LM3477_14 Datasheet, PDF (22/34 Pages) Texas Instruments – High Efficiency High-Side N-Channel Controller for Switching Regulator

LM3477
SNVS141J – MAY 2004 – REVISED JULY 2004
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A 0.1µF or 1µF ceramic bypass capacitor is also recommended on the V_IN pin (pin 8) of the IC. This capacitor
must be connected very close to pin 8.
Compensation
VREF ₊
-
VC
Fc(s)
+
VO
-
Compensator
Fp(s) x Fh(s)
Power Stage
Loop (T)
H
Feedback
Figure 15. Control Block Diagram of a Current Mode Controlled Buck Converter
The LM3477/A is a current mode controller, therefore the control block diagram representation involves 2
feedback loops (see Figure 15). The inner feedback loop derives its feedback from the sensed inductor current,
while the outer loop monitors the output voltage. This section will not give a rigorous analysis of current mode
control, but rather a simplified but accurate method to determine the compensation network. The first part reveals
the results of the model, giving expressions for solving for component values in the compensation network.
The compensation network is designed around the power components, or the power stage. An isolated
schematic of the error amplifier and the various compensation components is shown in Figure 16. The error
amplifier in conjunction with the compensation network makes up the compensator block in Figure 15. The
purpose of the compensator block is to stabilize the control loop and achieve high performance in terms of the
transient response, audio susceptibility and output impedance.
VOUT
CFF
RFB1
LM3477
FB
RFB2
-_GM
+
RGM
BG
VC
RC
CC2
CC1
Figure 16. LM3477 Compensation Components
Figure 17 shows a bode plot of a typical current mode buck regulator. It is an estimate of the actual plot using the
asymptotic approach. The three plots shown are of the compensator, powerstage, and loop gain, which is the
product of the power stage, compensator, and feedback gain. The loop gain determines both static and dynamic
performance of the converter. The power stage response is fixed by the selection of the power components,
therefore the compensator is designed around the powerstage response to achieve a good loop response.
Specifically, the compensator is added to increase low frequency magnitude, extend the 0dB frequency
(crossover frequency), and improve the phase characteristic.
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