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LT6236/LT6237

623637f

APPLICATIONS INFORMATION

ESD

The LT6236/LT6237 have reverse-biased ESD protection

diodes on all inputs and outputs as shown in Figure 1.

If these pins are forced beyond either supply, unlimited

current will flow through these diodes. If the current is

transient and limited to 100mA or less, no damage to the

device will occur.

Noise

The noise voltage of the LT6236/LT6237 is equivalent to

that of a 75Ω resistor, and for the lowest possible noise it

is desirable to keep the source and feedback resistance

at or below this value, i.e. RS + RG||RFB ≤ 75Ω. With

RS + RG||RFB = 75Ω the total noise of the amplifier is:

eN = (1.1nV)2 +(1.1nV)2 = 1.55nV / Hz

Below this resistance value, the amplifier dominates the

noise, but in the region between 75Ω and about 3k, the

noise is dominated by the resistor thermal noise. As the

total resistance is further increased beyond 3k, the amplifier

noise current multiplied by the total resistance eventually

dominates the noise.

The product of eN • √ISUPPLY is an interesting way to gauge

low noise amplifiers. Most low noise amplifiers have high

ISUPPLY. In applications that require low noise voltage with

the lowest possible supply current, this product can be

helpful.

The LT6236/LT6237 have an eN • √ISUPPLY of only 1.9

per amplifier, yet it is common to see amplifiers with

similar noise specifications to have eN • √ISUPPLY as high

as 13.5. For a complete discussion of amplifier noise, see

the LT1028 data sheet.

ENABLE Pin

The LT6236 includes an ENABLE pin that shuts down the

amplifier to 10μA maximum supply current. For normal

operation, the ENABLE pin must be pulled to at least

2.7V below V+. The ENABLE pin must be driven high to

within 0.35V of V+ to shut down the amplifier. This can

be accomplished with simple gate logic; however care

must be taken if the logic and the LT6236 operate from

different supplies. If this is the case, open drain logic can

be used with a pull-up resistor to ensure that the ampli-

fier remains off. When the ENABLE pin is left floating, the

amplifier is inactive. However, care should be taken to

control the leakage current through the pin so the amplifier

is not inadvertently turned on. See Typical Performance

Characteristics.

The output leakage current when disabled is very low;

however, current can flow into the input protection diodes,

D1 and D2, if the output voltage exceeds the input voltage

by a diode drop.

Power Dissipation

The LT6237MS8 combines high speed with large output

current in a small package. Due to the wide supply volt-

age range, it is possible to exceed the maximum junction

temperature under certain conditions. Maximum junction

temperature (TJ) is calculated from the ambient tempera-

ture (TA) and power dissipation (PD) as follows:

TJ = TA + (PD • θJA)

The power dissipation in the IC is the function of the sup-

ply voltage, output voltage and the load resistance. For

a given supply voltage, the worst-case power dissipation

PD(MAX) occurs at the maximum quiescent supply current

and at the output voltage which is half of either supply

voltage (or the maximum swing if it is less than half the

supply voltage). PD(MAX) is given by:

PD(MAX) = (V+– V–)( IS(MAX)) + (V+/2)2/RL

Example: An LT6237HMS8 in the 8-Lead MSOP package

has a thermal resistance of θJA = 273°C/W. Operating

on ±5V supplies with one amplifier driving a 1k load, the

worst-case power dissipation is given by:

PD(MAX) = (10V)(11mA) + (2.5V)2/1000Ω= 116mW

In this example, the maximum ambient temperature that

the part is allowed to operate is:

TA = TJ - (PD(MAX) × 273°C/W)

TA = 150°C – (116mW)(273°C/W) = 118.3°C

To operate the device at a higher ambient temperature for

the same conditions, switch to using two LT6236 in the

6-Lead TSOT-23, or a single LT6237 in the 8-Lead DFN

package.

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