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LT1963AES8 数据表(PDF) 14 Page - Linear Dimensions Semiconductor |
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LT1963AES8 数据表(HTML) 14 Page - Linear Dimensions Semiconductor |
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14 / 28 page  LT1963A Series 14 1963afe The LT1963A series are 1.5A low dropout regulators opti- mized for fast transient response. The devices are capable of supplying 1.5A at a dropout voltage of 350mV. The low operating quiescent current (1mA) drops to less than 1µA in shutdown. In addition to the low quiescent current, the LT1963Aregulatorsincorporateseveralprotectionfeatures whichmakethemidealforuseinbattery-poweredsystems. The devices are protected against both reverse input and reverse output voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1963A-X acts like it has a diode in series with its output and prevents reverse current flow. Additionally, in dual supply applications where the regulator load is returned to a negative supply, the output can be pulled below ground by as much as 20V and still allow the device to start and operate. Adjustable Operation The adjustable version of the LT1963A has an output voltage range of 1.21V to 20V. The output voltage is set by the ratio of two external resistors as shown in Figure 2. The device servos the output to maintain the voltage at the ADJ pin at 1.21V referenced to ground. The current in R1 is then equal to 1.21V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 3µA at 25°C, flows through R2 into the ADJ pin. The output voltage can be calculated using the formula in Figure 2. The value of R1 should be less than 4.17k to minimize errors in the output voltage caused by the ADJ pin bias current. Note that in shutdown the output is turned off and the divider current will be zero. The adjustable device is tested and specified with the ADJ pin tied to the OUT pin for an output voltage of 1.21V. Specifications for output voltages greater than 1.21V will be proportional to the ratio of the desired output voltage to 1.21V: VOUT/1.21V. For example, load regulation for an output current change of 1mA to 1.5A is –3mV typical at VOUT = 1.21V. At VOUT = 5V, load regulation is: (5V/1.21V)(–3mV) = –12.4mV Output Capacitors and Stability The LT1963A regulator is a feedback circuit. Like any feedback circuit, frequency compensation is needed to make it stable. For the LT1963A, the frequency compensa- tion is both internal and external—the output capacitor. The size of the output capacitor, the type of the output capacitor, and the ESR of the particular output capacitor all affect the stability. In addition to stability, the output capacitor also affects the high frequency transient response. The regulator loop has a finite band width. For high frequency transient loads, recovery from a transient is a combination of the output capacitor and the bandwidth of the regulator. The LT1963A was designed to be easy to use and accept a wide variety of output capacitors. However, the frequency compensation is affected by the output capacitor and opti- mumfrequencystabilitymayrequiresomeESR,especially with ceramic capacitors. Foreaseofuse,lowESRpolytantalumcapacitors(POSCAP) are a good choice for both the transient response and stability of the regulator. These capacitors have intrinsic ESR that improves the stability. Ceramic capacitors have extremely low ESR, and while they are a good choice in many cases, placing a small series resistance element will sometimes achieve optimum stability and minimize ringing. In all cases, a minimum of 10µF is required while the maximum ESR allowable is 3Ω. The place where ESR is most helpful with ceramics is low output voltage. At low output voltages, below 2.5V, some ESR helps the stability when ceramic output capaci- tors are used. Also, some ESR allows a smaller capaci- tor value to be used. When small signal ringing occurs with ceramics due to insufficient ESR, adding ESR or increasing the capacitor value improves the stability and reduces the ringing. Table 1 gives some recommended values of ESR to minimize ringing caused by fast, hard current transitions. IN 1963A F02 R2 OUT VIN VOUT ADJ GND LT1963A R1 + Figure 2. Adjustable Operation VOUT =1.21V 1+ R2 R1 + IADJ ( ) R2 ( ) VADJ =1.21V IADJ =3µA AT 25°C OUTPUT RANGE = 1.21V TO 20V applicaTions inForMaTion |
类似零件编号 - LT1963AES8 |
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类似说明 - LT1963AES8 |
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