ZHCSK33G July 2008 – July 2019 LM3421 , LM3423
PRODUCTION DATA.
PRO control is used by the device to control ILED. It is a combination of average peak current control and a one-shot off-timer that varies with input voltage. The LM3421 and LM3423 use 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 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:
The only disadvantage is that synchronization to an external reference frequency is generally not available.