DAC081S101
SNAS323C –JUNE 2005–REVISED FEBRUARY 2013
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The minimum resistor value in the circuit of Figure 42 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
DAC081S101 draws zero current. The maximum resistor value must allow the LM4050 to draw more than its
minimum current for regulation plus the maximum DAC081S101 current in full operation. 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 DAC081S101 draws its maximum current. These conditions can be
summarized as
R(min) = ( VIN(max) − VZ(min) / (IA(min) + IZ(max))
(3)
and
R(max) = ( VIN(min) − VZ(max) / (IA(max) + IZ(min) )
(4)
where VZ(min) and VZ(max) are the nominal LM4050 output voltages ± the LM4050 output tolerance over
temperature, IZ(max) is the maximum allowable current through the LM4050, IZ(min) is the minimum current
required by the LM4050 for proper regulation, IA(max) is the maximum DAC081S101 supply current, and IA(min)
is the minimum DAC081S101 supply current.
LP3985
The LP3985 is a low noise, ultra low dropout voltage regulator with a 3% accuracy over temperature. It is a good
choice for applications that do not require a precision reference for the DAC081S101. It comes in 3.0V, 3.3V and
5V versions, among others, and sports a low 30 µV noise specification 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 SOT-23 and 5-bump DSBGA packages.
Input
Voltage
LP3985
0.1 mF
1 mF
0.01 mF
DAC081S101
SYNC
V
OUT
= 0V to 5V
DIN
SCLK
Figure 43. Using the LP3985 regulator
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 5mΩ to 500mΩ is required at the output. Careful interpretation
and understanding of the capacitor specification is required to ensure correct device operation.
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.
V
IN
OFF
Input
Voltage
V
OUT
LP2980
ON /
1 mF
DAC081S101
SYNC
V
OUT
= 0V to 5V
DIN
SCLK
Figure 44. Using the LP2980 regulator
18
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