Can anyone help me analyze this circuit?

SXYDDD

Can anyone help me analyze this circuit? [Copy link]

 

It looks like a constant current source. Is there any expert who can analyze it?

maychang

"It looks like a constant current source."

Not really.

How did you know it was a constant current source?

maychang

The collector of Q702 is similar to a constant current source, but the collector of this tube is connected to the power supply through L780 (is it an inductor?). Looking at the collector of Q702 from the left end of R781, it is difficult to say that it is a constant current source.

SXYDDD

maychang posted on 2019-12-3 19:22 The collector of Q702 is similar to a constant current source, but the collector of the tube is connected to the power supply through L780 (is it an inductor?). Looking at the collector of Q702 from the left end of R781, it is hard to say...

It looks like a constant current source, but I can't figure it out. Actually, I mainly want to know what the voltage of vmcu_buf is. You can check it in combination with the English above. Thank you for your reply.

蓝的天

vmcu_buf is equal to the VMCU on the left.

hungchou

Voltage Following

VMCU_BUF=VMCU

5V_SW controls whether VMCU_BUF power is established

maychang

SXYDDD posted on 2019-12-3 21:56 It looks like a constant current source, but I can't figure it out. In fact, I mainly want to know what the voltage of vmcu_buf is. You can combine...

"Actually, I mainly want to know what the voltage of vmcu_buf is."

If Q701B is turned on, VMCU is a DC voltage, then the voltage of VMCU-BUF is equal to VMCU.

maychang

SXYDDD posted on 2019-12-3 21:56 It looks like a constant current source, but I can't figure it out. In fact, I mainly want to know what the voltage of vmcu_buf is. You can combine...

"You can look at it in conjunction with the English above."

The English words in large font above say “VMCU voltage mirror”. This is consistent with the voltage at VMCU-BUF being equal to the voltage at VMCU.

SXYDDD

hungchou posted on 2019-12-4 06:08 Voltage follows VMCU_BUF=VMCU 5V_SW controls whether VMCU_BUF power is established

gmchen

In fact, it is clearly written in the picture, voltage buffer.

gmchen

vmcu is the input and vmcu_buf is the output.

蓝的天

When 5V_SW arrives, Q701B is turned on, and the VMCU voltage is sent to the 3-pin of U706 through Q701B and R772. Because of the existence of C721, the voltage of the 3-pin of U706 gradually increases (actually very quickly) and finally equals to VMCU; at the same time, the 1-pin of U706 generates a gradually increasing voltage (after the 3-pin of U706 equals to VMCU, the voltage of the 1-pin of U706 continues to rise);

The voltage of pin 1 of U706 acts on the base of Q702, and the emitter of Q702 outputs current, which charges C725. Voltage starts to be generated at both ends of C725 and gradually increases. At the same time, part of the current of the emitter flows through R774, generating voltage at both ends of R774. The voltage at both ends of R774 is equal to the voltage at both ends of C725, and they increase at the same time. The voltage at both ends of C725 and R774 is VMCU_BUF, and the VMCU_BUF voltage is sent to pin 4 of U706 at the same time.

At the beginning of charging, the voltage across C725 is lower than VMCU, and then gradually increases; at the same time, the voltage of U706 4pin also increases;

When the voltage across C725 (VMCU_BUF) rises to VMCU, the 4pin voltage of U706 is also equal to VMCU, so the 1pin voltage of U706 no longer increases, and Q702 enters a stable working state. At this time, the emitter current no longer changes, the current flowing through R774 no longer changes, and the voltage across R774 is stable and no longer changes, that is, VMCU _BUF is established and equal to the VMCU voltage value.

The key to the analysis is to understand the working principle of the operational amplifier U706. When the voltage of pin 4 of U706 is equal to the voltage of pin 3, the voltage across C725 (VMCU_BUF) is equal to VMCU and will not change.

埋土书生

It is a voltage follower. The op amp will short-circuit and the transistor will only amplify the current. To put it bluntly, it is a wide current amplifier to improve the driving ability.

Ardinantisy

Blue Sky published on 2019-12-4 20:31 When 5V_SW arrives, Q701B is turned on, and the VMCU voltage passes through Q701B and R772 and is sent to the 3pin of U706. Because C ...

The analysis is very detailed, I learned a lot, thank you

蓝的天

Ardinantisy posted on 2020-1-7 10:54 The analysis is very detailed, I learned a lot, thank you big brother

You're welcome! Everyone here helps and learns from each other, it's very nice!