Datasheet

LT1762 Series
12
Rev B
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The LT1762 series are 150mA low dropout regulators
with micropower quiescent current and shutdown. The
devices are capable of supplying 150mA at a dropout
voltage of 270mV. Output voltage noise can be lowered
to 20µV
RMS
over a 10Hz to 100kHz bandwidth with
the addition of a 0.01µF reference bypass capacitor.
Additionally, the reference bypass capacitor will improve
transient response of the regulator, lowering the set-
tling time for transient load conditions. The low operat-
ing quiescent current (25µA) drops to less than 1µA
in shutdown. In addition to the low quiescent current,
the LT1762 regulators incorporate several protection
features which make them ideal for use in battery-pow-
ered 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 LT1762-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 LT1762 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
APPLICATIONS INFORMATION
Figure2. Adjustable Operation
ADJ pin voltage at 1.22V referenced to ground. The cur-
rent 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 Characteristics 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 1.22V: V
OUT
/1.22V. For example, load regulation for an
output current change of 1mA to 150mA is –1mV typical
at V
OUT
= 1.22V. At V
OUT
= 12V, load regulation is:
(12V/1.22V)(–1mV) = –9.8mV
Bypass Capacitance and Low Noise Performance
The LT1762 regulators may be used with the addition
of a bypass capacitor from V
OUT
to the BYP pin to lower
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 lower the
output voltage noise to as low as 20µV
RMS
with the addi-
tion of a 0.01µF bypass capacitor. Using a bypass capaci-
tor has the added benefit of improving transient response.
With no bypass capacitor and a 10µF output capacitor,
a 10mA to 150mA 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 stay within 1%
for a 10mA to 150mA load step (see LT1762-5 Transient
Response in the Typical Performance 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.
IN
1762 F02
R2
OUT
V
IN
V
OUT
ADJ
GND
LT1762
R1
+
V
OUT
= 1.22V 1+
R2
R1
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
+ I
ADJ
( )
R2
( )
V
ADJ
= 1.22V
I
ADJ
= 30nA AT 25°C
OUTPUT RANGE = 1.22V TO 20V
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