LTC1844 Просмотр технического описания (PDF) - Linear Technology
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LTC1844 Datasheet PDF : 12 Pages
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LTC1844 Series
APPLICATIO S I FOR ATIO
The LTC1844 family are a series of 150mA ultralow
dropout regulators with micropower quiescent current
and shutdown. The devices are capable of supplying
150mA at a dropout voltage of 90mV (LTC1844-3.3, see
Electrical Characteristics for dropout voltage of other
versions). Output voltage noise is as low as 30µVRMS over
a 10Hz to 100kHz bandwidth with the addition of a 0.1µF
bypass capacitor. The low operating quiescent current
(35µA) drops to 10nA in shutdown.
In addition to the low quiescent current, the LTC1844
regulators incorporate several protection features which
make them ideal for use in battery-powered systems. The
devices are protected against both reverse input voltages
and reverse voltages from output to input (reverse current
protection). The devices also include current limit and
thermal overload protection, and will survive an output
short circuit indefinitely. The fast transient response over-
comes the traditional tradeoff between dropout voltage,
quiescent current and load transient response inherent in
most regulators by using a proprietary new architecture
(see Figure 1).
Adjustable Operation
The adjustable version of the LTC1844 has an output
voltage range of 1.25V to 6V. 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 1.25V (referenced to ground). The current in R1 is then
equal to 1.25V/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 1M 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 once COUT is
discharged.
Adjustable devices are tested and specified with the ADJ
pin tied to the OUT pin for an output voltage of 1.25V.
Specifications for output voltages greater than 1.25V will
be proportional to the ratio of the desired output voltage to
1.25V: VOUT/1.25V. For example, load regulation for an
VIN = 3V
0.04 CIN = 1µF
0.02 COUT = 1µF
0
–0.02
–0.04
50
0
0 10 20 30 40 50 60 70 80 90 100
TIME (µs)
1844 F01
Figure 1. LTC1844-2.5 Transient Response 1mA to 50mA to 1mA
IN
OUT
VIN
LTC1844
R2
ADJ
GND
R1
+
CFF
VOUT
1844 F02
VOUT = 1.25V1+ RR21 + (IADJ)(R2)
VADJ = 1.25V
IADJ = 30nA AT 25°C
OUTPUT RANGE = 1.25V TO 6V
CFF OPTIONAL
Figure 2. Adjustable Operation
1844ia
6
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