LT6003CDC datasheet(14/20 Pages) LINER

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LT6003/LT6004/LT6005

600345f

APPLICATIONS INFORMATION

Supply Voltage

The positive supply of the LT6003/LT6004/LT6005 should

be bypassed with a small capacitor (about 0.01μF) within

an inch of the pin. When driving heavy loads, an additional

4.7μFelectrolyticcapacitorshouldbeused.Whenusingsplit

supplies, the same is true for the negative supply pin.

Rail-to-Rail Characteristics

The LT6003/LT6004/LT6005 are fully functional for an input

signal range from the negative supply to the positive sup-

ply. Figure 1 shows a simpliﬁed schematic of the ampliﬁer.

The input stage consists of two differential ampliﬁers, a

PNP stage Q3/Q6 and an NPN stage Q4/ Q5 that are active

over different ranges of the input common mode voltage.

The PNP stage is active for common mode voltages,

VCM, between the negative supply to approximately 0.9V

below the positive supply. As VCM moves closer towards

the positive supply, the transistor Q7 will steer Q2’s tail

current to the current mirror Q8/Q9, activating the NPN

differential pair. The PNP pair becomes inactive for the

rest of the input common mode voltage range up to the

positive supply.

The second stage is a folded cascode and current mir-

ror that converts the input stage differential signals into

a single ended output. Capacitor C1 reduces the unity

cross frequency and improves the frequency stability

without degrading the gain bandwidth of the ampliﬁer.

The complementary drive generator supplies current to

the output transistors that swing from rail to rail.

Input

Input bias current (IB) is minimized with cancellation

circuitry on both input stages. The cancellation circuitry

remains active when VCM is more than 300mV from either

rail. As VCM approaches V

– the cancellation circuitry turns

off and IB is determined by the tail current of Q2 and the

beta of the PNP input transistors. As VCM approaches V

devices in the cancellation circuitry saturate causing IB to

increase (in the nanoamp range). Input offset voltage errors

due to IB can be minimized by equalizing the noninverting

and inverting source impedances.

The input offset voltage changes depending on which input

stage is active; input offset voltage is trimmed on both

input stages, and is guaranteed to be 500μV max in the

PNP stage. By trimming the input offset voltage of both

input stages, the input offset voltage shift over the entire

common mode range (CMRR) is typically 160μV, maintain-

ing the precision characteristics of the ampliﬁer.

The input stage of the LT6003/LT6004/LT6005 incorpo-

rates phase reversal protection to prevent wrong polarity

outputs from occurring when the inputs are driven up to

9V below the negative rail. 600k protective resistors are

included in the input leads so that current does not become

excessive when the inputs are forced below V– or when

a large differential signal is applied. Input current should

be limited to 10mA when the inputs are driven above the

positive rail.

Output

The output of the LT6003/LT6004/LT6005 is guaranteed to

swing within 100mV of the positive rail and 50mV of the

negative rail with no load, over the industrial temperature

range. The LT6003/LT6004/LT6005 can typically source

8mA on a single 5V supply. Sourcing current is reduced

to 5mA on a single 1.8V supply as noted in the electrical

characteristics. However, when sourcing more than 250μA

with an output load impedance greater than 20kΩ, a 1μF

capacitor in series with a 2k resistor should be placed

from the output to ground to insure stability.

The normally reverse-biased substrate diode from the

output to V– will cause unlimited currents to ﬂow when

the output is forced below V–. If the current is transient

and limited to 100mA, no damage will occur.

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