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LM3421 Datasheet(PDF) 18 Page - Texas Instruments

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Part # LM3421
Description  N-Channel Controllers for Constant Current LED Drivers
PDF  65 Pages
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3421 Datasheet(HTML) 18 Page - Texas Instruments

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VCMP
0
tVCC
tCMP
tCO
t
0.9V
FREQUENCY (Hz)
80
60
40
20
0
-20
-40
-60
-80
90
45
0
-45
-90
-135
-180
-225
-270
1e-1
1e1
1e3
1e5
1e7
GAIN
60°Phase Margin
PHASE
öP2
öP3
öP1
öZ1
x
=
0
U
T
T
-
1
¸
¸
¹
·
¨
¨
©
§
s
Z
1
Z
x
x
+
1
¸
¸
¹
·
¨
¨
©
§
s
Z
3
P
+
1
¸
¸
¹
·
¨
¨
©
§
s
Z
2
P
+
1
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¨
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s
Z
1
P
1
3
P =
Z
FS
FS
C
R
x
1
2
P =
Z
CMP
6
C
e
5
x
:
LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011
www.ti.com
(19)
It may also be necessary to add one final pole at least one decade above the crossover frequency to attenuate
switching noise and, in some cases, provide better gain margin. This pole can be placed across RSNS to filter the
ESL of the sense resistor at the same time. Figure 23 shows how the compensation is physically implemented in
the system.
The high frequency pole (
ωP3) can be calculated:
(20)
The total system transfer function becomes:
(21)
The resulting compensated loop gain frequency response shown in Figure 24 indicates that the system has
adequate phase margin (above 45°) if the dominant compensation pole is placed low enough, ensuring stability:
Figure 24. Compensated Loop Gain Frequency Response
Figure 25. Start-Up Waveforms
18
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Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1



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