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LM3421 Datasheet(PDF) 36 Page - Texas Instruments

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Part # LM3421
Description  N-Channel Controllers for Constant-Current LED Drivers
PDF  72 Pages
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3421 Datasheet(HTML) 36 Page - Texas Instruments

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FS
P3
1
C
10 u Z
max
3
P =
Z
(
) 10
,
1
Z
1
P
x
Z
Z
CMP
6
P2
1
C
5 10
Z
u
u
1
Z
1
P
)
,
min(
Z
Z
5
x
0
U
T
2
P
=
Z
=
0
U
T
=
SNS
CSH
R
R
500V
D
x
x
x
?
620V
D
x
?
(
)
LIM
LED
R
I
D
1
x
x
+
(
)
LIM
HSP
R
R
D
1
x
x
+
=
0
U
T
=
SNS
CSH
R
R
500V
D
x
x
x
?
310V
D
x
?
LIM
LED
R
I
x
LIM
HSP
R
R
2
x
x
SNS
620V
R
R
500V
=
x
x
CSH
LIM
LED
R
I
x
0
U
T
=
LIM
HSP
R
R
x
=
D
r
2
D
c
x
1
Z
Z
L1
D x
=
D
r
2
D
c
x
1
Z
Z
L1
1
P =
Z
1+

D
O
D
C
r x
3
1
P =
Z
2
O
D
C
r x
3
LM3421, LM3421-Q1, LM3423, LM3423-Q1
SNVS574F – JULY 2008 – REVISED JULY 2015
www.ti.com
Typical Applications (continued)
Boost
(56)
Buck-Boost
(57)
And the RHP zero (
ωZ1) is approximated:
Boost
(58)
Buck-Boost
(59)
And the uncompensated DC loop gain (TU0) is approximated:
Buck
(60)
Boost
(61)
Buck-Boost
(62)
For all topologies, the primary method of compensation is to place a low frequency dominant pole (
ωP2), which
will ensure that there is ample phase margin at the crossover frequency. This is accomplished by placing a
capacitor (CCMP) from the COMP pin to AGND, which is calculated according to the lower value of the pole and
the RHP zero of the system (shown as a minimizing function):
(63)
(64)
If analog dimming is used, CCMP should be approximately 4× larger to maintain stability as the LEDs are dimmed
to zero.
A high-frequency compensation pole (
ωP3) can be used to attenuate switching noise and provide better gain
margin. Assuming RFS = 10 Ω, CFS is calculated according to the higher value of the pole and the RHP zero of
the system (shown as a maximizing function):
(65)
(66)
The total system loop gain (T) can then be written as:
36
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Copyright © 2008–2015, Texas Instruments Incorporated
Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1



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