AN855 Datasheet, PDF (1/3 Pages) Silicon Laboratories – ADJUSTING HYSTERESIS ON A DUAL COMPARATOR USING ONE RESISTOR

AN855
ADJUSTING HYSTERESIS ON A DUAL COMPARATOR
USING ONE RESISTOR
1. Introduction
Silicon Labs’ TS9002 low-power dual comparator incorporates an internal reference that, when used with a simple
feedback network, can adjust the hysteresis. Since only two resistors are needed for the feedback network, only
one resistor needs to be adjusted to alter the hysteresis band.
For this walk-through, only the A-comparator of the TS9002 will be considered. For the A-comparator, the output
will go high once the input to the A-comparator rises from below to above the upper threshold, while the output of
the A-comparator will go low once the input to the A-comparator goes from above to below the low threshold.
First, the hysteresis for the TS9002 comparator will be set to 30 mV. The voltage supplied by the internal reference,
or the REF pin, is 1.182 V. The voltage applied to the Hysteresis pin, HYST, should be half the hysteresis band
voltage, VHB, below the reference voltage. Equation 1 defines the required voltage at the HYST pin in terms of the
reference voltage and the desired hysteresis band. For an internal reference voltage of 1.182 V and a hysteresis
band of 30 mV, the required voltage at HYST results in 1.167 V.
HYST = REF – 0.5  VHB
Equation 1.
Before selecting the feedback network resistor values of R1 and R2, it is important to note that the current sourced
by the internal reference must be maintained below 20 µA and significantly greater than 20 pA. The current
sourced by the internal reference can be approximated by Equation 2. Therefore, to satisfy both conditions, the
value of 2 is selected to be 1 M. This results in a reference current of 1.16 µA.
I_REF
=
H_-----Y_-----S_----T_--
R2
Equation 2.
To calculate the required R1 value for the corresponding hysteresis band and internal reference current,
Equation 3 is provided on page 9 of the TS9002 data sheet. A 30 mV hysteresis band, with a reference current of
1.167 µA, results in a R1 value of 12.85 k.
R1 = ⁰----^.-⁵-----^-----^H-----^Y----^S-----^T--
I_REF
Equation 3.
This feedback network consisting of R2 as 1 M and R1 as 12.85 k, results in a 30 mV hysteresis band with the
lower threshold occurring at 1.168 V while the high threshold occurs at 1.198 V.
To easily adjust the hysteresis band to 40 mV, only the R1 value needs to be changed. Using a 40 mV hysteresis
band, the new voltage at the HYTST pin should be equal to 1.162 V, or 20 mV below the reference voltage as
defined by Equation 1. Since R2 remains equal to 1M, the new reference current is now equal to 1.162 µA.
Inserting the 40 mV hysteresis band and the corresponding reference current into Equation 3, the appropriate R1
value of 17.2 k can be calculated.
Rev. 1.0 1/15
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AN855