 13
LT1211/LT1212
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Supply Voltage
The LT1211/LT1212 op amps are fully functional and all
internal bias circuits are in regulation with 2.2V of supply.
The amplifiers will continue to function with as little as
1.5V, although the input common-mode range and the
phase margin are about gone. The minimum operating
supply voltage is guaranteed by the PSRR tests which are
done with the input common mode equal to 500mV and a
minimum supply voltage of 2.5V. The LT1211/LT1212 are
guaranteed over the full –55°C to 125°C range with a
minimum supply voltage of 2.5V.
The positive supply pin of the LT1211/LT1212 should be
bypassed with a small capacitor (about 0.01µF) within an
inch of the pin. When driving heavy loads and for good
settling time, an additional 4.7µF capacitor should be
used. When using split supplies, the same is true for the
negative supply pin.
Power Dissipation
The LT1211/LT1212 amplifiers combine high speed and
large output current drive into very small packages. Be-
cause these amplifiers work over a very wide supply range,
it is possible to exceed the maximum junction temperature
under certain conditions. To insure that the LT1211/
LT1212 are used properly, calculate the worst case power
dissipation, define the maximum ambient temperature,
select the appropriate package and then calculate the
maximum junction temperature.
The worst case amplifier power dissipation is the total of
the quiescent current times the total power supply voltage
plus the power in the IC due to the load. The quiescent
supply current of the LT1211/LT1212 has a positive tem-
perature coefficient. The maximum supply current of each
amplifier at 125°C is given by the following formula:
I
SMAX
= 2.5 + 0.036 • (V
S
– 5) in mA
V
S
is the total supply voltage.
The power in the IC due to the load is a function of the
output voltage, the supply voltage and load resistance. The
worst case occurs when the output voltage is at half
supply, if it can go that far, or its maximum value if it
cannot reach half supply.
For example, calculate the worst case power dissipation
while operating on ±15V supplies and driving a 500 load.
I
SMAX
= 2.5 + 0.036 • (30 – 5) = 3.4mA
P
DMAX
= 2 • V
S
• I
SMAX
+ (V
S
– V
OMAX
) • V
OMAX
/R
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P
DMAX
= 2 • 15V
× 3.4mA + (15V – 7.5V) • 7.5V/500
= 0.102 + 0.113 = 0.215W per Amp
If this is the quad LT1212, the total power in the package
is four times that, or 0.860W. Now calculate how much the
die temperature will rise above the ambient. The total
power dissipation times the thermal resistance of the
package gives the amount of temperature rise. For this
example, in the SO surface mount package, the thermal
resistance is 100°C/W junction-to-ambient in still air.
Temperature Rise = P
DMAX
θ
JA
= 0.860W • 100°C/W
= 86°C
The maximum junction temperature allowed in the plastic
package is 150°C. Therefore the maximum ambient al-
lowed is the maximum junction temperature less the
temperature rise.
Maximum Ambient = 150°C – 86°C = 64°C
That means the SO quad can only be operated at or below
64°C on ±15V supplies with a 500 load.
As a guideline to help in the selection of the LT1211/
LT1212, the following table describes the maximum sup-
ply voltage that can be used with each part based on the
following assumptions:
1. The maximum ambient is 70°C or 125°C depending
on the part rating.
2. The load is 500, includes the feedback resistors.
3. The output can be anywhere between the supplies.
PART MAX SUPPLIES MAX POWER AT MAX T
A
LT1211MJ8 19.5V or ±16.4V 500mW
LT1211CN8 25.2V or ±18.0V 800mW
LT1211CS8 20.3V or ±17.1V 533mW
LT1212CN 21.0V or ±17.8V 1143mW
LT1212CS 17.3V or ±14.4V 800mW 14
LT1211/LT1212
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positive rail, is about 100 as the output starts to source
current; this resistance drops to about 25 as the current
increases. Therefore when the output sources 1mA, the
output will swing to within 0.7V of the positive supply.
While sourcing 20mA, it is within 1.1V of the positive
supply.
The output of the LT1211/LT1212 will swing to within 3mV
of the negative supply while sinking zero current. Thus, in
a typical single supply application with the load going to
ground, the output will go to within 3mV of ground. The
open-loop output resistance when the output is driven
hard into the negative rail is about 44 at low currents and
reduces to about 24 at high currents. Therefore, when
the output sinks 1mA, the output is about 42mV above the
negative supply and while sinking 20mA, it is about
480mV above it.
The output of the LT1211/LT1212 has reverse-biased
diodes to each supply. If the output is forced beyond either
supply, unlimited currents will flow. If the current is
transient and limited to several hundred mA, no damage
will occur.
Feedback Components
Because the input currents of the LT1211/LT1212 are less
than 125nA, it is possible to use high value feedback
resistors to set the gain. However, care must be taken to
insure that the pole that is formed by the feedback resis-
tors and the input capacitance does not degrade the
stability of the amplifier. For example, if a single supply,
noninverting gain of two is set with two 20k resistors, the
LT1211/LT1212 will probably oscillate. This is because
the amplifier goes open-loop at 3MHz (6dB of gain) and
has 50° of phase margin. The feedback resistors and the
10pF input capacitance generate a pole at 1.6MHz that
introduces 63° of phase shift at 3MHz! The solution is
simple; use lower value resistors or add a feedback
capacitor of 10pF or more.
Inputs
Typically, at room temperature, the inputs of the LT1211/
LT1212 can common mode 400mV below ground (V
)
and to within 1.2V of the positive supply with the amplifier
still functional. However the input bias current and offset
voltage will shift as shown in the characteristic curves. For
full precision performance, the common-mode range
should be limited between ground (V
) and 1.5V below the
positive supply.
When either of the inputs is taken below ground (V
) by
more than about 700mV, that input bias current will
increase dramatically. The current is limited by internal
100 resistors between the input pins and diodes to each
supply. The output will remain low (no phase reversal) for
inputs 1.3V below ground (V
). If the output does not have
to sink current, such as in a single supply system with a 1k
load to ground, there is no phase reversal for inputs up to
8V below ground.
There are no clamps across the inputs of the LT1211/
LT1212 and therefore each input can be forced to any
voltage between the supplies. The input current will re-
main constant at about 60nA over most of this range.
When an input gets closer than 1.5V to the positive supply,
that input current will gradually decrease to zero until the
input goes above the supply, then it will increase due to the
previously mentioned diodes. If the inverting input is held
more positive than the noninverting input by 200mV or
more, while at the same time the noninverting input is
within 300mV of ground (V
), then the supply current will
increase by 1mA and the noninverting input current will
increase to about 10µA. This should be kept in mind in
comparator applications where the inverting input stays
above ground (V
) and the noninverting input is at or near
ground (V
).
Output
The output of the LT1211/LT1212 will swing to within
0.60V of the positive supply with no load. The open-loop
output resistance, when the output is driven hard into the 15
LT1211/LT1212
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Comparator Applications
Sometimes it is desirable to use an op amp as a compara-
tor. When operating the LT1211/LT1212 on a single 3.3V
or 5V supply, the output interfaces directly with most TTL
and CMOS logic.
The response time of the LT1211/LT1212 is a strong
function of the amount of input overdrive as shown in the
4
2
0
100
0
V
S
= 5V 1211/12 AI01
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=
4
2
0
100
0
5µs/DIV
V
S
= 5V 1211/12 AI02
R
L
=
LT1211 Comparator Response (+)
20mV, 10mV, 5mV, 2mV Overdrives
LT1211 Comparator Response (–)
20mV, 10mV, 5mV, 2mV Overdrives
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following photos. These amplifiers are unity-gain stable
op amps and not fast comparators, therefore, the logic
being driven may oscillate due to the long transition time.
The output can be speeded up by adding 20mV or more of
hysteresis (positive feedback), but the offset is then a
function of the input direction.
INPUT (mV)
OUTPUT (V)
OUTPUT (V)
INPUT (mV)
C
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Q5
Q10
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OUT
V
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–IN
+IN
1211/12 SS
Q7
Q9
Q8
Q11
Q12
Q14
Q15
Q13
Q16
Q6
Q3
Q4
Q1
Q2
5µs/DIV

#### LT1212CN#PBF

Mfr. #:
Manufacturer:
Analog Devices Inc.
Description:
Precision Amplifiers 14MHz, 7V/ s, 1x S 2x & 4x Prec Op Amps
Lifecycle:
New from this manufacturer.
Delivery:
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