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LCYG Datasheet(PDF) 10 Page - Linear Technology |
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LCYG Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LT3505 10 3505fc Now the required on time has decreased below the minimum on time of 130ns. Instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fixed at 130ns. In Figure 2 the inductor current ramps up to a value exceeding the load current and the output ripple increases to ~200mV. The part then remains off until the output voltage dips below 100% of the programmed value before it begins switching again. For switching frequencies above 750kHz, the input voltage must not exceed VIN(PS).Seethe“InputVoltageFrequency Foldback” section of the datasheet for a circuit solution that provides safe operation above VIN(PS) at switching frequencies exceeding 750kHz. For switching frequencies below 750kHz, operation above VIN(PS) is safe and will not damage the part as long as the output voltage stays in regulation and the inductor does not saturate. Figure 3 shows the switching waveforms of a 750kHz applica- tion when the input voltage is increased to its absolute maximum rating of 40V. As the input voltage increases, the inductor current ramp rate increases, the number of skipped pulses increases and the output voltage ripple increases. The part is robust enough to survive prolonged operation under these condi- tions as long as the programmed switching frequency is less than 750kHz and the peak inductor current does not exceed 2.2A. Inductor current saturation may further limit performance in this operating regime. Frequency Selection The maximum frequency that the LT3505 can be pro- grammed to is 3MHz. The minimum frequency that the LT3505 can be programmed to is 200kHz. The switching frequencyisprogrammedbytyinga1%resistorfromtheRT pin to ground. Table 1 can be used to select the value of RT. Minimumon-timeandedgelossmustbetakenintoconsid- erationwhenselectingtheintendedfrequencyofoperation. Higher switching frequency increases power dissipation andlowersefficiency. APPLICATIONS INFORMATION VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10 F VOUT = 3V VIN = 35V ILOAD = 0.75A L = 10 H RT = 75.0k 2 s/DIV 3505 F02 IL 0.5A/DIV Figure 2 Figure 3 VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10µF VOUT = 3V VIN = 40V ILOAD = 0.75A L = 10 µH RT = 75.0k 2 µs/DIV 3505 F03 IL 0.5A/DIV VSW 20V/DIV VOUT 200mV/DIV AC COUPLED COUT = 10 F VOUT = 3V VIN = 30V ILOAD = 0.75A L = 10 H RT = 75.0k 2 s/DIV 3505 F01 IL 0.5A/DIV Figure 1 |
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