Datasheet

(1-D
MID
)DII
LEDRMSCIN
xx=
- MID
C
IN
=
'V
IN-PP
x f
SW
I
LED
x D
C
IN
=
8 x '
V
IN-PP
x f
SW
'
i
L-PP
C
IN
=
I
LED
x (1 - D) x D
'V
IN-PP
x f
SW
x=
0U
TT
-1
¸
¸
¹
·
¨
¨
©
§
s
Z
1
Z
xx +1
¸
¸
¹
·
¨
¨
©
§
s
Z
3P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
2P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
1P
x=
0U
TT
1
xx +1
¸
¸
¹
·
¨
¨
©
§
s
Z
3P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
2P
+1
¸
¸
¹
·
¨
¨
©
§
s
Z
1P
1
=
C
FS
3P
10
x
Z
max
3P
=
Z
( )
10,
1Z1P
x
ZZ
1
CMP
C
=
6
2P
e5
x
Z
LM3429, LM3429-Q1
SNVS616G –APRIL 2009–REVISED MAY 2013 www.ti.com
(57)
If analog dimming is used, C
CMP
should be approximately 4x larger to maintain stability as the LEDs are dimmed
to zero.
A high frequency compensation pole (ω
P3
) can be used to attenuate switching noise and provide better gain
margin. Assuming R
FS
= 10Ω, C
FS
is calculated according to the higher value of the pole and the RHP zero of
the system (shown as a maximizing function):
(58)
(59)
The total system loop gain (T) can then be written as:
Buck
(60)
Boost and Buck-boost
(61)
8. INPUT CAPACITANCE
Set the nominal input voltage ripple (Δv
IN-PP
) by solving for the required capacitance (C
IN
):
Buck
(62)
Boost
(63)
Buck-boost
(64)
Use D
MAX
to set the worst case input voltage ripple, when solving for C
IN
in a buck-boost regulator and D
MID
= 0.5
when solving for C
IN
in a buck regulator.
The minimum allowable RMS input current rating (I
CIN-RMS
) can be approximated:
Buck
(65)
28 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated
Product Folder Links: LM3429 LM3429-Q1