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LT1529CT Datasheet(PDF) 8 Page - Linear Technology |
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LT1529CT Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LT1529 LT1529-3.3/LT1529-5 PI FU CTIO S function of voltage appears in the Typical Performance Characteristics. If the SHDN pin is not used it can be left open circuit. The device will be active, output on, if the SHDN pin is not connected. VIN (Pin 5): Input Pin. Power is supplied to the device through the VIN pin. The VIN pin should be bypassed to ground if the device is more than six inches away from the main input filter capacitor. In general, the output imped- ance of a battery rises with frequency so it is advisable to S APPLICATI I FOR ATIO The LT1529 is a 3A low dropout regulator with mi- cropower quiescent current and shutdown capable of supplying 3A of output current at a dropout voltage of 0.6V. The device operates with very low quiescent current (50 µA). In shutdown the quiescent current drops to only 16 µA. In addition to the low quiescent current the LT1529 incorporates several protection features which make it ideal for use in battery-powered systems. The device is protected against reverse input voltages. In battery backup applications where the output can be held up by a backup battery when the input is pulled to ground, the LT1529 acts like it has a diode in series with its output and prevents reverse current flow. Adjustable Operation The adjustable version of the LT1529 has an output voltage range of 3.75V to 14V. The output voltage is set by the ratio of two external resistors as shown in Figure 2. The device servos the output voltage to maintain the voltage at the ADJ pin at 3.75V. The current in R1 is then equal to 3.75V/R1. The current in R2 is equal to the sum of the current in R1 and the ADJ pin bias current. The ADJ pin bias current, 150nA at 25 °C, flows through R2 into the ADJ pin. The output voltage can be calculated according to the formula in Figure 2. The value of R1 should be less than 400k 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. Curves of ADJ Pin Voltage vs Temperature and ADJ Pin include a bypass capacitor in battery-powered circuits. A bypass capacitor in the range of 1 µF to 10µF is sufficient. The LT1529 is designed to withstand reverse voltages on the VIN pin with respect to ground and OUTPUT pin. In the case of a reversed input, which can happen if a battery is plugged in backwards, the LT1529 will act as if there is a diode in series with its input. There will be no reverse current flow into the LT1529 and no reverse voltage will appear at the load. The device will protect both itself and the load. + VIN VOUT = 3.75V + (IADJ × R2) VIN VOUT R2 R1 1 2 3 LT1529 • F02 5 4 OUTPUT SENSE LT1529 SHDN GND () 1 + R2 R1 VADJ = 3.75V IADJ = 150nA AT 25°C OUTPUT RANGE = 3.3V TO 14V Figure 2. Adjustable Operation Bias Current vs Temperature appear in the Typical Perfor- mance Characteristics. The reference voltage at the ADJ pin has a positive temperature coefficient of approxi- mately 15ppm/ °C.TheADJpinbiascurrenthasanegative temperature coefficient. These effects will tend to cancel each other. The adjustable device is specified with the ADJ pin tied to the OUTPUT pin. This sets the output voltage to 3.75V. Specifications for output voltage greater than 3.75V will be proportional to the ratio of the desired output voltage to 3.75V (VOUT/3.75V). For example: load regulation for an output current change of 1mA to 3A is – 0.5mV typical at VOUT = 3.75V. At VOUT = 12V, load regulation would be: 12 375 05 16 V V mV mV . –. – . () = () |
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