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