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LTC1872 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1872 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1872 Figure 4. Setting Output Voltage 3 VFB VOUT LTC1872 R1 1872 F04 R2 APPLICATIONS INFORMATION Setting Output Voltage The LTC1872 develops a 0.8V reference voltage between the feedback (Pin 3) terminal and ground (see Figure 4). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set the overall output voltage. The regulated output voltage is determined by: VV R R OUT =+ 08 1 2 1 . where f is the operating frequency, COUT is the output capacitance and IRIPPLE is the ripple current in the induc- tor. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance through- hole capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR (size) product of any aluminum electrolytic at a somewhat higher price. The output capacitor RMS current is approxi- mately equal to: IDC DC PK •− 2 where IPK is the peak inductor current and DC is the switch duty cycle. When using electrolytic output capacitors, if the ripple and ESR requirements are met, there is likely to be far more capacitance than required. In surface mount applications, multiple capacitors may have to be paralleled to meet the ESR or RMS current handling requirements of the application. Aluminum elec- trolytic and dry tantalum capacitors are both available in surface mount configurations. An excellent choice of tantalum capacitors is the AVX TPS and KEMET T510 series of surface mount tantalum capacitors. Also, ceramic capacitors in X5R pr X7R dielectrics offer excel- lent performance. Low Supply Operation Although the LTC1872 can function down to approxi- mately 2.0V, the maximum allowable output current is reduced when VIN decreases below 3V. Figure 3 shows the amount of change as the supply is reduced down to 2V. Also shown in Figure 3 is the effect of VIN on VREF as VIN goes below 2.3V. Figure 3. Line Regulation of VREF and VITH INPUT VOLTAGE (V) 2.0 105 100 95 90 85 80 75 2.2 2.4 2.6 2.8 1872 F03 3.0 VREF VITH |
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Similar Description - LTC1872 |
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