LT1763 Series
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PIN FUNCTIONS
Note that the voltage drop across the external PC traces
willaddtothedropoutvoltageoftheregulator. TheSENSE
pin bias current is 10µA at the nominal rated output
voltage. The SENSE pin can be pulled below ground (as in
a dual supply system where the regulator load is returned
to a negative supply) and still allow the device to start and
operate.
SHDN (Pin 5): Shutdown. The SHDN pin is used to put the
LT1763 regulators into a low power shutdown state. The
output will be off when the SHDN pin is pulled low. The
SHDN pin can be driven either by 5V logic or open-
collector logic with a pull-up resistor. The pull-up resistor
is required to supply the pull-up current of the open-
collector gate, normally several microamperes, and the
SHDN pin current, typically 1µA. If unused, the SHDN pin
must be connected to VIN. The device will be in the low
power shutdown state if the SHDN pin is not connected.
ADJ (Pin 2): Adjust. For the adjustable LT1763, this is the
input to the error amplifier. This pin is internally clamped
to ±7V. It has a bias current of 30nA which flows into the
pin (see curve of ADJ Pin Bias Current vs Temperature in
theTypicalPerformanceCharacteristicssection).TheADJ
pin voltage is 1.22V referenced to ground and the output
voltage range is 1.22V to 20V.
IN (Pin 8): Input. Power is supplied to the device through
the IN pin. A bypass capacitor is required on this pin if the
device is more than six inches away from the main input
filter capacitor. In general, the output impedance of a
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
LT1763 regulators are designed to withstand reverse
voltages on the IN pin with respect to ground and the OUT
pin. In the case of a reverse input, which can happen if a
battery is plugged in backwards, the device will act as if
there is a diode in series with its input. There will be no
reverse current flow into the regulator and no reverse
voltage will appear at the load. The device will protect both
itself and the load.
BYP (Pin 4): Bypass. The BYP pin is used to bypass the
reference of the LT1763 regulators to achieve low noise
performance from the regulator. The BYP pin is clamped
internally to ±0.6V (one VBE). A small capacitor from the
output to this pin will bypass the reference to lower the
output voltage noise. A maximum value of 0.01µF can be
usedforreducingoutputvoltagenoisetoatypical20µVRMS
over a 10Hz to 100kHz bandwidth. If not used, this pin
must be left unconnected.
GND (Pins 3, 6, 7): Ground.
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APPLICATIONS INFORMATION
TheLT1763seriesare500mAlowdropoutregulatorswith
micropowerquiescentcurrentandshutdown.Thedevices
are capable of supplying 500mA at a dropout voltage of
300mV. Output voltage noise can be lowered to 20µVRMS
over a 10Hz to 100kHz bandwidth with the addition of a
0.01µFreferencebypasscapacitor. Additionally, therefer-
ence bypass capacitor will improve transient response of
the regulator, lowering the settling time for transient load
conditions. The low operating quiescent current (30µA)
drops to less than 1µA in shutdown. In addition to the low
quiescentcurrent, theLT1763regulatorsincorporatesev-
eral protection features which make them ideal for use in
battery-powered systems. 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 LT1763-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 re-
turnedtoanegativesupply,theoutputcanbepulledbelow
groundbyasmuchas20Vandstillallowthedevicetostart
and operate.
Adjustable Operation
The adjustable version of the LT1763 has an output
voltage range of 1.22V to 20V. The output voltage is set by
theratiooftwoexternalresistorsasshowninFigure2.The
device servos the output to maintain the ADJ pin voltage
at 1.22V referenced to ground. The current in R1 is then
equalto1.22V/R1andthecurrentinR2isthecurrentinR1
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Linear [ Linear ]
