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LT1304CS8-5 Datasheet(PDF) 8 Page - Linear Technology |
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LT1304CS8-5 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1304/LT1304-3.3/LT1304-5 OPERATIO If the full power capability of the LT1304 is not required, peak switch current can be limited by connecting a resistor RLIM from the ILIM pin to ground. With RLIM = 22k, peak switch current is reduced to approximately 500mA. Smaller power components can then be used. The graph in Figure 6 shows switch current vs RLIM resistor value. VSHDN 10V/DIV IIN 500mA/DIV bypass capacitor is required. If the input supply is close to the IC, a 1 µF ceramic capacitor can be used instead. The LT1304 switches current in 1A pulses, so a low impedance supply must be available. If the power source (for example, a 2 AA cell battery) is within 1 or 2 inches of the IC, the battery itself provides bulk capacitance and the 1 µF ce- ramic capacitor acts to smooth voltage spikes at switch turn-on and turn-off. If the power source is far away from the IC, inductance in the power source leads results in high impedance at high frequency. A local high capacitance bypass is then required to restore low impedance at the IC. Figure 5. Start-Up Response with 1 µF/1MΩ Components in Figure 2 Added. Input Current Is More Controlled. VOUT Reaches 5V in 6ms. Output Drives 20mA Load Layout/Input Bypassing The LT1304’s high speed switching mandates careful attention to PC board layout. Suggested component place- ment is shown in Figure 7. The input supply must have low impedance at AC and the input capacitor should be placed as indicated in the figure. The value of this capacitor depends on how close the input supply is to the IC. In situations where the input supply is more than a few inches away from the IC, a 47 µF to 100µF solid tantalum Figure 6. Peak Switch Current vs RLIM Value RLIM (kΩ) 10 1000 900 800 700 600 500 400 100 1000 1304 F06 Low-Battery Detector The LT1304 contains an independent low-battery detector that remains active when the device is shut down. This detector, actually a hysteretic comparator, has an open collector output that can sink up to 500 µA. The compara- tor also operates below the switcher’s undervoltage lock- out threshold, operating until VIN reaches approximately 1.4V. Figure 8 illustrates the input/output characteristic of the detector. Hysteresis is clearly evident in the figure. Figure 7. Suggested Layout for Best Performance. Input Capacitor Placement as Shown Is Highly Recommended. Switch Trace (Pin 4) Copper Area Is Minimized 1304 F07 8 7 6 5 4 3 2 1 LT1304 COUT SHUTDOWN VOUT VIN CIN GND (BATTERY AND LOAD RETURN) 1ms/DIV 1304 F05 VOUT 2V/DIV |
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Similar Description - LT1304CS8-5 |
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