47 If it is a single discrete device, can I use an isolated oscilloscope to measure it at both ends of the object? Of course, I can measure it with an isolated probe.

Shouldn't the isolated probe mentioned here be an isolated oscilloscope? Aren't probes only divided into high voltage and low voltage? Oscilloscopes are divided into isolated and non-isolated ones?

395qav

1. System gnd should be connected to the gnd (0V) of your input source. 2. If the "common ground on the PCB" is 0V, connect them together. 3. Use R1/R2 voltage divider to determine the output voltage. There is a table in the manual (Rt also has one). Isolation probe - separates the oscilloscope and the object being measured (the two ends of the probe are different from those of ordinary probes) Similar to the differential input method
   

shaorc

395qav posted on 2019-3-27 00:36 1. System gnd should be connected to the gnd (0V) of your input source 2. If the "common ground on the PCB board" is 0V, then connect them together 3. Benefit...

Thank you, isolation probe and isolation oscilloscope are two different concepts. Because when using high voltage, use high voltage probe and isolation oscilloscope, so I don't know if the probe is isolated.

shaorc

maychang posted on 2019-3-26 17:46 If you don't know where the "ground" of the object being measured is on the PCB board, but you want to use an oscilloscope to measure the voltage, where should the ground clip be loaded? Oscilloscope ground...

By the way, in this case, [1] Can't you use an isolated probe or an isolated oscilloscope to directly add it to the DS poles of the upper tube to measure the voltage? [2] The source of the upper tube of the half-bridge circuit is not grounded, so the oscilloscope generally cannot directly measure the voltage waveform between the drain and source of the half-bridge upper tube. The ground of the upper tube (that is, the source is floating) is how to measure the DS voltage waveform?

maychang

shaorc posted on 2019-3-27 11:28 By the way, in this case, [1] Can't we directly use an isolated probe or an isolated oscilloscope to measure the voltage directly across the DS pole of the upper tube...

[1] Can't we directly use an isolated probe or an isolated oscilloscope to measure the voltage directly across the DS pole of the upper tube? You can use an isolated probe to measure the voltage between the DS poles of the half-bridge upper tube.

maychang

shaorc posted on 2019-3-27 11:28 By the way, in this case, [1] Can't we use an isolated probe or an isolated oscilloscope to directly add voltage to the DS pole of the upper tube to measure the voltage...

[2] The source of the upper tube of the half-bridge circuit is not grounded, so the oscilloscope generally cannot directly measure the voltage waveform between the drain and source of the half-bridge upper tube. The ground of the upper tube (that is, the source is floating) is how to measure the DS voltage waveform? Some old-fashioned oscilloscopes without isolated probes, if they are dual-channel and can measure the difference between the two channel voltages, can use the probes of the two channels to simultaneously measure the drain voltage and source voltage of the half-bridge upper tube, and read the difference between the two channel voltages on the oscilloscope. If it does not have this function, you can only measure the drain voltage and source voltage separately at the same time, read them from the waveform, and then calculate the voltage difference between the two points yourself. If it is a single-power-off oscilloscope, you can only measure twice and calculate the difference between the two readings yourself.

shaorc

maychang published on 2019-3-27 14:29 [2] The source of the upper tube of the half-bridge circuit is not grounded, so the oscilloscope cannot directly measure the voltage waveform between the drain and source of the upper tube of the half-bridge. The upper tube...

[1] Some old-fashioned oscilloscopes do not have isolated probes. Since there is no isolated probe, does it require the oscilloscope to be isolated? [2] If it is a dual-channel oscilloscope and the difference between the two channel voltages can be measured, then the two-channel probes can be used to measure the drain voltage and source voltage of the upper tube of the half-bridge at the same time, and the difference between the two channel voltages can be read on the oscilloscope. The two probes measure the drain and source voltages of the upper tube of the half-bridge respectively, so where should the ground clips of these two probes be added? Is it directly connected to the ground of the lower tube (S pole of the lower tube)?

maychang

shaorc posted on 2019-3-27 15:54 [1] Some old-fashioned oscilloscopes do not have isolated probes. Since there is no isolated probe, does the oscilloscope need to be isolated? [2] If it is...

Since there is no isolated probe, does the oscilloscope need to be isolated? This is mainly because the high-power switching power supply uses the DC directly obtained from the AC mains rectifier and filter as the power supply, so the ground wire of the high-power switching power supply (the negative end of the DC power supply) is connected to the AC mains. The shell of the oscilloscope is connected to the earth, and the ground wire clamp of the oscilloscope probe is not allowed to be clamped to any place connected to the AC mains, of course, including any part of the switching power supply. If you need to clamp the probe ground wire clip to the ground wire of the switching power supply, the oscilloscope housing must be disconnected from the ground before the ground wire clip is attached. At this time, the oscilloscope housing is energized, so pay attention to personal safety!

maychang

shaorc posted on 2019-3-27 15:54 [1] Some old-fashioned oscilloscopes do not have isolated probes. Since there is no isolated probe, does it require the oscilloscope to be isolated? [2] If it is...

Two probes measure the drain and source voltages of the upper tube of the half-bridge respectively, so where are the ground clips of these two probes added? Directly connected to the ground of the lower tube (the S pole of the lower tube)? At this time, the oscilloscope housing must be disconnected from the ground, and the ground clips of the two probes are clamped to the ground wire of the switching power supply (the negative end after the AC mains is rectified and filtered).