Datasheet

and Rt × Ct =

Delay × �

Note: Rt is in kΩ, Ct is in nF, Delay is in seconds. K values are obtained from Figure 3-8.

To ensure correct operation it is recommended that the value of

Rt × Ct

�

is between 4 and 240.

Values outside this range may be interpreted as the hard wired options TIME linked to OUT and TIME

linked to “off” respectively, causing the QT1012 to use the relevant predefined auto-off delays.

Table 3-5. Programmable Auto-off Delay (Example)Vm = Vss (delay multiplier = 1), Vdd = 3.5 V

Output Type Auto-off Delay (Seconds)

Active high (Rt × Ct × 15) / 19

Active low (Rt × Ct × 15) / 22

K values (19 and 22) are obtained from Figure 3-8.

Note: Rt is in kΩ, Ct is in nF.

Figure 3-8. Typical Values of K Versus Supply Voltage

The charts in Figure 3-8 show typical values of K versus supply voltage for a QT1012 with active high or

active low output.

Example using the formula to calculate Rt and Ct

Requirements:

• Active high output (Vop connected to VSS)

• Auto-off delay nominal 45 minutes

• VDD = 3.5 V

Proceed as follows:

1. Calculate Auto-off delay in seconds 45 x 60 = 2700

2. Obtain K from Figure 3-8, K = 22.8

3. Calculate Rt × Ct =

2700 × 22.8

= 4104

4. Decide on a value for Rt or Ct (for example, Ct = 47 nF)

5. Calculate Rt =

4104

= 87 k

6. Verify that

RtxCt

�

= 179 (which is between 4 and 240)

AT42QT1012

Datasheet

DS40001948A-page 16