2.0 Applications Information

(Continued)

LM4050, I

LM4050 for proper regulation, and I

mum DAC104S085 supply current.

2.1.3 LP3985

The LP3985 is a low noise, ultra low dropout voltage regu-

lator with a 3% accuracy over temperature. It is a good

choice for applications that do not require a precision refer-

ence for the DAC104S085. It comes in 3.0V, 3.3V and 5V

versions, among others, and sports a low 30 µV noise speci-

fication at low frequencies. Since low frequency noise is

relatively difficult to filter, this specification could be important

for some applications. The LP3985 comes in a space-saving

5-pin SOT23 and 5-bump micro SMD packages.

An input capacitance of 1.0µF without any ESR requirement

is required at the LP3985 input, while a 1.0µF ceramic

capacitor with an ESR requirement of 5m

Ω to 500mΩ is

required at the output. Careful interpretation and under-

standing of the capacitor specification is required to ensure

correct device operation.

2.1.4 LP2980

The LP2980 is an ultra low dropout regulator with a 0.5% or

1.0% accuracy over temperature, depending upon grade. It

is available in 3.0V, 3.3V and 5V versions, among others.

Like any low dropout regulator, the LP2980 requires an

output capacitor for loop stability. This output capacitor must

be at least 1.0µF over temperature, but values of 2.2µF or

more will provide even better performance. The ESR of this

capacitor should be within the range specified in the LP2980

data sheet. Surface-mount solid tantalum capacitors offer a

good combination of small size and ESR. Ceramic capaci-

tors are attractive due to their small size but generally have

ESR values that are too low for use with the LP2980. Alumi-

num electrolytic capacitors are typically not a good choice

due to their large size and have ESR values that may be too

high at low temperatures.

2.2 BIPOLAR OPERATION

The DAC104S085 is designed for single supply operation

and thus has a unipolar output. However, a bipolar output

may be obtained with the circuit in Figure 9. This circuit will

amplifier should be used if the amplifier supplies are limited

The output voltage of this circuit for any code is found to be

V

where D is the input code in decimal form. With V

R1 = R2,

V

A list of rail-to-rail amplifiers suitable for this application are

indicated in Table 2.

TABLE 2. Some Rail-to-Rail Amplifiers

AMP

PKGS

Typ V

OS

Typ I

SUPPLY

LMC7111

DIP-8

SOT23-5

0.9 mV

25 µA

LM7301

SO-8

SOT23-5

0.03 mV

620 µA

LM8261

SOT23-5

0.7 mV

1 mA

20195315

FIGURE 7. Using the LP3985 regulator

20195316

FIGURE 8. Using the LP2980 regulator

20195317

FIGURE 9. Bipolar Operation

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