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 .
|
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