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LM3421, LM3421-Q1
LM3423, LM3423-Q1
Z P 2 x 5 e 6
www.ti.com
300
C CMP =
1
SNVS574E – JULY 2008 – REVISED MAY 2013
(64)
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 = 10 ? , C FS is calculated according to the higher value of the pole and the RHP zero of
the system (shown as a maximizing function):
10 x Z P 3
Z P 3 = max ( Z P 1 , Z Z 1 ) x 10
300
1
C FS =
The total system loop gain (T) can then be written as:
Buck
(65)
(66)
¨ 1 +
? x
¨ 1 +
? x ¨ 1 +
Z P 2 ?
s ·
Z P 3 ?
T = T U 0 x
§
¨
?
s ·
1
Z P 1 ?
§
¨
?
1
s · §
1 ?
¨
?
1
(67)
Boost and Buck-boost
¨ 1 +
¨ 1 - s ?
Z Z 1 ? 1
? x ¨ 1 +
? x ¨ 1 +
Z P 1 ? Z P 2 ?
s ·
Z P 3 ?
T = T U 0 x
§
¨
?
s · § s · §
1 ? 1 ?
§ ·
¨
?
¨ ¨
?
1
(68)
8. INPUT CAPACITANCE
Set the nominal input voltage ripple ( Δ v IN-PP ) by solving for the required capacitance (C IN ):
Buck
C IN =
Boost
C IN =
I LED x (1 - D) x D
' V IN-PP x f SW
' i L-PP
8 x ' V IN-PP x f SW
(69)
(70)
Buck-boost
C IN =
I LED x D
' V IN-PP x f SW
(71)
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
I CIN - RMS = I LED x D MID x (1-D MID )
(72)
Copyright ? 2008–2013, Texas Instruments Incorporated
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