DS1239 datasheet(2/4 Pages) DALLAS

2 / 4 page

DS1239

2 of 4

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 VCC (+5V), the PSI is internally held at a high

level at all times with the VBAT supply. When PSI is forced low via a keypad or other source, the PSO is

connected to the VBAT 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 VCC. The timing is illustrated in Figure 2. Holding the PSI input low, the PSO

output will supply a connection to the VBAT pin until the VCC reaches VBAT, or a maximum of 500 ms. If

the supply voltage on VCC rises above the VBAT level before the tPSI timeout, the PSO pin will remain high

and track the VCC input. If VCC does not rise above VBAT before either tPSI or PSI is allowed to return to a

high level, the PSO output will return to tristate. Once the PSO output and VCC 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

DS1239 Datasheet(PDF) 2 Page - Dallas Semiconductor