LM3429_14 Datasheet, PDF (10/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
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i_L (t)
I_L-MAX
I_L
Âi_L-PP
t_ON = DT_S t_OFF = (1-D)T_S
0
T_S
I_L-MIN
t
Figure 2. Ideal CCM Regulator Inductor Current i_L(t)
The average output LED current (I_LED) is proportional to the average inductor current (I_L) , therefore if I_L is tightly
controlled, I_LED will be well regulated. As the system changes input voltage or output voltage, the ideal duty cycle
(D) is varied to regulate I_L and ultimately I_LED. For any current regulator, D is a function of the conversion ratio:
Buck
D
=
V_O
V_IN
(1)
Boost
D=
V_O - V_IN
V_O
(2)
Buck-boost
D
=
V_O
V_O + V_IN
(3)
PREDICTIVE OFF-TIME (PRO) CONTROL
PRO control is used by the LM3429 to control I_LED. It is a combination of average peak current control and a one-
shot off-timer that varies with input voltage. The LM3429 uses peak current control to regulate the average LED
current through an array of HBLEDs. This method of control uses a series resistor in the LED path to sense LED
current and can use either a series resistor in the MosFET path or the MosFET R_DS-ON for both cycle-by-cycle
current limit and input voltage feed forward. D is indirectly controlled by changes in both t_OFF and t_ON, which vary
depending on the operating point.
Even though the off-time control is quasi-hysteretic, the input voltage proportionality in the off-timer creates an
essentially constant switching frequency over the entire operating range for boost and buck-boost topologies.
The buck topology can be designed to give constant ripple over either input voltage or output voltage, however
switching frequency is only constant at a specific operating point .
This type of control minimizes the control loop compensation necessary in many switching regulators, simplifying
the design process. The averaging mechanism in the peak detection control loop provides extremely accurate
LED current regulation over the entire operating range.
PRO control was designed to mitigate “current mode instability” (also called “sub-harmonic oscillation”) found in
standard peak current mode control when operating near or above 50% duty cycles. When using standard peak
current mode control with a fixed switching frequency, this condition is present, regardless of the topology.
However, using a constant off-time approach, current mode instability cannot occur, enabling easier design and
control.
Predictive off-time advantages:
• There is no current mode instability at any duty cycle.
• Higher duty cycles / voltage transformation ratios are possible, especially in the boost regulator.
The only disadvantage is that synchronization to an external reference frequency is generally not available.
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