LT6232 datasheet(16/26 Pages) AD

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LTC6246/LTC6247/LTC6248

Rev C

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APPLICATIONS INFORMATION

Figure 2. 5pF Feedback Cancels Parasitic Pole

Feedback Components

When feedback resistors are used to set up gain, care

must be taken to ensure that the pole formed by the

feedback resistors and the parasitic capacitance at the

inverting input does not degrade stability. For example if

the amplifier is set up in a gain of +2 configuration with

gain and feedback resistors of 5k, a parasitic capacitance

of 5pF (device + PC board) at the amplifier’s inverting

input will cause the part to oscillate, due to a pole formed

at 12.7MHz. An additional capacitor of 5pF across the

feedback resistor as shown in Figure 2 will eliminate any

ringing or oscillation. In general, if the resistive feedback

network results in a pole whose frequency lies within the

closed loop bandwidth of the amplifier, a capacitor can be

added in parallel with the feedback resistor to introduce

a zero whose frequency is close to the frequency of the

pole, improving stability.

624678 F02

CPAR

VOUT

VIN

5pF

Power Dissipation

The LTC6246 and LTC6247 contain one and two ampli-

fiers respectively. Hence the maximum on-chip power

dissipation for them will be less than the maximum on-

chip power dissipation for the LTC6248, which contains

four amplifiers.

The LTC6248 is housed in a small 16-lead MS package

and typically has a thermal resistance (θJA) of 125°C/ W. It

is necessary to ensure that the die’s junction temperature

does not exceed 150°C. The junction temperature, TJ, is

calculated from the ambient temperature, TA, power dis-

sipation, PD, and thermal resistance, θJA:

TJ = TA + (PD • θJA)

The power dissipation in the IC is a function of the supply

voltage, output voltage and load resistance. For a given

supply voltage with output connected to ground or supply,

the worst-case power dissipation PD(MAX) occurs when

the supply current is maximum and the output voltage at

half of either supply voltage for a given load resistance.

PD(MAX) is approximately (since IS actually changes with

output load current) given by:

PD(MAX) =(VS •IS(MAX))+

⎛

⎝⎜

⎞

⎠⎟

/RL

Example: For an LTC6248 in a 16-lead MS package operat-

ing on ±2.5V supplies and driving a 100Ω load to ground,

theworst-casepowerdissipationisapproximatelygivenby

PD(MAX)/Amp = (5 • 1.3mA) + (1.25)2/100 = 22mW

If all four amplifiers are loaded simultaneously then the

total power dissipation is 88mW.

AttheAbsoluteMaximumambientoperatingtemperature,

the junction temperature under these conditions will be:

TJ = TA + PD • 125°C/W

= 125 + (0.088W • 125°C/W) = 136°C

which is less than the absolute maximum junction tem-

perature for the LTC6248 (150°C).

Refer to the Pin Configuration section for thermal resis-

tances of various packages.

Shutdown

The LTC6246 and LTC6247MS have SHDN pins that can

shut down the amplifier to 42µA typical supply current.

The SHDN pin needs to be taken below 0.8V above the

negative supply for the amplifier to shut down. When left

floating, the SHDN pin is internally pulled up to the posi-

tive supply and the amplifier remains on.

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