Datasheet

K
PWM
v
g
v
o
v
c
+
-
d
X
LC
V
ref
K
FB
Tv(s)
K
EA
( )
( )
)S(XKSKKT
LCPWMEAFBSV
´´´=
TPS40200-EP
www.ti.com SGLS371A –JANUARY 2007–REVISED JANUARY 2013
Frequency Compensation
The four elements that determine the system overall response are discussed in the following paragraphs. The
gain of the error amplifier (K
EA
) is the first of three 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 may be expressed by the following expression that represents the system transfer function
(see Figure 37).
(19)
Where:
K
FB
= Output voltage setting divider
K
EA
= Error amplifier feedback
K
PWM
= Modulator gain
X
LC
= Filter transfer function
Figure 37. Control Loop
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