Cover Story
powered from the automotive 9V – 16V input range and operates |
( FB1 and FB2) should be used for best EMI results. |
to 36V in the face of transients and down to 3V( cold-crank conditions). Its low EMI Silent Switcher architecture, SSFM, and controlled switching edges make it ideal for powering LEDs with low EMI. Its versatility makes it useful in boost, buck and boostincorporates buck applications for exterior daytime running lights, signal lights, tail lights, and headlight segments as well as interior dashboard and heads-up displays with high dimming ratio. Built-in flexibility and fault protection reduce the number of required components to protect against short and open LED strings. |
Linear’ s patented Silent Switcher architecture,
Built-in features for low EMI
The LT3922 includes a number of features that enable designers to easily achieve low EMI solutions. First of all, LT3922 where internal synchronous switches minimize hot-switching-loop size and controlled switching edges do not ring.
Figure 6 shows how the LT3922’ s pinout enables placement of small, high frequency capacitors near the two V
OUT pins to minimize hot-loop size and EMI.
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The switching edge rate is controlled by the LT3922, eliminating high frequency ringing that is common in switching converters without this feature. The LT3922’ s controlled switching edges reduce power switch high frequency EMI without degrading efficiency and power capability.
Figure 4. 400kHz automotive boost LED driver with filters for low EMI and option for 100 %, 10 % or 1 % internally generated PWM dimming. EMI tests( Figure 5) show that this solution passes CISPR 25 Class 5.
Figure 6. The dual-loop layout and high frequency 0402 split capacitors create small, opposing hot-loops that help reduce high frequency EMI
SSFM in the LT3922 spreads the resistor-set switching frequency up and down from 100 % to 125 % of its value at a rate of 1.6kHz for the 400kHz converter. This decreases both the peak and average EMI in the converter at low and high frequencies. The feature is easy to turn on and off by connecting the SYNC / SPRD pin to INTV CC or GND, respectively.
Internally generated PWM Dimming
Figure 5. EMI profile of the 400kHz LED driver shown in Figure 4, which passes CISPR 25 Class 5 with minimal EMI filters. A larger LC filter can be added to the input if further EMI reduction is needed for specific manufacturer EMI requirements.
The 400kHz automotive boost LED driver in Figure 4 passes CISPR 25 Class 5 EMI tests, as shown in Figure 5, which shows conducted and radiated EMI test results of the LT3922 along with class 5 EMI limits. This is a result of a combination of LT3922 low EMI features, including, but not limited to, controlled switching edges and spread spectrum frequency modulation( SSFM). Of Figure 7.( a) Externally generated 2000:1 or 4000:1 PWM dimming of course, proper layout and a small amount of ferrite bead filtering Figure 1 and( b) internally generated 128:1 PWM dimming of Figure 1.
ELE Times | 30 | December, 2016