TSM9117-TSM9120
hysteresis effectively forces one comparator input to
move quickly past the other input, moving the input
out of the region where oscillation occurs. 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.
1) Setting R2. As the leakage current at the IN
pin is less than 2nA, the current through R2
should be at least 0.2μA 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
or
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 = 0.2μA is chosen, then the
formulae above produce two resistor values:
6.26Mꢀ and 10.24Mꢀ - the 6.2Mꢀ standard
value for R2 is selected.
Figure 2: TSM9117-TSM9120 Threshold Hysteresis
2) Next, the desired hysteresis band (VHYSB) is
set. In this example, VHYSB is set to 100mV.
Band
3) Resistor R1 is calculated according to the
following equation:
Adding Hysteresis to the TSM9117/TSM9119
The TSM9117/TSM9119 exhibit an internal
R1 = R2 x (VHYSB/VCC)
hysteresis band (VHYSB
)
of 4mV. Additional
hysteresis can be generated with three external
resistors using positive feedback as shown in Figure
3. Unfortunately, this method also reduces the
and substituting the values selected in 1)
and 2) above yields:
R1 = 6.2Mꢀ x (100mV/3.3V) = 187.88kꢀ.
The 187kꢀ 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:
`
Figure 3: Using Three Resistors Introduces Additional
R3 = 1/[VTHR/(VREF x R1) - (1/R1) - (1/R2)]
Hysteresis in the TSM9117 & TSM9119.
R3 = 1/[3V/(1.252V x 187kꢀ)
- (1/187kꢀ) - (1/6.2Mꢀ)] = 136.9kꢀ
hysteresis response time. Use the following
procedure to calculate resistor values.
TSM9117_20DS r1p0
Page 13
RTFDS
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