Datasheet

LTC6246/LTC6247/LTC6248
15
624678fa
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
C
PAR
5k
–
+
V
OUT
V
IN
5k
5pF
Power Dissipation
The LTC6246 and LTC6247 contain one and two amplifiers
respectively. Hence the maximum on-chip power dis-
sipation 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 (θ
JA
) of 125°C/ W. It is
necessary to ensure that the die’s junction temperature
does not exceed 150°C. The junction temperature, T
J
, is
calculated from the ambient temperature, T
A
, power dis-
sipation, PD, and thermal resistance, θ
JA
:
T
J
= T
A
+ (P
D
• θ
JA
)
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 P
D(MAX)
occurs when
the supply current is maximum and the output voltage at
half of either supply voltage for a given load resistance.
P
D(MAX)
is approximately (since I
S
actually changes with
output load current) given by:
P
D(MAX)
=(V
S
•I
S(MAX)
)+
V
S
2






2
/ R
L
Example: For an LTC6248 in a 16-lead MS package operating
on ±2.5V supplies and driving a 100Ω load to ground, the
worst-case power dissipation is approximately given by
P
D(MAX)
/Amp = (5 • 1.3mA) + (1.25)
2
/100 = 22mW
If all four amplifiers are loaded simultaneously then the
total power dissipation is 88mW.
At the Absolute Maximum ambient operating temperature,
the junction temperature under these conditions will be:
T
J
= T
A
+ P
D
• 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 positive
supply and the amplifier remains on.