Datasheet LT1521, LT1521-3, LT1521-3.3, LT1521-5 (Analog Devices) - 9

LT1521/LT1521-3 LT1521-3.3/LT1521-5
U U U PIN FUNCTIONS
R 5 1 P VBE). This allows the shutdown pin to be driven directly by IN OUT 5V logic or by open collector logic with a pull-up resistor. LT1521 The pull-up resistor is only required to supply the leakage + + 4 2 V LOAD current of the open collector gate, normally several micro- IN SHDN SENSE GND amperes. Pull-up current must be limited to a maximum of 3 5mA. A curve of the shutdown pin input current as a RP function of voltage appears in the Typical Performance LT1521 • F01 Characteristics. If the shutdown pin is not used it can be left open circuit. The device will be active (output on) if the
Figure 1. Kelvin Sense Connection
shutdown pin is not connected.
IN (Pin 8):
Power is supplied to the device through the
ADJ (Pin 2):
For adjustable LT1521, the adjust pin is the input pin. The input pin should be bypassed to ground if input to the error amplifier. This pin is internally clamped the device is more than six inches away from the main to 6V and – 0.6V (one VBE). It has a bias current of 50nA input filter capacitor. In general, the output impedance of which flows into the pin. See Adjust Pin Bias Current vs battery rises with frequency, so it is advisable to include a Temperature in the Typical Performance Characteristics bypass capacitor in battery-powered circuits. A bypass section. The adjust pin reference voltage is 3.75V refer- capacitor in the range of 1µF to 10µF is sufficient. The enced to ground. The output voltage range that can be LT1521 is designed to withstand reverse voltages on the produced by this device is 3.75V to 20V. input pin with respect to ground and the output pin. In the
SHDN (Pin 5):
The shutdown pin is used to put the device case of reversed input, which can happen if a battery is into shutdown. In shutdown the output of the device is plugged in backwards, the LT1521 will act as if there is a turned off. This pin is active low. The device will be shut diode in series with its input. There will be no reverse down if the shutdown pin is pulled low. The shutdown pin current flow into the LT1521 and no reverse voltage will current with the pin pulled to ground will be 1.7µA. The appear at the load. The device will protect both itself and shutdown pin is internally clamped to 7V and – 0.6V (one the load.
U U W U APPLICATIONS INFORMATION
The LT1521 is a 300mA low dropout regulator with
Adjustable Operation
micropower quiescent current and shutdown. The device The adjustable version of the LT1521 has an output is capable of supplying 300mA at a dropout of 0.5V and voltage range of 3.75V to 20V. The output voltage is set by operates with very low quiescent current (12µA). In shut- the ratio of two external resistors as shown in Figure 2. The down, the quiescent current drops to only 6µA. In addition device servos the output voltage to maintain the voltage at to the low quiescent current, the LT1521 incorporates the adjust pin at 3.75V. The current in R1 is then equal to several protection features which make it ideal for use in 3.75V/R1. The current in R2 is equal to the sum of the battery-powered systems. The device is protected against current in R1 and the adjust pin bias current. The adjust pin both reverse input voltages and reverse output voltages. bias current, 50nA at 25°C, flows through R2 into the In battery backup applications where the output can be adjust pin. The output voltage can be calculated using the held up by a backup battery when the input is pulled to formula in Figure 2. The value of R1 should be less than ground, the LT1521 acts like it has a diode in series with 400k to minimize errors in the output voltage caused by its output and prevents reverse current flow. the adjust pin bias current. Note that in shutdown the 1521335fb 9