Datasheet LT3492 (Analog Devices) - 10

LT3492
APPLICATIONS INFORMATION
is similar to your application and tune the compensation
Input Capacitor Selection
network to optimize the performance. The stability, PWM For proper operation, it is necessary to place a bypass dimming waveforms and the start-up time should be capacitor to GND close to the V checked across all operating conditions. IN pin of the LT3492. A 1μF or greater capacitor with low ESR should be used. A
Open-LED Protection
ceramic capacitor is usually the best choice. The LT3492 has open-LED protection for all the three In the buck mode confi guration, the capacitor at PVIN has converters. As shown in Figure 1, the OVP1 pin receives large pulsed currents due to the current returned though the output voltage (the voltage across the output capacitor) the Schottky diode when the switch is off. For the best feedback signal from an external resistor divider. OVP1 reliability, this capacitor should have low ESR and ESL voltage is compared with a 1V internal voltage reference by and have an adequate ripple current rating. The RMS comparator A6. In the event the LED string is disconnected input current is: or fails open, converter 1 output voltage will increase, caus- I ( )•D ing OVP1 voltage to increase. When OVP1 voltage exceeds IN(RMS) =ILED • 1–D 1V, the power switch Q1 will turn off, and cause the output where D is the switch duty cycle. A 1μF ceramic type ca- voltage to decrease. Eventually, OVP1 will be regulated to pacitor placed close to the Schottky diode and the ground 1V and the output voltage will be limited. In the event one plane is usually suffi cient for each channel. of the converters has an open-LED protection, the other converters will continue functioning properly.
Output Capacitor Selection Switching Frequency and Soft-Start
The selection of output fi lter capacitor depends on the load and converter confi guration, i.e., step-up or step-down. The LT3492 switching frequency is controlled by FADJ For LED applications, the equivalent resistance of the LED pin voltage. Setting FADJ voltage to be less than 1V will is typically low, and the output fi lter capacitor should be reduce switching frequency. large enough to attenuate the current ripple. If FADJ voltage is higher than 1V, the default switch- To achieve the same LED ripple current, the required fi lter ing frequency is 2.1MHz. In general, a lower switching capacitor value is larger in the boost and buck-boost mode frequency should be used where either very high or very applications than that in the buck mode applications. For low switch duty cycle is required or higher effi ciency is the LED buck mode applications at 1.3MHz, a 0.22μF ce- desired. Selection of a higher switching frequency will ramic capacitor is usually suffi cient for each channel. For allow use of low value external components and yield a the LED boost and buck-boost applications at 1.3MHz, a smaller solution size and profi le. 1μF ceramic capacitor is usually suffi cient for each chan- As a cautionary note, operation of the LT3492 at a com- nel. Lower switching frequency requires proportionately bination of high switching frequency with high output higher capacitor values. If higher LED current ripple can voltage and high switch current may cause excessive be tolerated, a lower output capacitance can be selected internal power dissipation. Consideration should be given to reduce the capacitor’s cost and size. to selecting a switching frequency less than 1MHz if these Use only ceramic capacitors with X7R or X5R dielectric, conditions exist. as they are good for temperature and DC bias stability of Connecting FADJ pin to a lowpass fi lter (R5 and C4 in the capacitor value. All ceramic capacitors exhibit loss of Figure 1) from the REF pin provides a soft-start function. capacitance value with increasing DC voltage bias, so it During start-up, FADJ voltage increases slowly from 0V may be necessary to choose a higher value capacitor to get to the setting voltage. As a result, the switching frequency the required capacitance at the operation voltage. Always increases slowly to the setting frequency. This function check that the voltage rating of the capacitor is suffi cient. limits the inrush current during start-up. Table 1 shows some recommended capacitor vendors. 3492fa 10