VOUT

VREF

R1

R2

VIN

+

-

O

1

2

V

R

R

1

1.25V

é

ù

=

-

ê

ú

ë

û

VO = VREF

R1

R2

+ 1

12

LP3982

SNVS185F – FEBRUARY 2002 – REVISED APRIL 2017

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8.2.2.2 Output Voltage Setting (ADJ Version Only)

The output voltage is set according to the amount of negative feedback (the pass transistor inverts the feedback

signal.) Figure 15 simplifies the topology of the LP3982. This type of regulator can be represented as an op amp

configured as non-inverting amplifier and a fixed DC Voltage (VREF) for its input signal. The special characteristic

of this op amp is its extra-large output transistor that only sources current. In terms of its non-inverting

configuration, the output voltage equals VREF times the closed loop gain:

(5)

Use Equation 6 for adjusting the output to a particular voltage:

(6)

Choose R2 = 100 kΩ to optimize accuracy, power supply rejection, noise, and power consumption.

Figure 15. Regulator Topology Simplified

Similarity in the output capabilities exists between op amps and linear regulators. Just as rail-to-rail output op

amps allow their output voltage to approach the supply voltage, low dropout regulators (LDOs) allow their output

voltage to operate close to the input voltage. Both achieve this by the configuration of their output transistors.

Standard operational amplifiers and regulator outputs are at the source (or emitter) of the output transistor. Rail-

to-rail op amp and LDO regulator outputs are at the drain (or collector) of the output transistor. This replaces the

threshold (or diode drop) limitations on the output with the less restrictive source-to-drain (or VSAT) limitations.

There is a trade-off; the output impedance become significantly higher, thus providing a critically lower pole when

combined with the capacitive load. That is why rail-to-rail operational amplifiers are usually poor at driving

capacitive loads and a series output resistor recommended when doing so. LDOs require the same series

resistance except that the internal resistance of the output capacitor will usually suffice. Refer to the Output

Capacitance section for more information.

8.2.2.3 Output Capacitance

The LP3982 is specifically designed to employ ceramic output capacitors as low as 2.2 μF. Ceramic capacitors

below 10 μF offer significant cost and space savings, along with high frequency noise filtering. Higher values and

other types and of capacitor may be used, but their equivalent series resistance (ESR) must be maintained below

0.5

Ω.