DS1239 Datasheet, PDF (2/4 Pages) Dallas Semiconductor – MicroManager Chip

DS1239
DS1239 is identical to the DS1236 with two exceptions. The RC and RST pins have been replaced with
PSI and PSO, respectively. Other pins and functions operate exactly as the DS1236 in NMOS mode.
POWER SUPPLY CONTROL
The DS1239 facilitates the power-up and power-down sequencing of a main power supply from a
keyboard or pushbutton. The Power Supply Control Input ( PSI ) and Power Supply Control Output (PSO)
are used for this purpose. Prior to establishing a voltage on V_CC (+5V), the PSI is internally held at a high
level at all times with the V_BAT supply. When PSI is forced low via a keypad or other source, the PSO is
connected to the V_BAT to provide a high level. As shown in Figure 1, this active high signal can be wired
directly to an optically isolated SCR to initiate an AC to DC power-up sequence. This in turn will provide
the supply voltage for V_CC. The timing is illustrated in Figure 2. Holding the PSI input low, the PSO
output will supply a connection to the V_BAT pin until the V_CC reaches V_BAT, or a maximum of 500 ms. If
the supply voltage on V_CC rises above the V_BAT level before the t_PSI timeout, the PSO pin will remain high
and track the V_CC input. If V_CC does not rise above V_BAT before either t_PSI or PSI is allowed to return to a
high level, the PSO output will return to tristate. Once the PSO output and V_CC are set at a high level, a
subsequent falling edge on PSI will tristate PSO to initiate a shut down condition. The 10 microamp
current supplied by the PSI pin allows the use of a 0.1 µF capacitor as a simple pushbutton debounce
circuit. The battery size for this application must be selected to provide the SCR on-current for the power
supply response time and is consequently application-specific.
POWER SUPPLY CONTROL Figure 1
2 of 4