LM3429_14 Datasheet, PDF (14/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
www.ti.com
LM3429
COMP
Q1
GATE
800 mV
IS
PWM
I_T
R_LIM
LEB
245 mV
PGND
Figure 7. Current Sense / Current Limit Circuitry
There are two possible methods to sense the transistor current. The R_DS-ON of the main power MosFET can be
used as the current sense resistance because the IS pin was designed to withstand the high voltages present on
the drain when the MosFET is in the off state. Alternatively, a sense resistor located in the source of the MosFET
may be used for current sensing, however a low inductance (ESL) type is suggested. The cycle-by-cycle current
limit (I_LIM) can be calulated using either method as the limiting resistance (R_LIM):
I_LIM
=
245 mV
R_LIM
(12)
In general, the external series resistor allows for more design flexibility, however it is important to ensure all of
the noise sensitive low power ground connections are connected together local to the controller and a single
connection is made to the high current PGND (sense resistor ground point).
CONTROL LOOP COMPENSATION
The LM3429 control loop is modeled like any current mode controller. Using a first order approximation, the
uncompensated loop can be modeled as a single pole created by the output capacitor and, in the boost and
buck-boost topologies, a right half plane zero created by the inductor, where both have a dependence on the
LED string dynamic resistance. There is also a high frequency pole in the model, however it is above the
switching frequency and plays no part in the compensation design process therefore it will be neglected. Since
ceramic capacitance is recommended for use with LED drivers due to long lifetimes and high ripple current
rating, the ESR of the output capacitor can also be neglected in the loop analysis. Finally, there is a DC gain of
the uncompensated loop which is dependent on internal controller gains and the external sensing network.
A buck-boost regulator will be used as an example case. See the Design Guide section for compensation of all
topologies.
The uncompensated loop gain for a buck-boost regulator is given by the following equation:
¨¨©§^{1 -}
s
Z
Z1
¸¸¹·
T_U
=
T_U0
x
¨¨©§¹⁺
s
Z
P1
¸¸¹·
(13)
Where the uncompensated DC loop gain of the system is described as:
T_U0
=
Dcx 500V x R_CSH x R_SNS
(1+ D) x R_HSP x R_LIM
= ^{Dc x 620V}
(1+ D) x I_LED x R_LIM
(14)
And the ou₃tput pole (ω_P1) is approximated:
Z_P1 =
1+D
r_D x C_O
(15)
And the rig₃ht half plane zero (ω_Z1) is:
Z_Z1
=
r_D x Dc²
D xL1
(16)
14
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