Datasheet LT1585A, LT1585A-3.3 (Analog Devices) - 7
| 制造商 | Analog Devices |
| 描述 | 5A Low Dropout Fast Response Positive Regulators Adjustable and Fixed |
| 页数 / 页 | 12 / 7 — APPLICATIONS INFORMATION. Ripple Rejection. Output Voltage. Figure 3. … |
| 文件格式/大小 | PDF / 169 Kb |
| 文件语言 | 英语 |
APPLICATIONS INFORMATION. Ripple Rejection. Output Voltage. Figure 3. Basic Adjustable Regulator. Figure 2
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LT1585A/LT1585A-3.3
U U W U APPLICATIONS INFORMATION
A protection diode between the input and output pins is
Ripple Rejection
usually not needed. An internal diode between the input The typical curve for ripple rejection reflects values for the and output pins on the LT1585A/LT1585A-3.3 can handle LT1585A-3.3 fixed output voltage part. In applications microsecond surge currents of 50A to 100A. Even with that require improved ripple rejection, use the adjustable large value output capacitors it is difficult to obtain those device. A bypass capacitor from the adjust pin to ground values of surge currents in normal operation. Only with reduces the output ripple by the ratio of V large values of output capacitance, such as 1000µF to OUT/1.25V. The impedance of the adjust pin capacitor at the ripple fre- 5000µF, and with the input pin instantaneously shorted to quency should be less than the value of R1 (typically in the ground can damage occur. A crowbar circuit at the input range of 100Ω to 120Ω) in the feedback divider network of the LT1585A/LT1585A-3.3 can generate those levels of in Figure 2. Therefore, the value of the required adjust pin current, and a diode from output to input is then recom- capacitor is a function of the input ripple frequency. For mended. This is shown in Figure 2. Usually, normal power example, if R1 equals 100Ω and the ripple frequency supply cycling or system “hot plugging and unplugging” equals 120Hz, the adjust pin capacitor should be 22µF. At will not generate current large enough to do any damage. 10kHz, only 0.22µF is needed. The adjust pin can be driven on a transient basis ±7V with respect to the output, without any device degradation. As
Output Voltage
with any IC regulator, exceeding the maximum input-to- The LT1585A adjustable regulator develops a 1.25V ref- output voltage differential causes the internal transistors erence voltage between the output pin and the adjust pin to break down and none of the protection circuitry is then (see Figure 3). Placing a resistor R1 between these two functional. terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. D1 1N4002 Normally, this current is the specified minimum load (OPTIONAL) current of 10mA. The current out of the adjust pin adds to the current from R1 and is typically 55µA. Its output LT1585A-3.3 voltage contribution is small and only needs consider- VIN IN OUT VOUT + + C1 GND C2 ation when very precise output voltage setting is required. 10µF 100µF LT1585A D1 VIN IN OUT VOUT 1N4002 + + C1 C2 (OPTIONAL) ADJ VREF 10µF R1 100µF IADJ 55µA LT1585A R2 V V IN IN OUT VOUT + + OUT = VREF (1 + R2/R1) + IADJ (R2) C1 ADJ C2 LT1585A • F03 R1 10µF 100µF +
Figure 3. Basic Adjustable Regulator
R2 CADJ LT1585A • F02
Figure 2
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U U W U APPLICATIONS INFORMATION
A protection diode between the input and output pins is
Ripple Rejection
usually not needed. An internal diode between the input The typical curve for ripple rejection reflects values for the and output pins on the LT1585A/LT1585A-3.3 can handle LT1585A-3.3 fixed output voltage part. In applications microsecond surge currents of 50A to 100A. Even with that require improved ripple rejection, use the adjustable large value output capacitors it is difficult to obtain those device. A bypass capacitor from the adjust pin to ground values of surge currents in normal operation. Only with reduces the output ripple by the ratio of V large values of output capacitance, such as 1000µF to OUT/1.25V. The impedance of the adjust pin capacitor at the ripple fre- 5000µF, and with the input pin instantaneously shorted to quency should be less than the value of R1 (typically in the ground can damage occur. A crowbar circuit at the input range of 100Ω to 120Ω) in the feedback divider network of the LT1585A/LT1585A-3.3 can generate those levels of in Figure 2. Therefore, the value of the required adjust pin current, and a diode from output to input is then recom- capacitor is a function of the input ripple frequency. For mended. This is shown in Figure 2. Usually, normal power example, if R1 equals 100Ω and the ripple frequency supply cycling or system “hot plugging and unplugging” equals 120Hz, the adjust pin capacitor should be 22µF. At will not generate current large enough to do any damage. 10kHz, only 0.22µF is needed. The adjust pin can be driven on a transient basis ±7V with respect to the output, without any device degradation. As
Output Voltage
with any IC regulator, exceeding the maximum input-to- The LT1585A adjustable regulator develops a 1.25V ref- output voltage differential causes the internal transistors erence voltage between the output pin and the adjust pin to break down and none of the protection circuitry is then (see Figure 3). Placing a resistor R1 between these two functional. terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. D1 1N4002 Normally, this current is the specified minimum load (OPTIONAL) current of 10mA. The current out of the adjust pin adds to the current from R1 and is typically 55µA. Its output LT1585A-3.3 voltage contribution is small and only needs consider- VIN IN OUT VOUT + + C1 GND C2 ation when very precise output voltage setting is required. 10µF 100µF LT1585A D1 VIN IN OUT VOUT 1N4002 + + C1 C2 (OPTIONAL) ADJ VREF 10µF R1 100µF IADJ 55µA LT1585A R2 V V IN IN OUT VOUT + + OUT = VREF (1 + R2/R1) + IADJ (R2) C1 ADJ C2 LT1585A • F03 R1 10µF 100µF +
Figure 3. Basic Adjustable Regulator
R2 CADJ LT1585A • F02
Figure 2
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