6 Detailed Description

6.1 Overview

The LPV521 is fabricated with Texas Instruments' state-of-the-art VIP50 process. This proprietary process

dramatically improves the performance of Texas Instruments' low-power and low-voltage operational amplifiers.

The following sections showcase the advantages of the VIP50 process and highlight circuits that enable ultra-low

power consumption.

6.2 Functional Block Diagram

Figure 6-1. Block Diagram

6.3 Feature Description

The amplifier differential inputs consist of a noninverting input (+IN) and an inverting input (–IN). The amplifier

amplifies only the difference in voltage between the two inputs, which is called the differential input voltage. The

output voltage of the op-amp VOUT is given by Equation 1:

VOUT = AOL (IN+ – IN–)

(1)

where AOL is the open-loop gain of the amplifier, typically around 132dB (4,000,000 ×, or 0.25µV/V).

6.4 Device Functional Modes

6.4.1 Input Stage

The LPV521 has a rail-to-rail input that provides more flexibility for the system designer. Rail-to-rail input is

achieved by using in parallel, one PMOS differential pair and one NMOS differential pair. When the common

mode input voltage (VCM) is near V+, the NMOS pair is on and the PMOS pair is off. When VCM is near V−, the

NMOS pair is off and the PMOS pair is on. When VCM is between V+ and V−, internal logic decides how much

current each differential pair get. This special logic maintains stable and low-distortion amplifier operation within

the entire common-mode voltage range.

Both input stages have an offset voltage (VOS) characteristic; therefore, the offset voltage of the LPV521

becomes a function of VCM. VOS has a crossover point at 1.0V less than V+. See the Input Offset Voltage

vs Input Common Mode curves in the Typical Characteristics. Take care in situations where the input signal

amplitude is comparable to the VOS value or the design requires high accuracy. In these situations, the input

signal must avoid the crossover point. In addition, parameters such as PSRR and CMRR that involve the input

offset voltage are also affected by changes in VCM across the differential-pair transition region.

6.4.2 Output Stage

The LPV521 output voltage swings 3mV from rails at a 3.3V supply, which provides the maximum possible

dynamic range at the output. This feature is particularly important when operating on low supply voltages.

The LPV521 maximum output voltage swing defines the maximum swing possible under a particular output load.

The LPV521 output swings 50mV from the rail at a 5V supply with an output load of 100kΩ.

LPV521

SNOSB14E – AUGUST 2009 – REVISED JULY 2024

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