LT1763 Series
APPLICATIONS INFORMATION
The LT1763 series are 500mA low dropout regulators with
micropowerquiescentcurrentandshutdown.Thedevices
are capable of supplying 500mA at a dropout voltage of
to 1.22V: V /1.22V. For example, load regulation for an
OUT
output current change of 1mA to 500mA is –2mV typical
at V
= 1.22V. At V
= 12V, load regulation is:
OUT
OUT
300mV. Output voltage noise can be lowered to 20μV
RMS
(12V/1.22V)(–2mV) = –19.6mV
over a 10Hz to 100kHz bandwidth with the addition of
a 0.01μF reference bypass capacitor. Additionally, the
referencebypasscapacitorwillimprovetransientresponse
oftheregulator,loweringthesettlingtimefortransientload
conditions. The low operating quiescent current (30μA)
drops to less than 1μA in shutdown. In addition to the
low quiescent current, the LT1763 regulators incorporate
several 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
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.
IN
OUT
ADJ
V
OUT
+
V
IN
R2
LT1763
GND
R2
⎞
⎟
⎠
R1
⎛
⎝
VOUT = 1.22V 1+
+ I R2
ADJ)( )
(
⎜
V
ADJ = 1.22V
ADJ = 30nA AT 25°C
OUTPUT RANGE = 1.22V TO 20V
R1
I
1763 F02
Figure 2. Adjustable Operation
Bypass Capacitance and Low Noise Performance
The LT1763 regulators may be used with the addition of
a bypass capacitor from V
to the BYP pin to lower
OUT
output voltage noise. A good quality low leakage capacitor
is recommended. This capacitor will bypass the reference
of the regulator, providing a low frequency noise pole.
The noise pole provided by this bypass capacitor will
Adjustable Operation
lower the output voltage noise to as low as 20μV
TheadjustableversionoftheLT1763hasanoutputvoltage
range of 1.22V to 20V. The output voltage is set by the
ratio of two external resistors, as shown in Figure 2. The
device servos the output to maintain the ADJ pin voltage
at 1.22V referenced to ground. The current in R1 is then
equal to 1.22V/R1 and the current in R2 is the current
in R1 plus the ADJ pin bias current. The ADJ pin bias
current, 30nA at 25°C, flows through R2 into the ADJ pin.
The output voltage can be calculated using the formula in
Figure 2. The value of R1 should be no greater than 250k
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 Bias
CurrentvsTemperatureappearintheTypicalPerformance
Characteristics section.
RMS
with the addition of a 0.01μF bypass capacitor. Using
a bypass capacitor has the added benefit of improving
transient response. With no bypass capacitor and a 10μF
output capacitor, a 10mA to 500mA load step will settle
to within 1% of its final value in less than 100μs. With
the addition of a 0.01μF bypass capacitor, the output will
settle to within 1% for a 10mA to 500mA load step in less
than 10μs, with total output voltage deviation of less than
2.5% (see the LT1763-5 Transient Response curve in the
Typical Performance Characteristics section). However,
regulator start-up time is proportional to the size of the
bypass capacitor, slowing to 15ms with a 0.01μF bypass
capacitor and 10μF output capacitor.
The adjustable device is tested and specified with the ADJ
pin tied to the OUT pin for an output voltage of 1.22V.
Specifications for output voltages greater than 1.22V will
be proportional to the ratio of the desired output voltage
1763fe
14
Linear [ Linear ]
