Fast Fixed Frequency (F3) Architecture
As discussed earlier, a control loop that measures average current is free from peak-to-average current error, leading to a superior transient response. Figure 5 shows the simplified average current control scheme implemented by the IC in buck operation.
Here, the pulse tpw (Figure 6), between the turn-off of N1 and the time RSENSE x IL hits Vε is measured, and a delay equal to tpw is introduced before the turn-on of N1 by the pulse doubler. Hence, the error voltage, Vε, comes to represent exactly the average current Iav.
Naturally, the same strategy is adopted in boost mode. Average current mode ultimately results in higher fidelity of LED brightness and color.
The IC controls LED string voltages from 0 to 60V and provides both analog and digital PWM dimming. It has built-in analog PWM dimming at a 200Hz dimming frequency. The device allows adjustable 200kHz to 700kHz fixed-frequency operation. In addition, ±6% triangular spread spectrum is internally added to the oscillator to improve EMI performance. A fault flag indicates open LED, shorted LED, or thermal shutdown conditions. The adjustable soft-start feature limits the current peaks and voltage overshoots at startup. It also features robust output, open and short protection and is AEC qualified.
Driving high-power automotive LED exterior lighting poses challenges in terms of power dissipation and accurate current control, which affects the LED color and brightness.
We introduced a new LED controller IC, which, thanks to smooth buck-boost operation, minimizes system losses while its average current control enhances transient response, preserving LED color and brightness.
Article originally published in Design Solution No. 170, March 2019
About the Authors:
Nazzareno (Reno) Rossetti is an Analog and Power Management expert at Maxim Integrated. He is a published author and holds several patents in this field. Reno holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.
Yin Wu, MBA, MSEE at Maxim Integrated, is a semiconductor business professional. He holds a Master in Business Administration from Santa Clara University and a Master in Electrical Engineering from San Jose State University.