參數(shù)資料
型號(hào): LTC3536EMSE#PBF
廠商: LINEAR TECHNOLOGY CORP
元件分類(lèi): 穩(wěn)壓器
英文描述: 4 A SWITCHING REGULATOR, 1440 kHz SWITCHING FREQ-MAX, PDSO12
封裝: LEAD FREE, PLASTIC, MSOP-12
文件頁(yè)數(shù): 13/28頁(yè)
文件大?。?/td> 1088K
代理商: LTC3536EMSE#PBF
LTC3536
20
3536f
APPLICATIONS INFORMATION
occuratthesamefrequency,fZ,andbothhigherorderpoles
(fPOLE2 and fPOLE3) occur at the common frequency, fP .
This is a good starting point for determining the compen-
sation network. However the Bode plot for the complete
loop should be checked overall operating conditions and
for variations in components values to ensure that suf-
ficient phase margin and gain margin exists in all cases.
A reasonable choice is to pick the frequency of the poles,
fP, to be about 50 times higher than the frequency of the
zeros, fZ, which provides a peak phase boost of approxi-
mately
ΦMAX = 60° as was assumed previously. Next, the
phase boost must be centered so that the peak phase
occurs at the target crossover frequency. The frequency
of the maximum phase boost, fC, is the geometric mean
of the pole and zero:
fC = fP fZ = 50 fZ2 = 7 fZ
Therefore, in order to center the phase boost given a factor
of 50 separation between the pole and zero frequencies,
the zeros should be located at one-seventh of the cross-
over frequency and the poles should be located at seven
times the crossover frequency as given by the following
equations:
fZ =
1
7
fC =
1
7
37.8kHz
(
)=5.4kHz
fP = 7 fC = 7 37.8kHz
(
)=264.6kHz
Thisplacementofthepolesandzeroswillyieldapeakphase
boost of 60° that is centered at the crossover frequency,
fC. Next, in order to produce the desired target crossover
frequency, the gain of the compensation network at the
point of maximum phase boost, GCENTER, must be set to
+2dB. The gain of the compensated error amplifier at the
point of maximum phase gain is given by:
GCENTER =10log
2
πfP
2
πfZ
( )3 RTOPCFB
(
)2
Assuming a multiple of 50 separation between the pole
frequencies and zero frequencies this can be simplified
to the following expression:
GCENTER = 20log
50
2
πfC
(
) RTOPCFB
(
)
The first step in defining the compensation component
values is to pick a value for RTOP that provides an accept-
ably low quiescent current through the resistor divider.
A value of RTOP = 845k is a reasonable choice. Next, the
value of CFB can be found:
GCENTER = 2dB
CFB
50
2
π 37.8kHz
(
)845k10
2dB
20
= 198pF ≈180pF
The compensation poles can be set at 264.6kHz and the
zeros at 5.4kHz by using the expressions for the pole and
zero frequencies given in the previous section. Setting the
frequency of the first zero, fZERO1, to 5.4kHz results in the
following value for RFB:
RFB =
1
2
π 180pF
(
)5.4kHz
= 163k ≈162k
This leaves the free parameter, CPOLE, to set the frequency
fPOLE1tothecommonpolefrequencyof264.6kHzasgiven:
CPOLE =
1
2
π 162k
(
)264.6kHz
= 3.71pF≈ 3.9pF
Next, CFF can be chosen to set the second zero, fZERO2, to
the common zero frequency of 5.4kHz.
CFF =
1
2
π 845k
(
)5.4kHz
= 34.9pF≈ 33pF
Finally, the resistor value RFF can be chosen to place the
second pole at 264.6kHz:
RFF =
1
2
π 33pF
(
)264.6kHz
= 18.2k
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