Datasheet LT3509 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | Dual 36V, 700mA Step-Down Regulator |
| Seiten / Seite | 24 / 9 — applicaTions inForMaTion. Shutdown and Soft Start. Setting The Output … |
| Dateiformat / Größe | PDF / 328 Kb |
| Dokumentensprache | Englisch |
applicaTions inForMaTion. Shutdown and Soft Start. Setting The Output Voltage. Frequency Setting
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LT3509
applicaTions inForMaTion Shutdown and Soft Start Setting 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 Frequencies FREQUENCY TIMING 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
applicaTions inForMaTion Shutdown and Soft Start Setting 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 Frequencies FREQUENCY TIMING 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