L6258EX datasheet(19/32 Pages) STMICROELECTRONICS

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L6258EX

PWM current control loop

19/32

Before analysing the error amplifier block and the sense transconductance block, we have to

do this consideration:

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 amplifier and load blocks;

Ax|dB = (29,5) + (-31.4) = -1.9dB

The BODE analysis of the transfer function of Ax is:

Figure 7.

Ax bode plot

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.

3.4

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

voltage the output of the same, the voltage gain is given by:

Vsense

⋅

Vsense

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L6258EX bảng dữ liệu(PDF) 19 Page - STMicroelectronics