STLED325 datasheet(28/62 Pages) STMICROELECTRONICS

28 / 62 page

Functional description

STLED325

28/62

Doc ID 17576 Rev 1

Figure 10.

Power sense circuit

For the STLED325 itself, there is the normal operational mode where the supply is from the

5V VCC. When the VCC drops below a pre-defined low level, the supply source is switched

from the VCC to the battery or super-cap supply.

To conserve power and maintain long battery life in this battery supply mode, only the RTC

and the clock to the RTC remain operational.

The system will only go into battery mode while:

Vcc < 3.5 V and Vcc < Vbat.

So, it means that the system will only switch to battery mode when Vcc drop below 3.5V and

Battery voltage is higher VCC voltage.

The system will enter back into Vcc mode from battery mode while:

VCC > Vbat

It means that the system will switch back to Vcc mode as soon as the VCC is higher than

Vbat.

The STLED325 continually monitors Vcc for an out-of-tolerance condition. Should VCC fall

below the Switchover voltage (VSO = 3.5 V), the device goes into a low-power mode. Inputs

to the device will not be recognized at this time to prevent any erroneous data or outcome

from device. The device also automatically switches over to the battery and powers down

into an ultra low current mode of operation to maximize the super-cap or battery duration. As

system power returns and VCC rises above Vbat, the battery or super-cap is disconnected

and the power supply is switched to the external VCC. During the battery or super-cap back-

up mode, the clock registers of RTC are maintained by the attached battery or super-cap.

On power-up, when VCC returns to a nominal value, write protection continues for tREC (refer

to timing diagram in later part of spec).

Upon power-up, the device switches from battery to VCC when VCC > Vbat. When VCC rises

above Vbat, it will recognize the inputs.

STLED325 数据表(PDF) 28 Page - STMicroelectronics