TSM9119 datasheet(13/18 Pages) SILABS

13 / 18 page

TSM9117-TSM9120

TSM9117/20 Rev. 1.0

Page 13

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. To save cost and external pcb area,

an internal 4mV hysteresis circuit was added to the

TSM9117–TSM9120.

Adding Hysteresis to the TSM9117/TSM9119

The

TSM9117/TSM9119

exhibit

internal

hysteresis

band

(VHYSB)

4mV.

Additional

hysteresis can be generated with three external

resistors using positive feedback as shown in Figure

3. Unfortunately, this method also reduces the

hysteresis

response

time.

Use

the

following

procedure to calculate resistor values.

1) Setting R2. As the leakage current at the IN

pin is less than 2nA, the current through R2

should be at least 0.2

μA to minimize offset

voltage errors caused by the input leakage

current. The current through R2 at the trip

point is (VREF - VOUT)/R2.

In solving for R2, there are two formulas –

one each for the two possible output states:

R2 = VREF/IR2

R2 = (VCC - VREF)/IR2

From the results of the two formulae, the

smaller of the two resulting resistor values is

chosen. For example, when using the

TSM9117 (VREF = 1.252V) at a VCC = 3.3V

and if IR2 = 0.2

μA is chosen, then the

formulae above produce two resistor values:

6.26M

Ω and 10.24MΩ - the 6.2MΩ standard

value for R2 is selected.

2) Next, the desired hysteresis band (VHYSB) is

set. In this example, VHYSB is set to 100mV.

3) Resistor R1 is calculated according to the

following equation:

R1 = R2 x (VHYSB/VCC)

and substituting the values selected in 1)

and 2) above yields:

R1 = 6.2M

Ω x (100mV/3.3V) = 187.88kΩ.

The 187k

Ω standard value for R1 is chosen.

4) The trip point for VIN rising (VTHR) is chosen

such that VTHR > VREF x (R1 + R2)/R2 (VTHF

is the trip point for VIN falling). This is the

threshold voltage at which the comparator

switches its output from low to high as VIN

rises above the trip point. In this example,

VTHR is set to 3V.

5) With the VTHR from Step 4 above, resistor R3

is then computed as follows:

R3 = 1/[VTHR/(VREF x R1) - (1/R1) - (1/R2)]

R3 = 1/[3V/(1.252V x 187k

Ω)

- (1/187k

Ω) - (1/6.2MΩ)] = 136.9kΩ

In this example, a 137k

Ω, 1% standard

value resistor is selected for R3.

The last step is to verify the trip voltages

and hysteresis band using the standard

resistance values:

For VIN rising:

VTHR = VREF x R1 [(1/R1) + (1/R2) + (1/R3)]

= 3V

and, for VIN falling:

VTHF = VTHR - (R1 x VCC/R2) = 2.9V

Figure 3: Using Three Resistors Introduces Additional

Hysteresis in the TSM9117 & TSM9119.

Figure 2: TSM9117-TSM9120 Threshold Hysteresis

Band

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