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MAX16832CASA+ Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX16832CASA+ Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 10 page 2MHz, High-Brightness LED Drivers with Integrated MOSFET and High-Side Current Sense 8 _______________________________________________________________________________________ Freewheeling-Diode Selection For stability and best efficiency, a low forward-voltage drop diode with fast reverse-recovery time and low capacitance is recommended. A Schottky diode is a good choice as long as its breakdown voltage is high enough to withstand the maximum operating voltage. PCB Layout Guidelines Careful PCB layout is critical to achieve low switching losses and stable operation. In normal operation, there are two power loops. One is formed when the internal MOSFET is on and the high current flows from ground through the input cap, RSENSE, the LED load, the inductor, and the internal MOSFET back to ground. The second loop is formed when the internal MOSFET is off and the high current circulates from the input cap posi- tive terminal through RSENSE, the LED load, the induc- tor, and the freewheeling diode and back to the input cap positive terminal. Note that the current through RSENSE, the LED load, and the inductor is basically DC with some triangular ripple (low noise). The high-noise, large signal, fast transition switching currents only flow through the freewheeling diode to the input cap positive terminal, or through the MOSFET to ground and then to the input cap positive terminal. Without a proper PCB layout, these square-wave switching currents can cre- ate problems in a hysteretic LED driver. The current control depends solely on the voltage across RSENSE. Any noise pickup on this node induces erratic switching of the internal MOSFET (the IC will operate at a much higher frequency). To help prevent this, place RSENSE as close as possible to CS and IN and keep the sense traces short. It is especially impor- tant to keep the square-wave switching currents in the freewheeling diode away from RSENSE. To minimize interference, place the freewheeling diode on the oppo- site side of the IC as RSENSE and position the input capacitor near the diode so it can return the high fre- quency currents to ground. The layout in Figure 3 should be used as a guideline. The dashed line shows the path of the high frequency components that cause disruption in operation. For a good thermal design, the exposed pad on the IC should solder to a large pad with many vias to the backside ground plane. fSW t t t2 ∆I t1 AVG. LED CURRENT HYSTERETIC MODE ILED VDIM Figure 2. Current-Regulator Operation |
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