LT1762 LINER [Linear Technology], LT1762 Datasheet - Page 11

• Manufacturer Part Number: LT1762
• Description: 150mA, Low Noise, LDO Micropower Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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
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 (25 A)
drops to less than 1 A in shutdown. In addition to the low
quiescent current, the LT1762 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 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 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 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 ADJ pin voltage
V
IN
Figure 2. Adjustable Operation
I
V
V
OUTPUT RANGE = 1.22V TO 20V
IN
ADJ
OUT
ADJ
U
LT1762
30
GND
1 22
1 22 1
.
.
nA
V
V
INFORMATION
AT 25 C
OUT
ADJ
U
R
R
2
1
R2
I
R1
ADJ
W
R
+
2
1762 F02
V
OUT
U
RMS
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
1.22V: V
output current change of 1mA to 150mA is –1mV typical
at V
Bypass Capacitance and Low Noise Performance
The LT1762 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 20 V
addition of a 0.01 F bypass capacitor. Using a bypass
capacitor has the added benefit of improving transient
response. With no bypass capacitor and a 10 F output
capacitor, a 10mA to 150mA load step will settle to within
1% of its final value in less than 100 s. With the addition
of a 0.01 F bypass capacitor, the output will stay within
1% for a 10mA to 150mA load step (see LT1762-5
Transient Response in the Typical Performance Charac-
teristics). However, regulator start-up time is inversely
proportional to the size of the bypass capacitor, slowing to
15ms with a 0.01 F bypass capacitor and 10 F output
capacitor.
(12V/1.22V)(–1mV) = – 9.8mV
OUT
= 1.22V. At V
OUT
/1.22V. For example, load regulation for an
OUT
OUT
= 12V, load regulation is:
to the BYP pin to lower output
LT1762 Series
RMS
with the
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