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Please tell us the cause of the power supply potential rise.
An experiment was performed using 74AHC245 under the following conditions.
VCC = DIR = 3.3V.
/ OE = 0V.
A1 = 5V.
The stabilized power supply used for the IC power supply is PMM25-1TR manufactured by KIKUSUI ELECTRONICS CORPORATION.
Here, a part of the instruction manual of the used stabilized power supply device is described.
"This product cannot sink reverse current from the load. If a load that regenerates power to the power supply is connected, the output voltage will increase and the output will not be stabilized."
As an experimental result, a waveform before applying a 5V voltage to A1 is shown in FIG. 1, and a waveform when a 5V voltage is applied is shown in FIG. 2 (ch1: A1 terminal, ch2: B1 terminal).
Figure 1 Waveform before applying 5V voltage to A1(2.0V/division)
Figure 2 Waveform when 5V voltage is applied to A1(2.0V/division)
As shown in FIG. 2, the input voltage dropped from 5 V to 4.3 V, and the output voltage also rose from 3.3 V to 3.9 V. The power supply voltage also increased from 3.3V to about 4V. The current supplied to the IC changed from about 1 mA before input to minus 2.25 mA after input, and became a reverse current.
The 74AHC245 has an input tolerant, and the circuit shown in Fig. 3 is introduced in the catalog of the AHC series of TI.
Figure 3 Excerpt from Advanced High-Speed CMOS (AHC) Logic Family SCAA034C December 2002 document.
Why does the output potential rise and reverse current to the power supply causes a decrease?
If possible, please provide answers in Japanese.We apologize for the incorrect English.
CMOS devices draw very little supply current when idle (< 4 µA for the SN74AHC245). If the supply current was 1 mA before the experiment, then the device was damaged previously.
Thank you for your reply.
We will investigate the items you pointed out.
We are also looking forward to answering any questions you may have about other causes.
The most likely cause is ESD or other overvoltage (or reverse voltage). The exact cause would be very hard to determine afterwards.