L6258E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6258E' PDF : 20 Pages View PDF
L6258E
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 ampli-
fier and load blocks;
Ax|dB = (29,5) + (-31.4) = -1.9dB
The BODE analysis of the transfer function of Ax is:
In the case of no external RC network is used to com-
pensate 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 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 in-
creasing 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 voltage the out-
put of the same, the voltage gain is given by :
ib = Vsense ⋅ Gs = Vse nse ⋅ R---1--b--
Verr_out
= -(ic · Zc) so
ic = -(Verr_out
·
--1----
Zc
)
because ib = icwe have:
Vsense ·
---1----
Rb
= -(Verr_out ·
--1----
Zc
)
Bx = –V-----e----r--r---_---o---u----t = –-Z----c--
Vsense Rb
The BODE diagram shows together the error amplifi-
er open loop transfer function, the Ax function and the
resultant total Aloop given by the following 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 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 sys-
tem.
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 com-
pensation of the error amplifier.
The transfer function of the Bx block with the com-
pensation on the error amplifier is :
Bx = –R-Z----cb-- = –R-----c-----–----j--2--R------π----b------⋅-----1-f------⋅------C----------c--
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 = 2----π-----⋅---R---1--c----⋅---C-----c--
13/20
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