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Why is the OTA used in current-mode control for dc-dc

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jwfan

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I find that in most current mode controller, the OTA is used for the error amplifier instead of the OpAmp. What's the benefit of using OTA?

Thanks
 

suria3

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OTA provides high gain and high ouput impedance for constant current output. OTA is good in driving capacitive load wbereby OPAMP is good in driving resistive load as buffer is included at last stage of opamp.

Refer to this link......
 

jwfan

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Thank you so much. It's a good explaination of the difference between OTA and OpAmp.

But for the voltage-mode control of PWM control, All of the error amplifiers are built by OpAmp. When it come to Current-Mode control, all people use OTA. Is there particular consideration in this change?

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johnsmith101

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The compensation network (when using a Gm amplifier) is simplified and it only requires 1 pin. This allows you to build the resistor divider (feedback network) internal to the chip, so that the customer needs fewer external components. This in turn will require less board space for them, allowing them to make the form-factor of their product smaller.
 

jwfan

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Thank you, Johnsmith

Yes. I have searched for some papers about the current mode control. This control method can use the PI compensator, which means there are only capacitor load for the error amplifier. Then we can use the OTA instead of a OpAmp. Is it right?
 

johnsmith101

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Almost correct, you typically have an RC network from the output of the error amp to ground. Check out this MAXIM datasheet, it has a lot of useful information.

Pg. 16 shows a typical RC network (look at pins labelled CCSU, CCSD, CCM). The equations for choosing components can be seen on pg. 28 and 29.
 

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