How to use LM337 to generate negative voltage

lym1210

How to use LM337 to generate negative voltage [Copy link]

 

Recently I need to generate a -5V voltage, and I chose the LM337 chip. According to the picture in the manual below, I would like to ask you seniors, is the VIn in the picture a positive voltage input (for example, connected to the positive pole of 20V)? Will the polarity capacitor in the picture burn out? Another question I want to ask is that on this board I also use LM2596 to generate positive voltage. Can these two chips use the same input voltage?

maychang

"I would like to ask you seniors if the VIn in the figure is a positive voltage input." LM337 must be input with a negative voltage. If a positive voltage is input, the LM337 will be damaged quickly.

sysjtlwx

1) LM337 needs to be connected to negative voltage, which should be explained in the manual. 2) Pay attention to the direction of the polarity capacitor in the circuit diagram you screenshot, the GND side is the positive pole 3) If the input end is a negative voltage of 20V and the output is -5V, the voltage difference is relatively large. If the power is large, it is not recommended. 337 is a linear voltage regulator with low power and large loss. In this case, the chip is easily damaged after long-term operation. 4) As mentioned in 1), the input of 337 needs negative voltage.

lym1210

sysjtlwx posted on 2019-3-9 16:27 1) LM337 needs to be connected to negative voltage, which should be explained in the manual. 2) Pay attention to the direction of the polarity capacitor in the circuit diagram you screenshot, the GND side is the positive pole 3) ...

What I mean is to connect the positive pole of 20V to the ground of the schematic diagram, and the negative pole of 20V to VIn, but will this burn out the capacitor?

dcexpert

lym1210 posted on 2019-3-9 16:49 What I mean is to connect the positive pole of 20V to the ground of the schematic diagram, and the negative pole of 20V to VIn, but will this burn out the capacitor?

It depends on the rated voltage of the capacitor.

lym1210

dcexpert posted on 2019-3-9 16:53 You need to look at the rated voltage of the capacitor

Is a 50v100uF electrolytic capacitor okay, but I don't know what its reverse withstand voltage is

maychang

lym1210 posted on 2019-3-9 17:01 Is a 50v100uF electrolytic capacitor okay, but I don't know what its reverse withstand voltage is

Electrolytic capacitors are not allowed to withstand reverse voltage. Applying reverse voltage to an electrolytic capacitor will cause damage to the electrolytic capacitor or even explosion.

dcexpert

lym1210 posted on 2019-3-9 17:01 Is a 50v100uF electrolytic capacitor OK, but I don't know what its reverse withstand voltage is

Electrolytic capacitors cannot be reversed. If the negative voltage requirement is not high, it is more convenient to use a DC/DC than a three-terminal, or use a 7660 to convert from +5V to -5V.

sysjtlwx

lym1210 posted on 2019-3-9 16:49 I mean the positive pole of 20V is connected to the ground of the schematic diagram, and the negative pole of 20V is connected to VIn, but will this burn out the capacitor?

I don't know if I understand it correctly? As shown in the figure below, VI is connected to the GND of the actual voltage, and the ground in the circuit diagram is connected to the +20V of the actual circuit. 1) The polarity of the capacitor is as shown in the schematic diagram. It can be seen that the voltage of the positive pole of the capacitor is higher than that of the negative pole of the capacitor. As long as the withstand voltage value is correct, there is no problem in use. For example, a capacitor with a withstand voltage of 50V can be selected for 20V. 2) If it is true as I understand, then the reference ground plane of the 337 power supply part becomes the actual +20V, and the reference ground plane of the LM2596 mentioned in the article is still GND. In this way, the two "GNDs" are different, so they cannot share the same ground during use. If they are in the same solution, there will be problems.

lym1210

sysjtlwx posted on 2019-3-9 23:36 I don't know if I understand it correctly? As shown in the figure below, VI is connected to the GND of the actual voltage, and the ground in the circuit diagram is connected to the +20V of the actual circuit. 1) The polarity of the capacitor is as shown in the principle...

If you say so, are all the grounds in this schematic connected to 20V? Or is it just the input ground that is connected to 20V, and the output and resistor grounds are still connected to the actual ground?

sysjtlwx

lym1210 posted on 2019-3-10 13:07 If what you said is true, are all the grounds in this schematic connected to 20V? Or is it just the input ground that is connected to 20V, and the output and resistor grounds are still connected to the actual...

For the GND of the LM337 part of your entire circuit and its rear-end load electrical appliances, they need to be connected to the same GND network to achieve a common ground. However, if there are other power supplies in the entire circuit (such as the 2596 you mentioned), the reference ground planes used by these two parts of the power supply are different, so these two parts of the power supply and the loads brought by each power supply cannot be connected together.

lym1210

sysjtlwx posted on 2019-3-10 16:46 For the GND of the LM337 part of your entire circuit and its subsequent load electrical appliances, they need to be connected to the same GND network to achieve common ground. But if you...

If the LM337 module shares the ground, then the ground of this module is connected to my 20V, but why is the calculated output voltage VOut=-1.25*((R2/R1)+1), doesn't this formula assume that the ground is 0V? And I would like to ask if you know of a chip that has a positive input voltage and a negative output voltage?

lym1210

sysjtlwx posted on 2019-3-10 16:46 For your entire circuit, the GND of the LM337 part and its rear-end load appliances need to be connected to the same GND network to achieve a common ground. But if you...

Hello, my two LM337 and LM2596 power modules are independent, but the input power is the same 20V. According to what you said, can I connect the ground of LM337 to +20 and the input to -20, and the input of LM2596 to +20 and the ground to -20?

chunyang

lym1210 posted on 2019-3-10 17:50 Hello, my two LM337 and LM2596 power modules are independent, but the input power is the same 20V. According to what you said, can I put L...

Different loads must be completely independent. For independent loads, the positive and negative of the power supply are artificially defined. In fact, there is no such thing as positive voltage and negative voltage. Different reference points have different positive and negative voltages. However, once the loads are not independent of each other, because the reference point must be unique, the relationship between positive and negative voltages has been confirmed, and different reference bases cannot be used at the same time.

sysjtlwx

lym1210 posted on 2019-3-10 17:50 Hello senior, my two LM337 and LM2596 power modules are independent, but the input power is the same 20V. According to what you said, can I put L...

If the two power supplies work completely independently and there is no need to connect or communicate, then it is possible, otherwise it is not possible

lym1210

chunyang posted on 2019-3-10 19:43 Different loads must be completely independent. For independent loads, the positive and negative of the power supply are artificially defined. In fact, there is no such thing as positive voltage or negative voltage. The reference point is not...

I would like to ask if you know of a chip that can output a negative voltage with a positive input voltage? Or can multiple chips be used to generate negative voltage?

zhuyebb

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chunyang

lym1210 posted on 2019-3-11 08:32 I would like to ask if you know of a chip that can output a negative voltage with a positive input voltage? Or can multiple chips be used to generate negative voltage?

There are dedicated negative voltage output types and models that support both positive and negative voltages when using switching power supply chips. The only difference is that the positive and negative voltage circuits are different. You can go to the official website of TI or LT to select the model according to your needs.

xiaosong06

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