TPS40200-Q1
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SLUS739D –SEPTEMBER 2006–REVISED JULY 2011
Voltage Setting and Modulator Gain
Because the input current to the error amplifier is negligible, the feedback impedance can be selected over a
wide range. Knowing that the reference voltage is 708 mV, pick a convenient value for R1 and then calculate the
value of R2 from the following formula:
æ
ö
÷
÷
ø
R2
R1
ç
VOUT = 0.708 1+
ç
è
(3)
Vg
L
VOUT
d
KPWM
Vc
Cout
Rload
R2
+
Vref
R1
Figure 34. System Gain Elements
The error amplifier has a dc open-loop gain of at least 60 dB with a minimum of a 1.5-MHz gain bandwidth
product, which gives the user flexibility with respect to the type of feedback compensation used for the particular
application. The gain selected by the user at the crossover frequency is set to provide an overall unity gain for
the system. The crossover frequency should be selected so that the error amplifier's open-loop gain is high with
respect to the required closed-loop gain. This ensures that the amplifier's response is determined by the passive
feedback elements.
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