LT1962 Series
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applicaTions inForMaTion
The LT1962 series are 300mA low dropout regulators with
micropower quiescent current and shutdown. The devices
are capable of supplying 300mA at a dropout voltage of
300mV. 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 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 LT1962 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 LT1962-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 LT1962 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 cur
-
rent, 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.
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 300mA is –2
mV typical
at V
OUT
= 1.22V. At V
OUT
= 12V, load regulation is:
(12V/1.22V)(–2mV) = –19.7mV
Bypass Capacitance and Low Noise Performance
The LT1962 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 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
300mA 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 300mA load step in less than 10µs, with total output
voltage deviation of less than 2% (see LT1962-5 Transient
Response in the Typical Performance Characteristics sec
-
tion). 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.
Output Capacitance and T
ransient Response
The LT1962 regulators are designed to be stable with a
wide range of output capacitors. The ESR of the output
capacitor affects stability, most notably with small capaci
-
tors. A minimum output capacitor of 3.3µF with an ESR
of 3Ω or less is recommended to prevent oscillations.
Figure 2. Adjustable Operation
IN
R2
OUT
V
IN
V
ADJ
GND
LT1962
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
