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AN1518 数据表(PDF) 13 Page - STMicroelectronics

部件名 AN1518
功能描述  The L5973D is a step-down monolithic power switching regulator capable
PDF  30 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

AN1518 数据表(HTML) 13 Page - STMicroelectronics

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AN1518
PWM comparator
13/30
where RLOAD is defined as the ratio between VOUT and IOUT.
If RLOAD>>ESR, the previous expression of ALC can be simplified and becomes:
Equation 7
The zero of this transfer function is given by:
Equation 8
F0 is the zero introduced by the ESR of the output capacitor and it is very important to
increase the phase margin of the loop.
The poles of the transfer function can be calculated through the following expression:
Equation 9
In the denominator of ALC the typical second order system equation can be recognized:
Equation 10
If the damping coefficient
δ is very close to zero, the roots of the equation become a double
root whose value is
ωn.
Similarly for ALC the poles can usually be defined as a double pole whose value is:
Equation 11
8
PWM comparator
The PWM gain is given by the following formula:
Equation 12
where VOSCMAX is the maximum value of a sawtooth waveform and VOSCMIN is the
minimum value. A voltage feed forward is implemented to ensure a constant GPWM. This is
obtained by generating a sawtooth waveform directly proportional to the input voltage VCC.
Equation 13
A
LC s
()
1
ESR
C
OUT
•
s
•
+
LC
OUT
•
s
2
ESR
C
OUT
•
s1
+
•
+
•
------------------------------------------------------------------------------------------------
=
F
O
1
2
π
•
ESR
•
C
OUT
•
-----------------------------------------------------
=
F
PLC1 2
,
ESR
C
OUT
ESR
C
OUT
•
()
2
4L
•
C
OUT
•
–
±
•
–
2L
•
C
OUT
•
--------------------------------------------------------------------------------------------------------------------------------------------
=
s
2
2
δ
•ω
n
•
s
ω
2
n
+
•
+
F
PLC
1
2
π
•
LC
OUT
•
•
------------------------------------------------
=
G
PWM s
()
V
cc
V
OSCMAX
V
OSCMIN
–
()
-------------------------------------------------------------
=
V
OSCMAX
V
OSCMIN
–
KV
CC
•
=



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