LT1011_15 Datasheet, PDF (9/20 Pages) Linear Technology – Voltage Comparator

LT1011/LT1011A
Applications Information
This circuit is especially useful for general purpose
comparator applications because it does not force
any signals directly back onto the input signal source.
Instead, it takes advantage of the unique properties
of the BALANCE pins to provide extremely fast, clean
output switching even with low frequency input signals
in the millivolt range. The 0.003µF capacitor from Pin
6 to Pin 8 generates AC hysteresis because the voltage
on the BALANCE pins shifts slightly, depending on the
state of the output. Both pins move about 4mV. If one
pin (6) is bypassed, AC hysteresis is created. It is only
a few millivolts referred to the inputs, but is sufficient
to switch the output at nearly the maximum speed of
which the comparator is capable. To prevent problems
from low values of input slew rate, a slight amount of DC
hysteresis is also used. The sensitivity of the BALANCE
pins to current is about 0.5mV input referred offset for
each microampere of BALANCE pin current. The 15M
resistor tied from OUTPUT to Pin 5 generates 0.5mV DC
hysteresis. The combination of AC and DC hysteresis
creates clean oscillation-free switching with very small
input errors. Figure 2 plots input referred error versus
switching frequency for the circuit as shown.
Note that at low frequencies, the error is simply the
DC hysteresis, while at high frequencies, an addi-
tional error is created by the AC hysteresis. The high
frequency error can be reduced by reducing C_H, but
lower values may not provide clean switching with very
low slew rate input signals.
Input Protection
The inputs to the LT1011 are particularly suited to general
purpose comparator applications because large differential
and/or common mode voltages can be tolerated without
damage to the comparator. Either or both inputs can be
raised 40V above the negative supply, independent of the
positive supply voltage. Internal forward biased diodes
will conduct when the inputs are taken below the negative
supply. In this condition, input current must be limited to
1mA. If very large (fault) input voltages must be accom-
modated, series resistors and clamp diodes should be
used (see Figure 3).
8
C8 TO C6 = 0.003µF
7
6
5
4
3
2
1
OUTPUT “LO” TO “HI”
0
OUTPUT “HI” TO “LO”
–1
(50kHz)
(5kHz)
–2
1
10
100
1000
TIME/FREQUENCY (µs)
1011 F02
Figure 2. Input Offset Voltage vs Time to Last Transition
V⁺
R1**
D1
D2
R3*
300Ω
3_–
8
INPUTS
R4*
LT1011
R2**
300Ω _{2 +}
4
D3 D4
D1 TO D4: 1N4148
*MAY BE ELIMINATED FOR I_FAULT ≤ 1mA
**SELECT ACCORDING TO ALLOWABLE
FAULT CURRENT AND POWER DISSIPATION
V^–
1011 F03
Figure 3. Limiting Fault Input Currents
For more information www.linear.com/LT1011
1011afe
9