E-L6258EP STMICROELECTRONICS [STMicroelectronics], E-L6258EP Datasheet - Page 13

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E-L6258EP

Manufacturer Part Number
E-L6258EP
Description
PWM CONTROLLED - HIGH CURRENT DMOS UNIVERSAL MOTOR DRIVER
Manufacturer
STMICROELECTRONICS [STMicroelectronics]
Datasheet

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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.
4.4 Error Amplifier and Sense Amplifier
As explained before the gain of these two blocks is :
Bx
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 :
because ib = icwe have:
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 Ax
Figure 8.
The BODE diagram shows together the error amplifier open loop transfer function, the Ax function and the re-
sultant total Aloop given by the following equation :
Aloop
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 amplifier 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, using 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.
dB
= ACerr
dB
= AxdB + Bx
dB
+ ACsense
dB
dB
Verr_out = -(ic · Zc) so ic = -(Verr_out ·
ib
Vsense ·
=
Bx
Vsense Gs
=
------- -
Rb
1
Verr_out
------------------------
Vsense
= -(Verr_out ·
=
Vsense
=
------- -
Rb
Zc
------ -
Zc
1
------- -
Rb
)
1
------ -
Zc
1
)
dB
and Bx
L6258EP
dB
13/22
is :

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