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LT1300 Datasheet(PDF) 6 Page - Linear Technology |
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LT1300 Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page 6 LT1300 APPLICATIONS INFORMATION Output Voltage Selection The LT1300 can be selected to 3.3V or 5V under logic control or fixed at either by tying SELECT to ground or VIN respectively. It is permissible to tie SELECT to a voltage higher than VIN as long as it does not exceed 10V. Efficiency in 3.3V mode will be slightly less that in 5V mode due to the fact that the diode drop is a greater percentage of 3.3V than 5V. Since the bipolar switch in the LT1300 gets its base drive from VIN, no reduction in switch efficiency occurs when in 3.3V mode. When VIN exceeds the programmed output voltage the output will follow the input. This is characteristic of the simple step-up or “boost” converter topology. A circuit example that pro- vides a regulated output with an input voltage above or below the output (called a buck-boost or SEPIC) is shown in the Typical Applications section. Shutdown The converter can be turned off by pulling SHDN (pin 3) high. Quiescent current drops to 10 µA in this condition. Bias current of 3 µA to 5µA flows into the pin (at 2.5V input). It is recommended that SHDN not be left floating. Tie the pin to ground if the feature is not used. ILIM Function The LT1300’s current limit (ILIM) pin can be used for soft start. Upon start-up, switching regulators require maxi- mum current from the supply. The high currents flowing can create IR drops along supply and ground lines and are especially demanding on alkaline batteries. By in- stalling an R1 and C3 as shown in Figure 4, the switch current in the LT1300 is limited to 400mA until the 15 µA flowing out of the ILIM pin charges up the 0.1µF capaci- tor. Input current is held to under 500mA while the output voltage ramps up to 5V as shown in Figure 5. The 1Meg resistor provides a discharge path for the capacitor without appreciably decreasing peak switch current. When the full capability of the LT1300 is not required, peak current can be reduced by changing the value of R3 as shown in Figure 6. With R3 = 0, switch current is limited to approximately 400mA. Figure 4. Addition of R1 and C3 Limit Input Current at Startup 500 µs/DIV REP RATE = 1Hz LT1300 F5 VOUT 2VDIV IBATTERY 500mA/DIV VSHDN 10V/DIV C2 100µF C3 0.1µF R1 1M 5V/3.3V OUTPUT + + L1 10µH D1 C1 100µF SW ILIM PGND GND SENSE LT1300 SHDN SELECT VIN Figure 5. Startup Waveforms using Soft-Start Circuitry ILOAD = 100mA, VOUT = 5V RLIM (Ω) 500 700 800 1000 1100 100 10k 100k 1M LT1300 F1B 300 1k 900 600 400 1.6V ≤ VIN ≤ 5V Figure 6. Peak Switch Current vs. RLIM |
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