dm00050759

AN4071
Application note
Introduction to comparators, their parameters and basic
applications
By Radim Smat
Introduction
After operational amplifiers (op amps), comparators are the most generally used analog,
simple integrated circuits.
Operational amplifiers are well described in many publications and a lot of information can
be found regarding the design and proper use of these devices. On the other hand,
information concerning comparators is much harder to find as they are often considered as
simple devices. This note explains the main parameters of comparators and their limitations
from an application point of view.
Figure 1.
October 2012
TS332 and TS334 micropower low-voltage rail-to-rail comparators
Doc ID 022939 Rev 1
1/27
www.st.com
Contents
AN4071
Contents
1
Comparator substitution by an op amp . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Comparator parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Comparator classification by major parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
4
Datasheet parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1
Input common mode voltage range - VICM . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2
Input offset voltage - VIO (VTRIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3
CMRR and SVR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4
Voltage gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5
Propagation delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1
Built-in hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2
External hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3
Dynamical hysteresis, example of oscillation issue and the solution . . . . 19
5
Relaxation oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6
Window comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
TS332 and TS334 micropower low-voltage rail-to-rail comparators . . . . . . . . . . . . . . . . . . . 1
Comparator pinout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Signal comparison by TS3011 with built-in hysteresis
(blue IN+, green IN-, red OUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Open drain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Push-pull. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Wired-OR configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Voltage marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Input-trip point VTRIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Input offset voltage VIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
VIO (VTRIP) measurement circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
TS3011 trip point voltage vs. common mode voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
TPLH diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
TPHL diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
TS3011 TPD vs. overdrive voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
TS3011 TPD vs. input common mode voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Circuit for TPD measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Trip point voltage definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Input hysteresis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Non-inverting comparator with hysteresis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Inverting comparator with hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Hysteresis circuit example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Dynamical hysteresis circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Device oscillation without CFB capacitor (green OUT, blue IN+), time scale 500 ns/div. . . 20
Case with CFB (green OUT, blue IN+), time scale 500 ns/div. . . . . . . . . . . . . . . . . . . . . . . 20
Zoom of the signal (green OUT, blue IN+), time scale 20 ns/div. . . . . . . . . . . . . . . . . . . . . 21
Relaxation oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Window comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Temperature control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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Comparator substitution by an op amp
1
AN4071
Comparator substitution by an op amp
To use operational amplifiers in open loop as comparators is quite common. This especially
applies when an op amp is already used in the application, giving the user the opportunity to
use a dual channel (or quad channel) op amp which can save space in the application. This
is possible even if a better alternative is to use comparators that are optimized for this
purpose.
The op amp is a device which is designed to be used with negative feedback.
A major concern is to ensure the stability of such a configuration. Other parameters like slew
rate and maximum bandwidth are trade-offs with current consumption and the architecture
of an op amp.
Comparators, on the other hand, are designed to operate in open loop configuration without
any negative feedback. In most cases, they are not internally compensated. The speed
(propagation delay) and slew rate (rise and fall time) are maximized. The overall gain is also
usually higher. The use of an op amp as a comparator leads to an unoptimized situation,
where current consumption versus speed ratio is low. The opposite is even worse. Normally,
a comparator cannot be used instead of an op amp. Most probably, the comparator shows
instability under negative feedback. Generally speaking, comparators and operational
amplifiers cannot substitute each other except for low performance designs.
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2
Comparator parameters
Comparator parameters
Comparator classification by major parameters
●
Propagation delay
●
Current consumption
●
Output stage type (open collector/drain or push-pull)
●
Input offset voltage, hysteresis
●
Output current capability
●
Rise and fall time
●
Input common mode voltage range.
Besides major parameters, comparators are classified by other parameters such as input
bias current, common mode and power supply rejection ratio, sample/hold function, and
startup time.
Figure 2.
Comparator pinout example
/54
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A single device has, ordinarily, five pins: two for power supply VCC+,VCC-, two as inputs IN+,
IN- and one for the output OUT. It is possible to have an extra pin for standby function.
When VIN(+) > VIN(-), the output is in high state, if VIN(+) < VIN(-), the output is in low state.
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Comparator parameters
AN4071
Figure 3 shows the comparison made by a TS3011 comparator between a 20 m VPP input
signal (blue) applied on IN+ and a 50 mV DC reference voltage (green) applied at IN-. The
output signal (red) amplitude is 5 V. As can be seen, sometimes, the input signal exceeds
the reference voltage though the output remains low. This is caused by the internal
hysteresis voltage of the TS3011. Hysteresis is discussed in Section 4.
Figure 3.
Signal comparison by TS3011 with built-in hysteresis
(blue IN+, green IN-, red OUT)
Concerning the output configuration, there are two main types of comparators: with
push-pull and open collector (or open drain). Push-pull stage output levels are typically VCC
and 0 V (voltage drop from power supply pins can be neglected). In the case of open
collector configuration, an external pull-up resistor is used. Such a configuration allows an
external voltage to be applied, different from VCC, to drive the high level state. Pull-up
configuration can be used as a simple voltage level translator. The second advantage of
a device with open collector is that more outputs can be connected together. This is useful
for wired-OR configuration systems. It is also possible to find comparators with a differential
output stage. These devices are less common and mainly used in telecommunication
systems as transmission line drivers. For example, the TS3021, TS3011 and TS861 are
STMicroelectronics® comparators with push-pull output stage while the LMV331, TS7221
and TS331 are open drain configuration.
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Comparator parameters
Figure 4.
Open drain
Figure 5.
Push-pull
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Figure 6.
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Wired-OR configuration
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Datasheet parameters
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3
Datasheet parameters
3.1
Input common mode voltage range - VICM
The VICM is defined as the voltage range where both inputs must remain to guarantee the
functionality of the device. When the input common mode range covers both power supply
rails 0 V and VCC, it is a rail-to-rail input stage.
Regarding the use of a rail-to-rail (R2R) comparator, from a technical point of view, if the
R2R input capability is not needed, it is always better to choose a comparator without R2R
input stage. The power consumption is, in that case, obviously smaller and the device cost is
lower.
3.2
Input offset voltage - VIO (VTRIP)
The input offset voltage (VIO) can be defined as the differential input voltage to apply in
order to be at the toggling level. Input offset voltage limits the resolution of comparators.
Therefore, for very small signals (in the same order as the VIO), the comparator toggles at
an undesired value or does not toggle at all.
Figure 7.
Voltage marking
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Figure 8.
6).
Input-trip point VTRIP
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Input offset voltage VIO
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AN4071
Datasheet parameters
In other words, input offset voltage can be represented by a voltage source applied in series
with one input of an ideal comparator. Consequently, the output doesn't toggle when
VIN+ = VIN-, as in the case of an ideal comparator, but the threshold level is shifted by the
input offset value VIO. Input offset voltage rises in the input stage as a consequence of
transistor imbalance. For comparators with built-in hysteresis, VIO is defined as the average
value of VTRIP+ and VTRIP-, and the hysteresis of VHYST = VTRIP+ - VTRIP- where VTRIP+
(respectively VTRIP-) is the input differential voltage for which the output switches from low to
high state (respectively high to low state).
Measurement
Input offset voltage and trip points can be measured using the circuit shown in Figure 10.
The first DC source sets the power supply VCC and the second sets VICM, the common
mode voltage. A 100 m VPP triangle signal is applied on the voltage divider (1/101). The
voltage divider is necessary to obtain a good accuracy on the VIO reading by the scope, and
to use the function generator with an amplitude that it can handle. The triangle signal should
be low frequency (20 Hz); higher frequency can lead to error in the VIO measurement
caused by propagation delay of the device. When the output changes its state, the actual
input voltage value VIN is read from the scope, from VIN we can simply deduce VIO = VIN /
101.
Pay attention to the fact that the oscilloscope probes and waveform generator ground are on
the inverting input pin of the comparator. Therefore, VICM and VCC power supplies must be
floating from earth-ground, or an isolation transformer must be used. The advantage is that
the VICM voltage does not need to be subtracted from the VIO reading. VICM can be easily
changed without having to adjust the offset of the input signal, making the measurement
more comfortable.
Figure 10. VIO (VTRIP) measurement circuit
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Datasheet parameters
AN4071
Figure 11. TS3011 trip point voltage vs. common mode voltage
!-
Use passive oscilloscope probes with 10:1 dividing ratio. For increased accuracy on VIO
reading and decreased noise background, the probe applied at VIN can be replaced by
a coaxial cable. Unlike propagation delay (TPD) measurement, the cable capacity doesn't
have an impact here because the frequency of VIN is low.
Figure 11 is an example of the measured VIO vs. VICM for the TS3011 which is a high speed
rail-to-rail comparator from STMicroelectronics. The effect of the two input stages can be
clearly seen. Each stage has a different VIO with its own dependency on VICM. One stage is
operating up to VICM = VCC - 0.7 V while the second one covers the range from 0.7 V to VCC.
The overall curve is the merging of both stages. It can be understood from the presence of
two VTRIP curves that the TS3011 has a built-in hysteresis.
3.3
CMRR and SVR
The “common mode voltage rejection ratio” (CMRR) describes the relationship between
input offset voltage VIO and the input common voltage VICM. It is defined as VICM over VIO
variation ratio, and is usually represented in logarithmic scale.
Equation 1
CMRR [dB] = 20 • log (| ΔVICM /ΔVIO| )
CMRR is calculated with two values of input offset voltages measured for two different input
common mode voltages (usually 0 V and VCC).
The “supply voltage rejection” (SVR) is a parameter describing the relation between the
input offset voltage VIO and the power supply voltage. The power supply voltage
modification may affect, more or less, the bias of the input differential transistor pairs.
It means that the input offset voltage is also slightly modified.
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Datasheet parameters
Equation 2
SVR [dB] = 20 • log (| ΔVCC /ΔVIO| )
Note that, the higher CMMR and SVR are the better.
Measurement
The circuit is the same as the one used for VIO (Figure 10).
3.4
1.
CMRR
VIO at low (L) VICM rail and VIO at high (H) VICM rail are measured
then CMRR = 20 • log [(VICM(H) - VICM(L)) / (|VIO(H)-VIO(L)|)]
2.
SVR
VIO at low VCC and VIO at high VCC are measured
then SVR = 20 • log [(VCC(H)- VCC(L)) / (| VIO(H)-VIO(L)|)]
Voltage gain
Voltage gain AVD indicates the overall device gain. Higher gain means better small input
signal resolving capability which can be an advantage in certain applications. Common
comparators have an AVD in the range of 200 V/mV (106 dB). 1 mV input signal amplified by
106 dB leads to theoretical amplitude of 200 V. In reality, the output signal swing is limited by
VCC. Note that the AVD doesn’t affect external hysteresis as the output is always in high or
low state and never between (unlike an operational amplifier, a comparator is not used in its
linear region).
3.5
Propagation delay
Propagation delay TPD is one of the key parameters for many applications because it limits
the maximal input frequency which can be processed. Voltage comparison of analog signals
requires a minimum amount of time.
TPD is defined as the time difference between the moment the input signal crossing the
reference voltage and the moment the output state changes (usually when the output signal
crosses 50% of VCC, if nothing is specified).
A graphical interpretation is shown in Figure 12 and 13. For TPLH an input square signal
from -100 mV to +VOV, called overdrive voltage and referenced to VICM + VTRIP+, is applied
on the non-inverting input. The inverting input is connected at VICM voltage. Initial signal
condition 100 mV below VICM +VTRIP+ ensures a sufficient saturation for the input stage. For
the falling edge propagation delay measurement (TPHL), the input signal goes from
+100 mV to -VOV referenced to VICM + VTRIP-.
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Datasheet parameters
AN4071
Figure 12. TPLH diagram
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Figure 13. TPHL diagram
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Datasheet parameters
The input signal is referenced to VICM + VTRIP and not only to VICM because, in the case of
small signal overdrive, an error in TPD measurement can occur due to non-zero VIO (VTRIP+
and/or VTRIP- <> 0). In reality, we can neglect VIO without significant impact on TPD for VOV
values of 20 mV and higher, considering that the offset value reaches around 1 mV and
considering the exponential dependency of TPD on overdrive voltage, as shown in
Figure 14.
Figure 14. TS3011 TPD vs. overdrive voltage
!-
Figure 15. TS3011 TPD vs. input common mode voltage
!-
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Datasheet parameters
AN4071
Measurement
A basic circuit for TPD measurement is shown in Figure 16. One DC power supply is used
for VCC biasing, and a second one for VICM voltage. If the measurement is performed at
VICM = 0 V, the source can be removed and the inverting pin directly connected to ground.
To keep low source impedance and prevent from parasitic oscillation during switching, a 100
nF bypass capacitor is connected close to the positive supply pin VCC+ of the comparator.
A second bypass capacitor should be connected to the comparator input pin IIN- when VICM
source is used. A 50 Ω resistor minimizes the effect of input pin capacitance and avoids
signal reflection on the line matching the impedance at inputs of the comparator with the
impedance of generator (VIN).
It is necessary to define exactly the measurement conditions, especially the load
capacitance CLOAD which has a big impact on output signal edge speed, consequently, also
on the TPD value (measured at 50% of VOUT). The C LOAD represents the overall capacitive
loading at the comparator output including loading capacitor, oscilloscope probe capacity
and parasitic capacity of the PCB track.
Figure 16. Circuit for TPD measurement
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Many pulse generators, despite great performance in the time-base, are not able to provide
accurate signal amplitude, especially for low output voltage. Generating 5 mV overdrive may
be a problem because such a value is often below the generator accuracy. Therefore, for
small overdrive measurement, it is more suitable to place a 50 Ω attenuator (divider bridge),
instead of a single 50 Ω resistor, and increase the generator amplitude. In this way, a good
amplitude accuracy can be obtained with low overdrive values.
High speed signal processing
For high-speed signal applications, more attention (at PCB level) must be paid to: proper low
resistive grounding, short tracks and quality SMD capacitors having low ESR. Bypass
capacitor stores charge and provides supplementary source when spikes occur on the VCC
line. Each real capacitor has resonant frequency where its impedance reaches the lowest
value. If the input signal frequency is far from the resonant frequency, impedance strongly
increases and the capacitor loses bypassing capability. Placing different capacitors with
different resonant frequencies therefore allows a wide frequency bandwidth to be covered.
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Datasheet parameters
Such a bypass combination, made from 100 nF, 10 nF and 1 nF in parallel, eliminates
unwanted spikes on the VCC line much better than only one 100 nF capacitor.
Each mm of the track plays a role. Bypass capacitors must be placed as close as possible to
the comparator supply pin. Place the smallest capacitor closer to the supply pin than the
bigger one. Removing GND copper under signal track minimizes overall capacitive loading.
Input source impedance shouldn't exceed 1 kΩ, otherwise undesirable oscillations can
appear. In addition, too high input impedance in series with parasitic PCB capacity and input
comparator capacity produces additional RC constant which means additional propagation
delay.
Concerning time measurements on high-speed comparators, remember that for a high
speed signal the oscilloscope and probe can cause significant error in measurement
accuracy when their bandwidth is too low. The measured TRISE (TFALL) value is affected by
the rise time of the scope and the probe by:
Equation 3
TRISE = (TRISE SIGNAL2+ TRISE SCOPE2+ TRISE PROBE2)1/2
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Hysteresis
4
AN4071
Hysteresis
For slow time changing input signal, an output oscillation can appear while the input signal
remains close to the reference voltage. Also low amplitude signal on high impedance can
cause oscillations due to noise background. Such unwelcome behavior can be solved by
hysteresis. The principle of hysteresis consists of two different input threshold voltages
depending on actual output state.
4.1
Built-in hysteresis
Many comparators have built-in hysteresis. Typical hysteresis value is a few mV. This is
enough to suppress output undesired toggling in most cases but it doesn't impact
significantly the resolution of the comparator. For comparators with built-in hysteresis, the
average lower and upper threshold voltage is computed and referred as input offset voltage
VIO, the VTRIP+ and VTRIP- difference is referred as hysteresis voltage VHYST and is shown in
Figure 17.
Figure 17. Trip point voltage definition
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Figure 18. Input hysteresis
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4.2
Hysteresis
External hysteresis
If the device doesn't include built-in hysteresis, or if a large hysteresis is required, a positive
feedback network can be implemented. Figure 19 shows a non-inverting and Figure 20 an
inverting comparator with hysteresis.
Figure 19. Non-inverting comparator with hysteresis
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Figure 20. Inverting comparator with hysteresis
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For a non-inverting comparator circuit, neglecting input offset voltage and the effect of input
biasing current, the input threshold voltages are:
Equation 4
V TH +
RI
= ---------- • ( V R EF – V OL ) + V R EF
R FB
V TH –
RI
= ---------- • ( V REF – V OH ) + V REF
R FB
VOL is the saturation voltage in low state and VOH is the saturation voltage in high state at
the comparator output. The VTH+- VTH- difference determines the hysteresis voltage (VHYST)
while their average determines the middle of hysteresis (VTRIP).
Equation 5
V HYST = V TH + – V TH –
RI
= ---------- • ( V OH – V OL )
R FB
RI
V TH + + V T H –
V OL + V OH⎞
V TRIP = ----------------------------------------- = V REF + ---------- • ⎛ V REF – ---------------------------⎝
⎠
R FB
2
2
In Equation 5 the influence of reference voltage on the trigger voltage level can be seen. The
trip point voltage VTRIP (middle of hysteresis) is equal to the reference voltage VREF only
when the second part of the equation equals zero, it means when VREF is set just to the
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Hysteresis
AN4071
centre of output voltage swing. Otherwise VREF and VTRIP are different and the hysteresis is
not centred on VREF.
For calculating RI, RFB and VREF, two formulas, obtained from Equation 4 and 5, can be
used:
Equation 6
V OH – V OL
R FB
---------- = ------------------------V H YST
RI
R FB
RI
V REF = ------------------------------------- • ( V OH + V OL ) + ----------------------- • V TRI P
2 • ( R I + R FB )
R I + R FB
Example of real design of comparator using external hysteresis network:
Task: find resistor values and voltage reference in order to implement 400 mV hysteresis on
1.2 V threshold voltage. Use push-pull comparator TS3021 powered by +5 V source.
Solution: knowing the output voltage level swing and required hysteresis, calculate the
resistor ratio and choose the appropriate resistor (in the range of hundreds of kΩ). Then,
substituting resistor values and trip point voltage in the second formula, calculate the
reference voltage.
For 5 V output swing and hysteresis 400 mV, the ratio of feedback and input resistor is:
Equation 7
V OH – V OL
R FB
5V
---------= --------------------------- = ------------ = 12.5
0.4V
V HYST
RI
Appropriate resistors are RFB = 300 kΩ and RI= 24 kΩ. The VREF can now be easily
calculated:
Equation 8
R FB
R1
V R EF = ------------------------------------- • ( V OH + V OL ) + ----------------------- • V TRIP =
2 • ( R I + R FB )
R I + R FB
4
5
2.4 • 10
3 • 10
- • 5 + ------------------------------------------------- • 1.2 = 0.185 + 1.111V = 1.296V
= --------------------------------------------------------------4
5
4
5
2 • ( 2.4 • 10 + 3 • 10 )
2.4 • 10 + 3 • 10
Figure 21 shows the final circuit. A reference voltage is generated by the voltage divider
supplied from the +5 V source of the comparator. With resistors 68 kΩ and 24 kΩ from the
E24 series, the VREF is equal to 1.3 V.
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Hysteresis
Figure 21. Hysteresis circuit example
6
).
6
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4.3
Dynamical hysteresis, example of oscillation issue and the
solution
Dynamical hysteresis is another way to eliminate parasitic oscillation during the transition
period. When the input signal changes slowly around the reference voltage, an output
oscillation may occur. A small capacitor C FB applied between the output and the noninverting pin boosts up the input signal to go over (or below) the reference voltage faster and
in consequence to help reduce oscillations.
The circuit example in Figure 22 shows the usage of the CFB. To induce some output
oscillations (on purpose only), consider the case of the application shown in Figure 22: the
impedance of the 0.9 V reference voltage is too high because of the 6.8 kΩ resistor. First,
consider the situation without CFB when VIN+ exceeds VIN- and the output toggles to high
state. The fast output edge, together with the parasitic PCB capacity Cparasitic between the
IN- and OUT pin, causes a positive voltage peak to IN-. VIN- now becomes higher than VIN+.
Consequently, the output is returning to low state. The negative peak now goes back to
IN-, VIN- is lower than VIN+ and the output returns to high. This leads to repetitive
oscillations, as shown in Figure 23. The frequency of the oscillations is related to the TPD
of the comparator, here it is ~ 10 MHz (TPD = 50 ns).
Figure 22. Dynamical hysteresis circuit
#&"
6
P& 6
6 K(Z
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Hysteresis
AN4071
Figure 23. Device oscillation without CFB capacitor (green OUT, blue IN+), time scale
500 ns/div.
Applying a 22 pF feedback capacitor between the IN+ and OUT pin stops the oscillations, as
shown in Figure 24 and Figure 25. This feedback capacitor creates a peak (~150 mV for
20 ns) on the IN+ pin which securely eliminates the effects of the peak on VIN- coming from
Cparasitic. The feedback capacitor CFB implements a dynamic hysteresis.
Figure 24. Case with CFB (green OUT, blue IN+), time scale 500 ns/div.
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Hysteresis
Figure 25. Zoom of the signal (green OUT, blue IN+), time scale 20 ns/div.
Main general contributors leading to device oscillation
●
Parasitic capacity/inductance onboard: long narrow wires, signal and output track close
together, device plugged into socket
●
Fast (sharp) output edges: faster edge means higher dV/dT and therefore bigger
impact of parasitic capacities and inductances on the board
●
High impedance on input pins: sensitivity to noise, increased effect of parasitic
structures and signal crosstalk
●
Poor grounding
●
High power supply impedance: inappropriate or missing bypass capacitor
●
No hysteresis (static or dynamic) implemented
●
Use of high-speed comparator where it isn't necessary.
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Relaxation oscillator
5
AN4071
Relaxation oscillator
A relaxation oscillator belongs to the regenerative circuits group. One subgroup is
multivibrators which are furthermore classified as monostable, bistable and astable. The
relaxation oscillator is an astable multivibrator.
Figure 26. Relaxation oscillator
6
2
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The circuit in Figure 26 shows a representative circuit of relaxation oscillator based on the
TS3021 comparator. It uses both positive and negative feedback. Positive feedback
produces voltage hysteresis which has already been described in Section 4.2 Threshold
voltages VLOW (from low to high) and VHIGH (from high to low) on the inverting input are
given by R2, R3 and R4 resistors together with output voltage given by power supply voltage.
Considering zero voltage drop at the output see Equation 9:
Equation 9
VLOW = VCC • R2 | | R4 / (R3 + R2 | | R 4) and VHIGH = VCC • R2 / (R2 + R3 | | R4)
For R2 = R3 = R4: VLOW = 1/3 • VCC and VHIGH = 2/3 • VCC
Voltage on the non-inverting input is generated by charging and discharging capacitor C1
from the comparator output via resistor R1 in the feedback:
= V - (V - V
) • e-t/τ = V - 2/3 • V • e-t/τ
1. While C is charging: V
1
C1(t)
CC
CC
LOW
Capacitor C1 is charged at voltage VHIGH after time t1:
Equation 10
VHIGH = VCC - 2/3 • VCC • e-t1/τ
2/3 • VCC = VCC - 2/3 VCC • e-t1/τ
-1/3 • V = -2/3 • V • e-t1/τ
CC
-t1/τ
CC
1=2• e
-t1/τ = ln (1/2)
t1 = τ • ln (2)
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CC
CC
AN4071
Relaxation oscillator
2.
While C1 is discharging: VC1(t) = VHIGH • e-t/τ = 2/3 • VCC • e -t/τ
Capacitor C1 is discharged at voltage VLOW after time t2:
Equation 11
VLOW = 2/3 • VCC • e-t2/τ
1/3 • VCC = 2/3 • VCC • e-t2/τ
1 = 2 • e-t2/τ
-t2/τ = ln (1/2)
t2 = τ • ln (2)
Because t1 = t2, the output square signal has 50% duty cycle. The output signal period T is
the sum of the time t1 and the time t2:
T = t1 + t2= τ • ln (4), where τ = R1 • C1
In the configuration described in Figure 11, the output frequency is around 72 kHz.
Output frequency doesn't depend on the power supply voltage. As the TS3021 circuit can
work with a wide power supply voltage range from 1.8 V to 5 V, the output signal amplitude
has the same value as the power supply due to its rail-to-rail output stage. Frequency can be
adjusted simply by changing the value of R1 or C1. In case a different duty cycle is required,
R2, R3 and R 4 should not be kept equal.
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Window comparator
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Window comparator
The window comparator is a circuit utilizing two single open drain /collector comparators
operating together. It has three inputs. Two of them are dedicated for reference voltage and
one for signal input. Figure 27 illustrates the function of the window comparator. Output is
high (VOUT = VPULL) when input voltage lies above VTH- and below VTH+, otherwise the
output is low (0 V).
Figure 27. Window comparator
605,,
64(
6).
64(
2 05,,
64(
6/54
6).
6/54
64(
T
605,,
T
OUTPUT(WHEN
64( 6).64(
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Figure 28. Temperature control circuit
6
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34,-
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43
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An example of the window comparator application represents a simple temperature
controller circuit (Figure 28). Based on the TS332 dual comparator and temperature sensor
STLM20, the circuit monitors if the temperature holds in the required range (25 ± ~10 °C).
The STLM20 is a precision analog output temperature sensor for low current applications.
The maximum temperature accuracy of the STLM20 is ± 1.5 °C (typ. ±0.5 °C) at ambient
temperature of 25 °C and VCC of 2.7 V. The STLM20 has a maximum quiescent supply
current of 8 μA, therefore self-heating is negligible.
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Window comparator
The TS332 is a dual micropower and low-voltage comparator. It can operate with a supply
voltage ranging from 1.6 to 5.5 V with only 20 μA current consumption per operator. In
addition, rail-to-rail inputs and a temperature range of -40 °C to +125 °C makes it ideal for
a wide range of applications.
With good accuracy about 25 °C, the STLM20 transfer function can be described by the
parabolic transfer function expressed in Equation 12:
Equation 12
VOUT = (-3.88 • 10-6 • T2) + (-1.15 • 10-2 • T) + 1.8639 [V, °C]
Based on the STLM20 transfer function equation, for the considered temperature range
25 ± 10 °C, the minimum output voltage VTH- is obtained at the higher temperature (35 °C)
and equals 1.457 V. The maximum output voltage VTH+ is at the lowest temperature (15 °C)
and equals 1.691 V.
VTH voltages are generated by a voltage divider. 1 μA current through the divider (IDIV) is
a compromise between noise sensitivity and low impact on the supply current of the
application. The resistors in the divider are:
R1 (lower resistor) = VTH- / IDIV = 1.457 MΩ
R2 (middle resistor) = (VTH+ - VTH-) / IDIV = 234 kΩ
R3 (upper resistor) = (VCC - VTH+) / IDIV = 1.009 MΩ
The closest values from the E24 resistor series are: R1 = 1.5 MΩ, R2 = 240 kΩ, R3 = 1 MΩ.
A recalculation of the temperature range can be made with these resistors.
Equation 13
IDIV = VCC/(R1 + R2 + R3) = 2.7 V/(1.5 MΩ + 240 kΩ + 1 MΩ) = 0.985 μA
VTH- = IDIV • R1 = 1.478 V
VTH+ = IDIV • (R1+ R2) = 1.714 V
The temperature can be expressed from the voltage of the STLM20:
Equation 14
6 1.8639 – V OUT
- ( ° C, V )
T = – 1481.96 + 2.1962 • 10 + -------------------------------------–6
3.88 • 10
After substitution we get: TLOW = 13 °C and THIGH = 33.2 °C.
For better range setting precision, fixed resistors in the voltage divider bridge can be
replaced by variable resistors. Note that, temperature setting depends on the supply
voltage. Thanks to the micro power TS332 comparator, the power current consumption of
the application is typically only 50 μA at 25 °C and max. 74 μA over full temperature range.
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Revision history
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Revision history
Table 1.
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Document revision history
Date
Revision
09-Oct-2012
1
Changes
Initial release.
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