LT1962 Linear Technology, LT1962 Datasheet - Page 5

• Manufacturer Part Number: LT1962
• Description: 300mA/ Low Noise/ Micropower LDO Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 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 (30 A)
drops to less than 1 A in shutdown. In addition to the low
quiescent current, the LT1962 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 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 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 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 current,
30nA at 25 C, flows through R2 into the ADJ pin. The
V
IN
Figure 2. Adjustable Operation
IN
I
V
V
OUTPUT RANGE = 1.22V TO 20V
ADJ
OUT
ADJ
U
LT1962
GND
30
1 22
1 22 1
.
.
nA
OUT
V
ADJ
U
V
AT 25 C
R
R
2
1
R2
R1
I
ADJ
W
+
R
2
1962 F02
V
OUT
U
RMS
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
output current change of 1mA to 300mA is – 2mV typical
at V
Bypass Capacitance and Low Noise Performance
The LT1962 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 300mA 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 settle to
within 1% for a 10mA to 300mA load step in less than
10 s, with total output voltage deviation of less than 2.5%.
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.
Output Capacitance and Transient 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 3.3 F with an ESR of
3 or less is recommended to prevent oscillations. The
LT1962-X is a micropower device and output transient
response will be a function of output capacitance. Larger
values of output capacitance decrease the peak deviations
(12V/1.22V)(–2mV) = – 19.7mV
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
LT1962 Series
RMS
with the
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