L6258E(2004) View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
L6258E Datasheet PDF : 24 Pages
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L6258E
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 :
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 :
ib
=
Vsense ⋅ Gs
=
V
se
n
se
⋅
---1----
Rb
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
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 :
Figure 9.
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 :
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 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.
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