LM3424 Datasheet, PDF (35/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Typical Applications (continued)
3
Z_P1 =
1+D
r_D x C_O
(60)
And the RHP zero (ω_Z1) is approximated:
Boost
Z_Z1
=
r_D
x Dc²
L1
(61)
Buck-Boost
Z_Z1
=
r_D x Dc²
D xL1
(62)
And the uncompensated DC loop gain (T_U0) is approximated:
Buck
T_U0 =
500V x R_CSH x R_SNS
R_HSP x R_LIM
620V
=
I_LED x R_LIM
(63)
Boost
T_U0
=
D? x
500V x R_CSH x R_SNS
2 x R_HSP x R _LIM
=
D? x 310V
I_LED x R_LIM
(64)
Buck-Boost
T_U0
=
D?x 500V x R_CSH x R_SNS
(1+ D) x R_HSP x R_LIM
=
D? x 620V
(1+ D) x I_LED x R_LIM
(65)
For all topologies, the primary method of compensation is to place a low frequency dominant pole (ω_P2) which
will ensure that there is ample phase margin at the crossover frequency. This is accomplished by placing a
capacitor (C_CMP) from the COMP pin to GND, which is calculated according to the lower value of the pole and the
RHP zero of the system (shown as a minimizing function):
Z_P2
=
min(Z_P1, Z_Z1)
5 x T_U0
(66)
C_CMP
1
Z_P2 x 5x10⁶
(67)
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):
Z_P3 = max(Z_P1,Z_Z1) x 10
(68)
1
C_FS = _{10 x ZP3}
(69)
The total system loop gain (T) can then be written as:
Buck
T=
T_U0
x
¨¨©§¹⁺
Z^s_P1¸¸¹·
x
1
¨¨©§¹⁺ Z^s_P2¸¸¹·
x
¨¨©§¹⁺
Z^s_P3¸¸¹·
(70)
Boost and Buck-Boost
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