LT3509 applicaTions inForMaTionShutdown and Soft StartSetting The Output Voltage When the RUN/SS pins are pulled to ground, the part will The output voltage is programmed with a resistor divider shut down to its lowest current state of approximately between the output and the FB pin. Choose the resistors 9µA. If driving a large capacitive load it may be desirable according to: to use the current limiting soft-start feature. Connecting capacitors to ground from the RUN/SS pins will control V R1=R2 • OUT – 1 the delay until full current is available. The pull-up current 0.8 is 1µA and the full current threshold is 2V so the start-up time is given by: The designators correspond to Figure 1. R2 should be 20k or less to avoid bias current errors. T = 2 • C • 106 s For example a 0.005µF capacitor will give a time to full Frequency Setting current of 10ms. If both outputs can come up together The timing resistor, RT , for any desired frequency in the then the two inputs can be paralleled and tied to one ca- range 264kHz to 2.2MHz can be calculated from the pacitor. In this case use twice the capacitor value to obtain following formula: the same start-up time. During the soft-start time both the peak current threshold and the DA current threshold R 1.215 T = – 0.215 • 40.2 will track so the part will skip pulses as required to limit fSW the maximum inductor current. Starting up into a large capacitor is not much different to starting into a short- where fSW is in MHz and RT is in kΩ. circuit in this respect. Table 1. Standard E96 Resistors for Common FrequenciesFREQUENCYTIMING RESISTOR RT (kΩ) 264 kHz 178 VSW 10V/DIV 300 kHz 154 400kHz 113 500kHz 88.7 IL 1MHz 40.2 0.2A/DIV 2MHz 15.8 VOUT 5V/DIV 2.2MHz 13.7 TIME 1ms/DIV 3509 F02 Note: The device is specified for operation down to 300kHz. The 264kHz value is to allow external synchronization at 300kHz Figure 2. Soft-Start 3509fd For more information www.linear.com/LT3509 9 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Related Parts
