Datasheet LT3496 (Analog Devices) - 10

LT3496 applications inForMation dimming waveforms and the start-up time should be
Input Capacitor Selection
checked across all operating conditions. For proper operation, it is necessary to place a bypass
Open-LED Protection
capacitor to GND close to the VIN pin of the LT3496. A 1µF or greater capacitor with low ESR should be used. A The LT3496 has open-LED protection for all the three ceramic capacitor is usually the best choice. converters. As shown in Figure 1, the OVP1 pin receives the output voltage (the voltage across the output capacitor) In the buck mode configuration, the capacitor at PVIN has feedback signal from an external resistor divider. OVP1 large pulsed currents due to the current returned though voltage is compared with a 1V internal voltage reference by the Schottky diode when the switch is off. For the best comparator A6. In the event the LED string is disconnected reliability, this capacitor should have low ESR and ESL or fails open, converter 1 output voltage wil increase, caus- and have an adequate ripple current rating. The RMS ing OVP1 voltage to increase. When OVP1 voltage exceeds input current is: 1V, the power switch Q1 will turn off, and cause the output I ( )•D voltage to decrease. Eventually, OVP1 will be regulated to IN(RMS) =ILED • 1– D 1V and the output voltage will be limited. In the event one where D is the switch duty cycle. A 1µF ceramic type ca- of the converters has an open-LED protection, the other pacitor placed close to the Schottky diode and the ground converters will continue functioning properly. plane is usually sufficient for each channel.
Switching Frequency and Soft-Start Output Capacitor Selection
The LT3496 switching frequency is controlled by fADJ pin The selection of output filter capacitor depends on the load voltage. Setting fADJ voltage to be less than 1V will reduce and converter configuration, i.e., step-up or step-down. switching frequency. For LED applications, the equivalent resistance of the LED If fADJ voltage is higher than 1V, the default switching is typically low, and the output filter capacitor should be frequency is 2.1MHz. In general, a lower switching fre- large enough to attenuate the current ripple. quency should be used where either very high or very To achieve the same LED ripple current, the required filter low switch duty cycle is required or higher efficiency is capacitor value is larger in the boost and buck-boost mode desired. Selection of a higher switching frequency will applications than that in the buck mode applications. For the allow use of low value external components and yield a LED buck mode applications, a 0.22µF ceramic capacitor smaller solution size and profile. is usually sufficient for each channel. For the LED boost Connecting fADJ pin to a lowpass filter (R5 and C4 in and buck-boost applications, a 1µF ceramic capacitor is Figure 1) from the REF pin provides a soft-start function. usually sufficient for each channel. If higher LED current During start-up, fADJ voltage increases slowly from 0V to ripple can be tolerated, a lower output capacitance can be the setting voltage. As a result, the switching frequency selected to reduce the capacitor’s cost and size. increases slowly to the setting frequency. This function Use only ceramic capacitors with X7R or X5R dielectric, limits the inrush current during start-up. as they are good for temperature and DC bias stability of
Undervoltage Lockout
the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it The LT3496 has an undervoltage lockout circuit that may be necessary to choose a higher value capacitor to get shuts down all the three converters when the input volt- the required capacitance at the operation voltage. Always age drops below 2.4V. This prevents the converter from check that the voltage rating of the capacitor is sufficient. switching in an erratic mode when powered from a low Table 1 shows some recommended capacitor vendors. supply voltage. 3496ff 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts