LT1011/LT1011A
U
W U U
APPLICATIONS INFORMATION
15V
8
5V
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:
lightlyloadedsuppliesmaybeforcedtohighervoltagesby
large fault currents flowing through D1-D4.
R
L
–
+
7
LT1011
OUTPUT
1
6
4
TTL OR
CMOS DRIVE
(5V SUPPLY)
–15
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.
3k
1011 F04
Figure 4. Typical Strobe Circuit
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, thisdoesnotsimulatemanyrealworldsituations
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.
At5Vstepsize,forinstance,responsetimeincreasesfrom
150ns to 360ns. See the curve “Response Time vs Input
Step Size for more detail.
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.”
Thepin(6)usedforstrobingisalsooneoftheoffsetadjust
pins.CurrentflowintooroutofPin6mustbekeptverylow
(<0.2µA)whennotstrobingtopreventinputoffsetvoltage
shifts.
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.
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.
Strobing
+
V
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.ThevoltageattheSTROBEpinisabout150mVbelow
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.
I
1
0.5mA
D1
D2
COLLECTOR
(OUTPUT)
Q1
R1
170Ω
OUTPUT
TRANSISTOR
Q2
–
V
R2
470Ω
EMITTER
(GND PIN) 1011 F05
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
Figure 5. Output Transistor Circuitry
8
Linear [ Linear ]
