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LT3504 Datasheet(PDF) 11 Page - Linear Technology |
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LT3504 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3504 11 3504fa For more information www.linear.com/LT3504 applicaTions inForMaTion Frequency Selection The maximum frequency that the LT3504 can be pro- grammedtois2.5MHz.Theminimumfrequencyis250kHz. The switching frequency can be programmed in two ways. The first method is by tying a 1% resistor (RT) from the RT/SYNC pin to ground. Table 1 can be used to select the value of RT. The second method is to synchronize (sync) the internal oscillator to an external clock. The external clock must have a minimum amplitude from 0V to 1.6V and a minimum pulse-width of 50ns. Table 1. RT/SYNC Pin Resistance to Program Oscillator Frequency FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ) 0.20 140 0.3 82.5 0.4 56.2 0.5 43.2 0.6 34.8 0.7 28.0 0.8 23.7 0.9 20.5 1.0 18.2 1.1 16.9 1.2 14.7 1.3 13.0 1.4 11.5 1.5 10.7 1.6 9.76 1.7 8.66 1.8 8.06 1.9 7.32 2.0 6.81 2.1 6.34 2.2 6.04 2.3 5.62 2.4 5.36 2.5 4.99 In certain applications, the LT3504 may be required to be alive and switching for a period of time before it begins to receive a sync signal. If the sync signal is in a high impedance state when it is inactive then the solution is to Figure 2. Driving the RT/SYNC Pin From a Port That Is in a High Impedance State When it Is Inactive Figure 3. Driving the RT/SYNC Pin from a Port That Is in a Low Impedance State When it Is Inactive simply tie an RT resistor from the RT/SYNC pin to ground (Figure 2). The sync signal should be capable of driving the RT resistor. If the sync signal is in a low impedance state or an unknown state when it is inactive, then the solution is to tie the RT resistor from the RT/SYNC pin to ground and then to drive the RT/SYNC pin with the sync signal through a 1nF capacitor as shown in Figure 3. 3504 F02 PORT GND LT3504 RT/SYNC RT 3504 F03 PORT 1nF GND LT3504 RT/SYNC RT BOOST Regulator and SKY Pin Considerations The on-chip boost regulator generates the SKY voltage to be 4.85V above VIN. The SKY voltage is the source of drive current for the buck regulators which is used to fully saturate the power switch. The boost regulator requires two external components: an inductor and a capacitor. A good first choice for an inductor is given by: L = 20.5µH f where f is in MHz. Thus, for a 250kHz programmed switching frequency, a good first choice for an inductor value is 82µH. For a 2.5MHz programmed switching frequency, a good first |
Similar Part No. - LT3504_15 |
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Similar Description - LT3504_15 |
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