TS12011/TS12012
falling). This is the threshold voltage at which
the comparator switches its output from low
to high as V
COMPIN+
rises above the trip point.
In this example, V
THR
is set to 2.
5)
With the V
THR
from Step 4 above, resistor R3
is then computed as follows:
R3 = 1/[V
THR
/(V
REFOUT
x R1) - (1/R1) - (1/R2)]
Figure 2.
Using Three Resistors Introduces
Additional Hysteresis in the TS12011
1)
R3 = 1/[2V/(0.58V x 160kΩ) - (1/160kΩ) -
(1/4.02MΩ)] = 66.43kΩ
In this example, a 69.8kΩ, 1% standard
value resistor is selected for R3.
6)
Setting R2. As the leakage current at the IN
pin is less than 20nA, the current through R2
should be at least 150nA to minimize offset
voltage errors caused by the input leakage
current. The current through R2 at the trip
point is (V
REFOUT
- V
COMPOUT
)/R2.
In solving for R2, there are two formulas –
one each for the two possible output states:
R2 = V
REFOUT
/I
R2
or
R2 = (V
DD
- V
REFOUT
)/I
R2
From the results of the two formulae, the
smaller of the two resulting resistor values is
chosen. For example, when using the
TS12011 (V
REFOUT
= 0.58V) at a V
DD
= 2.5V
and if I
R2
= 150nA is chosen, then the
formulae above produce two resistor values:
3.87MΩ and 12.8MΩ - a 4.02MΩ standard
value for R2 is selected.
The last step is to verify the trip voltages and
hysteresis band using the standard
resistance values:
For V
COMPIN+
rising:
V
THR
= V
REFOUT
x R1 [(1/R1) + (1/R2) + (1/R3)]
= 1.93V
For V
COMPIN+
falling:
V
THF
= V
THR
- (R1 x V
DD
/R2) = 1.83V
and Hysteresis Band = V
THR
– V
THF
= 100mV
2)
Next, the desired hysteresis band (V
HYSB
) is
set. In this example, V
HYSB
is set to 100mV.
Resistor R1 is calculated according to the
following equation:
R1 = R2 x (V
HYSB
/V
DD
)
and substituting the values selected in 1) and
2) above yields:
R1 = 4.02MΩ x (100mV/2.5V) = 160.8kΩ.
The 160kΩ standard value for R1 is chosen.
3)
4)
The trip point for COMPIN+ rising (V
THR
) is
chosen such that V
THR
> V
REFOUT
x (R1 +
R2)/R2 (V
THF
is the trip point for V
COMPIN+
TS12011_12DS r1p0
RTFDS
Page 8
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
