LT1011 Datasheet, PDF (8/16 Pages) Linear Technology – Voltage Comparator

LT1011/LT1011A
APPLICATIONS INFORMATION
The input resistors should limit fault current to a reason-
able value (0.1mA to 20mA). Power dissipation in the
resistors must be considered for continuous faults, espe-
cially when the LT1011 supplies are off. One final caution:
lightly loaded supplies may be forced to higher voltages by
large fault currents flowing through D1-D4.
R3 and R4 limit input current to the LT1011 to less than
1mA when the input signals are held below V^–. They may
be eliminated if R1 and R2 are large enough to limit fault
current to less than 1mA.
Input Slew Rate Limitations
The response time of a comparator is typically measured
with a 100mV step and a 5mV to 10mV overdrive. Unfor-
tunately, this does not simulate many real world situations
where the step size is typically much larger and overdrive
can be significantly less. In the case of the LT1011, step
size is important because the slew rate of internal nodes
will limit response time for input step sizes larger than 1V.
At 5V step size, for instance, response time increases from
150ns to 360ns. See the curve “Response Time vs Input
Step Size for more detail.
If response time is critical and large input signals are
expected, clamp diodes across the inputs are recom-
mended. The slew rate limitation can also affect perfor-
mance when differential input voltage is low, but both
inputs must slew quickly. Maximum suggested common
mode slew rate is 10V/µs.
Strobing
The LT1011 can be strobed by pulling current out of the
STROBE pin. The output transistor is forced to an “off”
state, giving a “hi” output at the collector (Pin 7). Currents
as low as 250µA will cause strobing, but at low strobe
currents, strobe delay will be 200ns to 300ns. If strobe
current is increased to 3mA, strobe delay drops to about
60ns. The voltage at the STROBE pin is about 150mV below
V⁺ at zero strobe current and about 2V below V⁺ for 3mA
strobe current. Do not ground the STROBE pin. It must be
current driven. Figure 4 shows a typical strobe circuit.
Note that there is no bypass capacitor between Pins 5 and
6. This maximizes strobe speed, but leaves the compara-
tor more sensitive to oscillation problems for slow, low
15V
5V
–⁸
LT1011
+₆
4
–15
RL
7
OUTPUT
1
TTL OR
CMOS DRIVE
(5V SUPPLY)
3k
1011 F04
Figure 4. Typical Strobe Circuit
level inputs. A 1pF capacitor between the output and Pin 5
will greatly reduce oscillation problems without reducing
strobe speed.
DC hysteresis can also be added by placing a resistor from
output to Pin 5. See step 5 under “Preventing Oscillation
Problems.”
The pin (6) used for strobing is also one of the offset adjust
pins. Current flow into or out of Pin 6 must be kept very low
(< 0.2µA) when not strobing to prevent input offset voltage
shifts.
Output Transistor
The LT1011 output transistor is truly floating in the sense
that no current flows into or out of either the collector or
emitter when the transistor is in the “off” state. The
equivalent circuit is shown in Figure 5.
V⁺
I₁
0.5mA
D1 D2
V^–
Q1
R1
170Ω
R2
470Ω
COLLECTOR
(OUTPUT)
Q2
OUTPUT
TRANSISTOR
EMITTER
(GND PIN) 1011 F05
Figure 5. Output Transistor Circuitry
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