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參數(shù)資料
| 型號(hào)： | LM2633MTDX/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 275 kHz SWITCHING FREQ-MAX, PDSO48 |
| 封裝： | TSSOP-48 |
| 文件頁數(shù)： | 25/40頁 |
| 文件大?。?/td> | 1350K |
| 代理商： | LM2633MTDX/NOPB |
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Control Loop Design (Continued)
(18)
Where
D’ = 1D
(19)
(20)
S
e =Vm f
(21)
(22)
R
i =Rds ρ
(23)
S
e is the correction ramp slope, Sn is the on-time slope of the
current sense waveform, V
m is the peak-peak value of the
correction ramp, f is the PWM frequency, V
in is input voltage,
R
i is the transfer resistance from inductor current to ramp
voltage, R
ds is the top FET on-resistance and ρ is the gain of
the current sense amplifier.
The coefficient of the first current source is:
(24)
and the coefficient of the second current source is:
(25)
The output capacitance of the PWM switch is:
(26)
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:
(27)
Where
α =LC
sC(R+Re)
(28)
β =g
oLC(R + Re)+Cs(CRRe + L)
(29)
γ = C(R+R
e)+go(CRRe +L)+CsR
(30)
δ = 1+g
oR
(31)
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
e, the power stage can
be reduced to the one shown in
The transfer function can be re-written as:
(32)
Where
(33)
(34)
All the R
e terms are omitted in the denominator because
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:
(35)
Where
(36)
and
(37)
where
(38)
and
200008D5
FIGURE 8. Simplified Power Stage at High Frequencies
LM2633
www.national.com
31
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