6

LTC1844 Series

1844ia

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 30

a 10Hz to 100kHz bandwidth with the addition of a 0.1

µF

bypass capacitor. The low operating quiescent current

(35

µA) 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

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

Figure 2. Adjustable Operation

TIME (

µs)

0

–0.02

0.02

80

1844 F01

50

–0.04

0

0.04

0

20

10

40

30

60 70

90

50

100

Figure 1. LTC1844-2.5 Transient Response 1mA to 50mA to 1mA

IN

1844 F02

R2

LTC1844

OUT

ADJ

GND

R1

+

VV

R

R

IR

VV

InA

OUT

ADJ

ADJ

ADJ

=+









+

()( )

=

=°

125

1

2

1

2

125

30

.

.

AT 25 C

OUTPUT RANGE = 1.25V TO 6V

C

OPTIONAL

FF