lm2633mtd National Semiconductor Corporation, lm2633mtd Datasheet - Page 31

• Manufacturer Part Number: lm2633mtd
• Description: Advanced Two-phase Synchronous Triple Regulator Controller For Notebook Cpus
• Manufacturer: National Semiconductor Corporation

Control Loop Design
Where
S
current sense waveform, V
correction ramp, f is the PWM frequency, V
R
voltage, R
the current sense amplifier.
The coefficient of the first current source is:
and the coefficient of the second current source is:
The output capacitance of the PWM switch is:
The DC resistance of the FET switches and of the inductor is
not included here because its value is usually much smaller
than the load resistance.
Control-Output Transfer Function
The control (COMPx pin) voltage in a peak-current mode
scheme such as that of the LM2633 is the current command.
At any instant that voltage determines the level of the induc-
tor current (from an average-model point of view). The
control-output transfer function is a description of the
small-signal behavior of the power stage and is obtained by
letting the small signal component of the input voltage be
zero. The expression for the control-output transfer function
is:
Where
e
i
is the correction ramp slope, S
is the transfer resistance from inductor current to ramp
ds
is the top FET on-resistance and
= g
o
LC(R + R
= LC
R
S
D’ = 1−D
i
e
= R
m
= V
s
e
C(R + R
is the peak-peak value of the
) + C
ds
m
n
•
• f
is the on-time slope of the
s
(CRR
e
(Continued)
)
e
in
+ L)
is input voltage,
is the gain of
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For a reasonable design, the output filter has large attenua-
tion at large complex frequencies (i.e. large s values). At s
values where 1/sC is smaller than R
be reduced to the one shown in Figure 8 .
The transfer function can be re-written as:
Where
All the R
their values are negligible compared to other terms.
Since the denominator of the control-output transfer function
is a third-order polynomial, and its coefficients are positive
real numbers, the transfer function either has one real pole
and two complex poles that are complex conjugates or has
three real poles. Thus it can be approximately written in the
following format:
Where
and
where
and
FIGURE 8. Simplified Power Stage at High Frequencies
e
terms are omitted in the denominator because
= C(R + R
e
) + g
= 1 + g
o
(CRR
o
R
e
e
+ L) + C
, the power stage can
200008D5
s
R
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