LM8261 datasheet(15/29 Pages) TI

15 / 29 page

LM8261

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SNOS469I – APRIL 2000 – REVISED MARCH 2013

APPLICATION HINTS

BLOCK DIAGRAM AND OPERATIONAL DESCRIPTION

A) Input Stage

Figure 51. Simplified Schematic Diagram

As can be seen from the simplified schematic in Figure 51, the input stage consists of two distinct differential

pairs (Q1-Q2 and Q3-Q4) in order to accommodate the full Rail-to-Rail input common mode voltage range. The

voltage drop across R5, R6, R7, and R8 is kept to less than 200mV in order to allow the input to exceed the

supply rails. Q13 acts as a switch to steer current away from Q3-Q4 and into Q1-Q2, as the input increases

beyond 1.4V of V+. This in turn shifts the signal path from the bottom stage differential pair to the top one and

causes a subsequent increase in the supply current.

In transitioning from one stage to another, certain input stage parameters (VOS, Ib, IOS, en, and in) are determined

based on which differential pair is "on" at the time. Input Bias current, IB, will change in value and polarity as the

input crosses the transition region. In addition, parameters such as PSRR and CMRR which involve the input

offset voltage will also be effected by changes in VCM across the differential pair transition region.

The input stage is protected with the combination of R9-R10 and D1, D2, D3, and D4 against differential input

over-voltages. This fault condition could otherwise harm the differential pairs or cause offset voltage shift in case

of prolonged over voltage. As shown in Figure 52, if this voltage reaches approximately ±1.4V at 25°C, the

diodes turn on and current flow is limited by the internal series resistors (R9 and R10). The Absolute Maximum

Rating of ±10V differential on VIN still needs to be observed. With temperature variation, the point were the

diodes turn on will change at the rate of 5mV/°C.

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