Datasheet LT3505 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 1.2A, Step-Down Switching Regulator in 3mm × 3mm DFN |
| Pages / Page | 24 / 10 — Figure 1. Figure 2. Frequency Selection. Figure 3 |
| File Format / Size | PDF / 832 Kb |
| Document Language | English |
Figure 1. Figure 2. Frequency Selection. Figure 3
Model Line for this Datasheet
Text Version of Document
LT3505 APPLICATIONS INFORMATION Now the required on time has decreased below the minimum on time of 130ns. Instead of the switch pulse VSW width becoming narrower to accommodate the lower duty 20V/DIV cycle requirement, the switch pulse width remains fi xed at 130ns. In Figure 2 the inductor current ramps up to a IL 0.5A/DIV value exceeding the load current and the output ripple increases to ~200mV. The part then remains off until the VOUT output voltage dips below 100% of the programmed value 200mV/DIV AC COUPLED before it begins switching again. 2 s/DIV 3505 F01 For switching frequencies above 750kHz, the input voltage COUT = 10 F ILOAD = 0.75A VOUT = 3V L = 10 H must not exceed VIN(PS). See the “Input Voltage Frequency VIN = 30V RT = 75.0k Foldback” section of the datasheet for a circuit solution that provides safe operation above V
Figure 1
IN(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 V in regulation and the inductor does not saturate. Figure SW 20V/DIV 3 shows the switching waveforms of a 750kHz applica- tion when the input voltage is increased to its absolute IL maximum rating of 40V. 0.5A/DIV As the input voltage increases, the inductor current ramp VOUT rate increases, the number of skipped pulses increases 200mV/DIV AC COUPLED and the output voltage ripple increases. The part is robust enough to survive prolonged operation under these condi- 2 s/DIV 3505 F02 C I tions as long as the programmed switching frequency is OUT = 10 F LOAD = 0.75A VOUT = 3V L = 10 H less than 750kHz and the peak inductor current does not VIN = 35V RT = 75.0k exceed 2.2A. Inductor current saturation may further limit
Figure 2
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 VSW 20V/DIV LT3505 can be programmed to is 200kHz. The switching frequency is programmed by tying a 1% resistor from the RT pin to ground. Table 1 can be used to select the value of RT. IL 0.5A/DIV Minimum on-time and edge loss must be taken into consid- eration when selecting the intended frequency of operation. VOUT Higher switching frequency increases power dissipation 200mV/DIV and lowers effi ciency. AC COUPLED 2µs/DIV 3505 F03 COUT = 10µF ILOAD = 0.75A VOUT = 3V L = 10µH VIN = 40V RT = 75.0k
Figure 3
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Figure 1
IN(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 V in regulation and the inductor does not saturate. Figure SW 20V/DIV 3 shows the switching waveforms of a 750kHz applica- tion when the input voltage is increased to its absolute IL maximum rating of 40V. 0.5A/DIV As the input voltage increases, the inductor current ramp VOUT rate increases, the number of skipped pulses increases 200mV/DIV AC COUPLED and the output voltage ripple increases. The part is robust enough to survive prolonged operation under these condi- 2 s/DIV 3505 F02 C I tions as long as the programmed switching frequency is OUT = 10 F LOAD = 0.75A VOUT = 3V L = 10 H less than 750kHz and the peak inductor current does not VIN = 35V RT = 75.0k exceed 2.2A. Inductor current saturation may further limit
Figure 2
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 VSW 20V/DIV LT3505 can be programmed to is 200kHz. The switching frequency is programmed by tying a 1% resistor from the RT pin to ground. Table 1 can be used to select the value of RT. IL 0.5A/DIV Minimum on-time and edge loss must be taken into consid- eration when selecting the intended frequency of operation. VOUT Higher switching frequency increases power dissipation 200mV/DIV and lowers effi ciency. AC COUPLED 2µs/DIV 3505 F03 COUT = 10µF ILOAD = 0.75A VOUT = 3V L = 10µH VIN = 40V RT = 75.0k
Figure 3
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