TSM917
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. To save cost and external pcb area,
an internal 4mV hysteresis circuit was added to the
TSM917.
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
TSM917 (VREF = 1.245V) at a VCC = 3.3V
and if IR2 = 0.2μA is chosen, then the
formulae above produce two resistor values:
6.23MΩ 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.
Figure 2: TSM917’s Threshold Hysteresis Band
3) Resistor R1 is calculated according to the
following equation:
Adding Hysteresis to the TSM917
R1 = R2 x (VHB/VCC)
The TSM917 exhibits an internal hysteresis band
(VHB) of 4mV. Additional hysteresis can be
generated with three external resistors using positive
feedbackas shown in Figure 3. Unfortunately, this
method also reduces the hysteresis response time.
The design procedure below can be used to
calculate resistor values.
and substituting the values selected in 1)
and 2) above yields:
R1 = 6.2MΩ x (100mV/3.3V) = 187.88kΩ
The 187kΩ standard value for R1 is
selected.
4) The trip point for VIN rising (VTHR) is chosen
such that VTHR > VREF x (R1 + R2)/R2 (where
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)]
Figure 3: Using Three Resistors Introduces Additional
Hysteresis in the TSM917.
R3 = 1/[3V/(1.245V x 187kΩ) - (1/187kΩ)
- (1/6.2MΩ)] = 135.56kΩ
1) Setting R2. As the leakage current at the IN
pin is under 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
In this example, a 137kΩ, 1% standard
value resistor is selected for R3.
TSM917DS r1p0
Page 11
RTFDS
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