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AN3060 Datasheet(PDF) 8 Page - STMicroelectronics |
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AN3060 Datasheet(HTML) 8 Page - STMicroelectronics |
8 / 23 page Undershoot AN3060 8/23 Doc ID 16253 Rev 2 3 Undershoot 3.1 Can undershoot affect operation of my real-time clock? How? What can be done to guard against it? Inherent in all field effect devices - all CMOS devices - are parasitic diodes resulting from the intersection of P and N type regions in the device. Every PN junction forms a diode which will conduct under any condition which biases the P region above the N region. In Figure 3, if VCC drops sufficiently below ground, the parasitic diode will be turned on thus drawing current through the device. Significant undershoot on VCC can result in enough current flow to cause some disturbance in the real-time clock registers and hence can adversely affect timekeeping. Therefore, undershoot should be avoided. Figure 3. Typical parasitic diode Most ST real-time clocks specify a minimum VCC of –0.3 V thereby limiting negative undershoot to 0.3 V below ground. Some supplies ramp very quickly when power cycling with the result that ringing can occur which produces an undershoot condition that can cause problems for real-time clocks. In the rare situations where this occurs, the best protection is a back biased Schottky diode on the VCC pin as shown in Figure 4. This addition has been shown to be a very reliable fix for undershoot problems. Figure 4. Undershoot protection N N+ N+ P D S G VCC AM04721v1 SDA RTC VSS SCL XI XO AM04722v1 VCC VCC 1N5817 (through hole) BAT54J (surface mount) |
Similar Part No. - AN3060 |
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Similar Description - AN3060 |
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