Datasheet

( )
( )
)S(XKSKKT
LCPWMEAFBSV
´´´=
K
PWM
v
g
v
o
v
c
+
-
d
X
LC
V
ref
K
FB
Tv(s)
K
EA
TPS40200
www.ti.com SLUS659F –FEBRUARY 2006–REVISED MARCH 2012
Frequency Compensation
The four elements that determine the system overall response are discussed below The gain of the error
amplifier (K
EA
) is the first of there elements. Its output develops a control voltage which is the input to the PWM.
The TPS40200 has a unique modulator that scales the peak to peak amplitude of the PWM ramp to be 0.1 times
the value of the input voltage. Since modulator gain is given by V
IN
divided by V
RAMP
, the modulator gain is 10
and is constant at 10 (20 dB) over the entire specified input voltage range.
The last two elements that affect system gain are the transfer characteristic of the output LC filter and the
feedback network from the output to the input to the error amplifier.
These four elements maybe expressed by the following expression that represents the system transfer function
as shown in Figure 36.
where
• K
FB
is the output voltage setting divider
• K
EA
is the error amplifier feedback
• K
PWM
is the modulator gain
• X
LC
is the filter transfer function
(17)
Figure 36. Control Loop
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