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LT1302CN8-5 Datasheet(PDF) 11 Page - Linear Technology |
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LT1302CN8-5 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LT1302/LT1302-5 APPLICATIONS INFORMATION 1ms/DIV 1302 F08b VOUT 100mV/DIV AC COUPLED Figure 8b. 3.3k Resistor from IT Pin to Ground Forces LT1302 into Current Mode Regardless of Load. Audio Frequency Component Eliminated 525mA 25mA ILOAD INDUCTOR CURRENT 1A/DIV The IT pin cannot be used as a soft-start. Large capacitors connected to the pin will cause erratic operation. If oper- ating the device in Burst Mode, let the pin float. Keep high dV/dt signals away from the pin. Figure 8c details efficiency with and without the addition of R1. Burst Mode operation keeps efficiency high at light load with IT floating. Efficiency falls off at light load with R1 added because the LT1302 cannot transition into Burst Mode. Layout The high speed, high current switching associated with the LT1302 mandates careful attention to layout. Follow the suggested component placement in Figure 9 for proper operation. High current functions are separated by the package from sensitive control functions. Feedback resis- tors R1 and R2 should be close to the feedback pin (pin4). Noise can easily be coupled into this pin if care is not taken. A small capacitor (100pF to 200pF) from FB to ground provides a high frequency bypass. If the LT1302 is oper- ated off a three-cell or higher input, R3 (2 Ω to 10Ω) in series with VIN is recommended. This isolates the device from noise spikes on the input supply. Do not put in R3 if the device must operate from a 2V input, as input current will cause the voltage at the LT1302’s VIN pin to go below 2V. The 0.1 µF ceramic bypass capacitor C3 (use X7R, not Z5U) should be mounted as close as possible to the package. When R3 is used, C3 should be a 1 µF tantalum unit. Grounding should be segregated as illustrated. C3’s ground trace should not carry switch current. Run a Figure 8c. 3.3k Resistor for IT to Ground Increases Efficiency at Moderate Load, Decreases at Light Load OUTPUT CURRENT (mA) 1 90 80 70 60 50 40 30 10 100 1000 1302 F08c IT FLOATING 3.3k IT TO GND Figure 9. Suggested Component Placement for LT1302 ally reaches audio frequencies, but at a much lighter load than without the IT feature. At some input voltage/load current combinations, some residual bursting may occur at frequencies out of the audio band. 1302 F09 4 3 2 1 8 7 6 5 LT1302 C3 R3 2 Ω D1 C2 C1 VOUT VIN L1 + 200pF R1 R2 RC CC SHUTDOWN GND (BATTERY AND LOAD RETURN) |
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