TLC770X Datasheet, PDF (10/27 Pages) Texas Instruments – BiCMOS SUPPLY VOLTAGE SUPERVISORS

The operation of the circuit can be described as follows.
The voltage to be monitored is applied to the SENSE input. This voltage is then subdivided by the voltage divider
R₁/R₂, and compared in the comparator 1 with a reference voltage Vref, which is internally generated in the device.
The resistance values given in the Data Sheet for the resistive voltage divider at the SENSE input are typical values
only, since they are liable to considerable scatter during the manufacturing process (≈ ± 20%). Of importance is only
the ratio of the resistors, and this is only liable to slight process scatter, and in addition can be adjusted during the
manufacture of the component. The 1 V version TLC7701 is however an exception: with this component, an external
voltage divider must be used to determine the threshold voltage. This should be of high resistance (100 - 200 kΩ/V),
in order that the current consumption does not become excessive. The final resulting threshold voltage V_t’ can then
be calculated as follows:
V_t ’ = V_ref ⋅ ^{(R1 + R2 )}
R₂
with V_ref = 1.1 V
If for example a voltage of 2.5 V (±5 % tolerance) is to be monitored, the lower tolerance limit must be chosen for the
calculation of the resistance values.V_t’ is then calculated as follows:
V_t ’ = 2.5 V ⋅ 0.95 = 2.375 V
The values of the resistors which result are then:
R₁ = 174 kΩ and R₂ = 150 kΩ
The resistors must be of very close tolerance (± 1%).
The reference voltage V_ref = 1.1 V is generated with a Band-Gap reference. This precise reference voltage has
excellent temperature stability, and allows a very close tolerance of the output voltage V_ref. The resulting tolerance
is typically better than 1%, with a maximum of 1.5%.
As long as the voltage at the SENSE input is less than the threshold voltage V_t, the thyristor T is conducting and the
capacitor C_t is discharged. If the voltage at the SENSE input increases and becomes more than the voltage V_t,, then
the thyristor T blocks as a result the low current flowing through it (less than its holding current), and the capacitor C_t
becomes charged via the current source I_Charge. In order to keep the current consumption of the circuit low, the
current source I_Charge is only switched on via the switch S when a charging of the capacitor really needs to be
performed. This condition is fulfilled when the voltage at the SENSE input has reached the threshold value V_t
(V_SENSE > V_t), and the charging process of the capacitor C_t has not yet been completed ( V_Ct ≤ V_{Re f} ). In all other
operating states, the current source I_Charge remains switched off.
The voltage across the capacitor C_t is in turn compared by the comparator 2 with the reference voltage V_ref. If the
capacitor voltage exceeds the reference voltage, then the subsequent logic circuitry switches the outputs RESET
and RESET into the inactive state. This results in RESET Low becoming, and RESET becoming High.
Figure 7. shows the relationship between the behavior of the voltage at the SENSE input and the output RESET .
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