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FIGURE 1. Ideal CCM Regulator Inductor Current iL(t)
The average output LED current (ILED) is proportional to the
average inductor current (IL) , therefore if ILis tightly con-
trolled, ILED will be well regulated. As the system changes
input voltage or output voltage, the ideal duty cycle (D) is var-
ied to regulate ILand ultimately ILED. For any current regulator,
D is a function of the conversion ratio:
Buck
Boost
Buck-boost
PREDICTIVE OFF-TIME (PRO) CONTROL
PRO control is used by the LM3429 to control ILED. 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
RDS-ON for both cycle-by-cycle current limit and input voltage
feed forward. D is indirectly controlled by changes in both
tOFF and tON, 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 con-
stant at a specific operating point .
This type of control minimizes the control loop compensation
necessary in many switching regulators, simplifying the de-
sign process. The averaging mechanism in the peak detec-
tion 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 con-
stant off-time approach, current mode instability cannot oc-
cur, 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.
SWITCHING FREQUENCY
An external resistor (RT) connected between the RCT pin and
the switch node (where D1, Q1, and L1 connect), in combi-
nation with a capacitor (CT) between the RCT and AGND pins,
sets the off-time (tOFF) as shown in Figure 2. For boost and
buck-boost topologies, the VIN proportionality ensures a vir-
tually constant switching frequency (fSW).
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FIGURE 2. Off-timer Circuitry for Boost and Buck-boost
Regulators
For a buck topology, RTand CTare also used to set tOFF,
however the VIN proportionality will not ensure a constant
switching frequency. Instead, constant ripple operation can
be achieved. Changing the connection of RTin Figure 2 from
VSW to VIN will provide a constant ripple over varying VIN.
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LM3429 LM3429Q1
