TS1002/TS1004
In the event that an external RLOAD in parallel with
CLOAD appears in the application, the use of an RISO
results in gain accuracy loss because the external
series RISO forms a voltage-divider with the external
load resistor RLOAD
.
Figure 8: Analog Comparator Hysteresis Band and Output
Switching Points.
VIN
(VREF) for the circuit at ½ the supply voltage, or
0.75V, while keeping the current drawn by this
resistor divider low. Capacitor C1 is used to filter any
extraneous noise that could couple into the
amplifer’s inverting input.
VOUT
In this application, the desired hysteresis band was
set to 100mV (VHYB) with a desired high trip-point
(VHI) set at 1V and a desired low trip-point (VLO) set
at 0.9V.
Figure 6: TS1002/TS1004 Transient Response for RISO = 50kΩ
Since these amplifers draw very little supply current
(0.6µA per amplifier, typical), it is desired that the
design of an analog comparator using these
amplfiers should also use as little current as
practical. The first step in the design, therefore, was
to set the feedback resistor R3:
and CLOAD = 500pF.
Configuring the TS1002 or the TS1004 into a
Nanowatt Analog Comparator
Although optimized for use as an operational
amplifier, these amplifiers can also be used as a rail-
to-rail I/O comparator as illustrated in Figure 7.
R3 = 10MΩ
Calculating a value for R1 is given by the following
expression:
R1 = R3 x (VHYB/VDD)
Substituting VHYB = 100mV, VDD = 1.5V, and R3 =
10MΩ into the equation above yields:
R1 = 667kΩ
The following expression was then used to calculate
a value for R2:
Figure 7: A NanoWatt Analog Comparator with User-
Programmable Hysteresis.
R2 = 1/[VHI/(VREF x R1) – (1/R1) – (1/R3)]
External hysteresis can be employed to minimize the
risk of output oscillation. The positive feedback
circuit causes the input threshold to change when
the output voltage changes state. The diagram in
Figure 8 illustrates the amplifiers’ analog comparator
hysteresis band and output transfer characteristic.
Substituting VHI = 1V, VREF = 0.75V, R1 = 667kΩ,
and R3 = 10MΩ into the above expression yields:
R2 = 2.5MΩ
The design of an analog comparator using the
TS1002 or the TS1004 is straightforward. In this
application, a 1.5-V power supply (VDD) was used
and the resistor divider network formed by RD1 and
RD2 generated a convenient reference voltage
Page 10
TS1002_4DS r1p0
RTFDS
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