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LTM8005 датащи(PDF) 15 Page - Linear Technology |
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LTM8005 датащи(HTML) 15 Page - Linear Technology |
15 / 26 page LTM8005 15 8005f For more information www.linear.com/LTM8005 APPLICATIONS INFORMATION Duty Cycle Considerations Switching duty cycle is a key variable defining converter operation; therefore, its limits must be considered when programming the switching frequency for a particular application. The fixed minimum on-time and minimum off-time and the switching frequency define the minimum and maximum duty cycle of the switch, respectively. When calculating operating limits, use typical room temperature values of 320ns and 290ns for minimum on-time and off-time, respectively. Setting Input Current Limit The LTM8005 has a standalone input current sense ampli- fier to limit the input current. The input current IIN shown in Figure 8 is converted to a voltage output at the IVINCOMP pin. When the IVINCOMP voltage exceeds 1.2V the inter- nal power switch is turned off, and the converter stops switching. The input current limit is calculated as follows: IIN = 60mV RINSNS the ISP/ISN regulation loop, which consists of the output capacitance COUT and the dynamic resistance of the LED load. The minimum CFILT value of 10nF is integrated into the LTM8005. Loop Compensation The LTM8005 uses an internal transconductance error amplifier whose VC output compensates the control loop. The external inductor, output capacitor and the compen- sation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop response and stability. For typical LED applications, a 10nF compensation capacitor at VC is adequate, and a series resistor should always be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on the input supply to the converter. Soft-Start Capacitor Selection For many applications, it is important to minimize the inrush current at start-up. The LTM8005 soft-start cir- cuit significantly reduces the start-up current spike and output voltage overshoot. The soft-start interval is set by the soft-start capacitor (CSS) selection according to the equation: TSS = CSS • 2V / 28µA A typical value for the soft-start capacitor is 0.1µF. The soft-start pin voltage reduces the oscillator frequency and the maximum current in the switch. Soft-start also oper- ates as fault protection, which forces the converter into hiccup or latchoff mode. Detailed information is provided in the Fault Protection: Hiccup Mode and Latchoff Mode section. Fault Protection: Hiccup Mode and Latchoff Mode If an LED overcurrent condition, internal INTVCC under- voltage, output short (FB ≤ 0.3V), or thermal limit hap- pens, the integrated PMOS disconnect switch disconnects the LED array from the power path, and the integrated power switching MOSFET is turned off. If the soft-start pin is charging and still below 1.7V, then it will continue Figure8. Apply a Current Sense Resistor Between IVINP and IVINN to Limit Input Current Filter capacitor CFILT shown in Figure 8 filters the voltage at the IVINCOMP pin to minimize ripple due to the input current. CFILT also compensates the input current regula- tion loop, and is selected based on the loop response in addition to the intended voltage ripple on IVINCOMP. The IVINCOMP pin resistance to ground and CFILT form a second pole in the input current regulation loop in addi- tion to the dominant pole at VC pin. Suggested values for CFILT of 10nF to 0.1µF will usually provide a second pole in the input current regulation loop that results in stable loop response and is equivalent to the second pole in LTM8005 IVINP IVINN IVINCOMP 8005 F08 TO LOAD RINSNS IIN VIN CFILT |
Аналогичный номер детали - LTM8005 |
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Аналогичное описание - LTM8005 |
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