LT1761ES5-1.5 Linear Technology, LT1761ES5-1.5 Datasheet - Page 12

• Manufacturer Part Number: LT1761ES5-1.5
• Description: 100mA/ Low Noise/ LDO Micropower Regulators in SOT-23
• Manufacturer: Linear Technology

PI FU CTIO S
APPLICATIO S I FOR ATIO
The LT1761 series are 100mA low dropout regulators with
micropower quiescent current and shutdown. The devices
are capable of supplying 100mA at a dropout voltage of
300mV. Output voltage noise can be lowered to 20 V
over a 10Hz to 100kHz bandwidth with the addition of a
0.01 F reference bypass capacitor. Additionally, the refer-
ence bypass capacitor will improve transient response of
the regulator, lowering the settling time for transient load
conditions. The low operating quiescent current (20 A)
drops to less than 1 A in shutdown. In addition to the low
quiescent current, the LT1761 regulators incorporate sev-
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
LT1761 Series
IN (Pin 1): 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 1 F to 10 F is sufficient. The
LT1761 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.
GND (Pin 2): Ground.
SHDN (Pin 3, Fixed/-SD Devices): Shutdown. The SHDN
pin is used to put the LT1761 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 1 A. If unused, the
SHDN pin must be connected to V
12
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IN
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. The device will not
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RMS
function if the SHDN pin is not connected. For the
LT1761-BYP, the SHDN pin is internally connected to V
BYP (Pins 3/4, Fixed/-BYP Devices): Bypass. The BYP
pin is used to bypass the reference of the LT1761 regula-
tors to achieve low noise performance from the regulator.
The BYP pin is clamped internally to 0.6V (one V
ground. A small capacitor from the output to this pin will
bypass the reference to lower the output voltage noise. A
maximum value of 0.01 F can be used for reducing output
voltage noise to a typical 20 V
bandwidth. If not used, this pin must be left unconnected.
ADJ (Pin 4, Adjustable Devices Only): Adjust Pin. For the
adjustable LT1761, 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 the Typical Performance Char-
acteristics section). The ADJ pin voltage is 1.22V referenced
to ground and the output voltage range is 1.22V to 20V.
OUT (Pin 5): Output. The output supplies power to the
load. A minimum output capacitor of 1 F is required to
prevent oscillations. Larger output capacitors will be
required for applications with large transient loads to limit
peak voltage transients. See the Applications Information
section for more information on output capacitance and
reverse output characteristics.
up by a backup battery when the input is pulled to ground,
the LT1761-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 LT1761 has an output
voltage range of 1.22V to 20V. The output voltage is set by
the ratio of two external resistors as shown in Figure 1. The
device servos the output to maintain the ADJ pin voltage
at 1.22V referenced to ground. The current in R1 is then
RMS
over a 10Hz to 100kHz
BE
) from
IN
.