TSM971C datasheet(13/19 Pages) SILABS

13 / 19 page

TSM971/72/73/82/84

TSM971/72/73/82/84 Rev. 1.0

Page 13

hysteresis effectively forces one comparator input to

move quickly past the other input, moving the input

out of the region where oscillation occurs. Figure 1

illustrates the case in which an IN- input is a fixed

voltage and an IN+ is varied. If the input signals

were reversed, the figure would be the same with an

inverted output.

Hysteresis (TSM971/973 and TSM982)

Hysteresis can be generated with two external

resistors using positive feedback as shown in

Figure 2. Resistor R1 is connected between REF

and HYST and R2 is connected between HYST and

V-. This will increase the trip point for the rising input

voltage, VTHR, and decrease the trip point for the

falling input voltage, VTHF, by the same amount. If no

hysteresis is required, connect HYST to REF. The

hysteresis band, VHB, is voltage across the REF and

HYST pin multiplied by a factor of 2. The HYST pin

can accept a voltage between REF and REF-50mV,

where a voltage of REF-50mV generates the

maximum voltage across R1 and thus, the maximum

hysteresis and hysteresis band of 50mV and

100mV, respectively. To design the circuit for a

desired hysteresis band, consider the equations

below to acquire the values for resistors R1 and R2:

R1 =

VHB

2 x IREF

R2 =

1.182 -

VHB

IREF

where IREF is the primary source of current out of the

reference pin and should be maintained within the

maximum current the reference can source. This is

typically in the range of 0.1

μA and 4μA. It is also

important to ensure that the current from reference is

much larger than the HYST pin input current. Given

R2 = 2.4M

Ω, the current sourced by the reference is

0.5

μA. This allows the hysteresis band and R1 to be

approximated as follows:

R1(k

Ω) = VHB(mv)

For the TSM973 and TSM982, the hysteresis is the

same for both comparators.

Hysteresis (TSM972 and TSM984)

Relative to adding hysteresis with the HYST pin as

was done for the TSM971, TSM973, and the

TSM982, the circuit in Figure 3 uses positive

feedback along with two external resistors to set the

desired hysteresis. The circuit consumes more

current and it slows down the hysteresis effect due

to the high impedance on the feedback. Due to the

pull-up resistor on the output and its inability to

Figure 1. Threshold Hysteresis Band

Figure 2. Programming the HYST Pin

Figure 3. External Hysteresis

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