TPS40200-EP_08 Datasheet, PDF (25/41 Pages) Texas Instruments – WIDE-INPUT-RANGE NONSYNCHRONOUS VOLTAGE-MODE CONTROLLER

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Frequency Compensation
TPS40200-EP
SGLS371 – JANUARY 2007
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 36).
( ) T_V(S) = K _FB ´ K _EA S ´ K _PWM ´ X_LC (S)
(19)
Where:
K_FB = Output voltage setting divider
K_EA = Error amplifier feedback
K_PWM = Modulator gain
X_LC = Filter transfer function
vg
+
V_ref
-
K_EA ^vc K_PWM ^d
X_LC
vo
Tv(s)
K_FB
Figure 36. Control Loop
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