lt1763cs8-trpbf Linear Technology Corporation, lt1763cs8-trpbf Datasheet - Page 13

• Manufacturer Part Number: lt1763cs8-trpbf
• Description: 500ma, Low Noise, Ldo Micropower Regulators
• Manufacturer: Linear Technology Corporation

APPLICATIONS
The LT1763 series are 500mA low dropout regulators with
micropower quiescent current and shutdown. The devices
are capable of supplying 500mA at a dropout voltage of
300mV. Output voltage noise can be lowered to 20µV
over a 10Hz to 100kHz bandwidth with the addition of a
0.01µF 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 (30µA)
drops to less than 1µA in shutdown. In addition to the low
quiescent current, the LT1763 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
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-
turned 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 LT1763 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 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 Current vs
Temperature appear in the Typical Performance Charac-
teristics section.
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 to
U
INFORMATION
U
W
U
RMS
1.22V: V
output current change of 1mA to 500mA is – 2mV typical
at V
Bypass Capacitance and Low Noise Performance
The LT1763 regulators may be used with the addition of a
bypass capacitor from V
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 lower the
output voltage noise to as low as 20µV
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 LT1763-5 Transient Response in the Typical Perfor-
mance Characteristics). 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.
(12V/1.22V)(–2mV) = – 19.6mV
V
OUT
IN
IN
= 1.22V. At V
OUT
LT1763
GND
/1.22V. For example, load regulation for an
OUT
ADJ
Figure 2. Adjustable Operation
OUT
R2
R1
OUT
1763 F02
= 12V, load regulation is:
+
to the BYP pin to lower output
LT1763 Series
V
OUT
V
V
I
OUTPUT RANGE = 1.22V TO 20V
ADJ
OUT
ADJ
=
=
=
30
1 22
1 22 1
.
.
nA
V
V
AT 25 C
RMS
⎛
⎜
⎝
+
R
R
°
2
1
⎞
⎟ +
⎠
with the
13
(
I
ADJ
1763fc
)( )
R
2