2.0 Applications Information

(Continued)

2.2.2 LM4050

Available with accuracy of 0.44%, the LM4050 shunt refer-

ence is also a good choice as a power regulator for the

DAC101S101. It does not come ina3Volt version, but

4.096V and 5V versions are available. It comes in a space-

saving 3-pin SOT23.

The minimum resistor value in the circuit of Figure 10 should

be chosen such that the maximum current through the

LM4050 does not exceed its 15 mA rating. The conditions for

maximum current include the input voltage at its maximum,

the LM4050 voltage at its minimum, the resistor value at its

minimum due to tolerance, and the DAC101S101 draws

zero current. The maximum resistor value must allow the

LM4050 to draw more than its minimum current for regula-

tion plus the maximum DAC101S101 current in full opera-

tion. The conditions for minimum current include the input

voltage at its minimum, the LM4050 voltage at its maximum,

the resistor value at its maximum due to tolerance, and the

DAC101S101 draws its maximum current. These conditions

can be summarized as

R(min)=(V

and

R(max)=(V

where V

I

is the maximum allowable current through the

LM4050, I

LM4050 for proper regulation, I

is the maximum

DAC101S101 supply current, and I

DAC101S101 supply current.

2.2.3 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 DAC101S101. 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.

2.2.4 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-

20154113

FIGURE 9. The LM4130 as a power supply

20154114

FIGURE 10. The LM4050 as a power supply

20154115

FIGURE 11. Using the LP3985 regulator

20154116

FIGURE 12. Using the LP2980 regulator

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