TSM921 datasheet(13/20 Pages) SILABS

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TSM921-TSM924

TSM921/24 Rev. 1.0

Page 13

APPLICATIONS INFORMATION

Hysteresis

As a result of circuit noise or unintended parasitic

feedback, many analog comparators often break into

oscillation within their linear region of operation

especially when the applied differential input voltage

approaches 0V (zero volt). Externally-introduced

hysteresis

well-established

technique

stabilizing analog comparator behavior and requires

external components. As shown in Figure 1, adding

comparator hysteresis creates two trip points: VTHR

(for the rising input voltage) and VTHF (for the falling

input voltage). The hysteresis band (VHB) is defined

as the voltage difference between the two trip points.

When a comparator’s input voltages are equal,

hysteresis effectively forces one comparator input to

move quickly past the other input, moving the input

out of the region where oscillation occurs. Figure 2

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 (TSM921 and TSM923)

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 the HYST pin to the

REF pin. 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 TSM923, the hysteresis is the same for both

comparators.

Hysteresis (TSM922 and TSM924)

Relative to adding hysteresis with the HYST pin as

was done for the TSM921 and the TSM923, the

circuit in Figure 3 uses positive feedback along with

two external resistors to set the desired hysteresis

for the TSM924. The circuit consumes more current

and it slows down the hysteresis effect due to the

Figure 1. Threshold Hysteresis Band

Figure 2. Programming the HYST Pin

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