LM3477MM National Semiconductor, LM3477MM Datasheet - Page 19

• Manufacturer Part Number: LM3477MM
• Description: Pulse Width Modulation (PWM) Controller IC
• Manufacturer: National Semiconductor

Specifications of LM3477MM

• Input Voltage Primary Min: 2.95V
• Mounting Type: Surface Mount
• Topology: Buck (Step Down)
• Control Mode: Current
• Duty Cycle Max: 93%
• Input Voltage Primary Max: 35V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Compensation
Figure 14 shows a bode plot of a typical current mode buck
regulator. It is an estimate of the actual plot using the as-
ymptotic approach. The three plots shown are of the com-
pensator, 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 selec-
tion 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 fre-
quency (crossover frequency), and improve the phase char-
acteristic.
Poles, Zeros and Important Measurements are Labeled
Power Stage Bode Plots for LM3477 Buck Circuits.
FIGURE 14. Typical Open Loop, Compensator, and
FIGURE 13. LM3477 Compensation Components
(Continued)
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There are several different types of compensation that can
be used to improve the frequency response of the control
loop. To determine which compensation scheme to use,
some information about the power stage is needed.
Use V
ing compensation components.
With the power stage known, a compensator can be de-
signed to achieve improved performance and stability. The
LM3477/A will typically require only a single resistor and
capacitor for compensation, but depending on the power
stage it could require three or four external components.
It is a good idea to check that Q is between 0.15 and 2, if it
was not already done when selecting the inductor. If Q is less
than 0.15 or greater than 2, skip to the SAMPLING POLE
QUALITY FACTOR section below before continuing with the
compensator design.
First, a target crossover frequency (f
be selected. The crossover frequency is the bandwidth of the
converter. A higher bandwidth generally corresponds to
faster response times and lower overshoots to load tran-
sients. However, the bandwidth should not be much higher
than 1/10 the switching frequency. The LM3477/A operates
with a 500kHz switching frequency, so it is recommended to
choose a crossover frequency between 10kHz - 50kHz.
The schematic of the LM3477/A compensator is shown in
Figure 13. The default design uses R
(type 2) compensator. The C
form an additional pole that is typically used to cancel out the
esr zero of the output capacitor. Finally, if extra phase margin
is needed, the Cff capacitor can be added (this does not help
at low output voltages, see below).
IN
= V
IN(MIN)
S
e
and R = R
= f
S
(V
SL
+ 50x10
C2
MIN
capacitor can be added to
(I
OUT(MAX)
c
−6
) for the loop gain must
c
and C
R
SL
)
) when calculat-
C1
to form a lag
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