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L6258 Fiches technique(PDF) 11 Page - STMicroelectronics |
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L6258 Fiches technique(HTML) 11 Page - STMicroelectronics |
11 / 18 page AloopdB = AxdB + BxdB Ax|dB = ACpw|dB + ACload|dB and Bx|dB = ACerr|dB + ACsense|dB this means that Ax|dB is the sum of the power am- plifier and load blocks; Ax|dB = (29,5) + (-31.4) = -1.9dB The BODE analysis of the transfer function of Ax is: The Bode plot of the Ax|dB function shows a DC gain of -1.9dB and a pole at 163Hz. It is clear now that (because of the negative gain of the Ax function), Bx function must have an high DC gain in order to increment the total open loop gain increasing the bandwidth too. Error Amplifier and Sense Amplifier As explained before the gain of these two blocks is : BxdB = ACerrdB + ACsensedB Being the voltage across the sense resistor the input of the Bx block and the error amplifier volt- age the output of the same, the voltage gain is given by : ib = Vsense ⋅ Gs = Vsense ⋅ 1 Rb Verr_out = -(ic ⋅ Zc) so ic = -(Verr_out ⋅ 1 Zc ) because ib = ic we have: Vsense ⋅ 1 Rb = -(Verr_out ⋅ 1 Zc ) Bx = − Verr_out Vsense = − Zc Rb In the case of no external RC network is used to compensate the error amplifier, the typical open loop transfer function of the error plus the sense amplifier is something with a gain around 80dB and a unity gain bandwidth at 400kHz. In this case the situation of the total transfer function Aloop, given by the sum of the AxdB and BxdB is : The BODE diagram shows together the error am- plifier open loop transfer function, the Ax function and the resultant total Aloop given by the follow- ing equation : AloopdB = AxdB + BxdB The total Aloop has an high DC gain of 78.1dB with a bandwidth of 15KHz, but the problem in this case is the stability of the system; in fact the total Aloop cross the zero dB axis with a slope of -40dB/decade. Now it is necessary to compensate the error am- plifier in order to obtain a total Aloop with an high DC gain and a large bandwidth. Aloop must have enough phase margin to guarantee the stability of the system. A method to reach the stability of the system, us- ing the RC network showed in the block diagram, is to cancel the load pole with the zero given by the compensation of the error amplifier. The transfer function of the Bx block with the compensation on the error amplifier is : Bx = − Zc Rb = − Rc − j 1 2 π ⋅ f ⋅ Cc Rb In this case the Bx block has a DC gain equal to the open loop and equal to zero at a frequency given by the following formula : Fzero = 1 2 π ⋅ Rc ⋅ Cc L6258 11/18 |
Numéro de pièce similaire - L6258 |
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Description similaire - L6258 |
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