STLED325 Datasheet, PDF (14/62 Pages) STMicroelectronics – Standby power management to host

Functional description
STLED325
3.8
LED drivers
The constant current capability is up to 40 mA per output segment and is set for all the
outputs using a single external resistor. When acting as digit drivers, the outputs source
current to the display anodes. When acting as segment drivers, the LED outputs sink current
from the display common cathodes. The outputs are high impedance when not being used
as digit or segment drivers.
Each port configured as a LED segment driver behaves as a digitally-controlled constant
current sink. The LED drivers are suitable for both discrete LEDs and common anode (CA)
numeric LED digits. When fully configured as a LED driver, the STLED325 controls up to 8
LED segments in a single digit with individual 8-step adjustment of the constant current
through each LED segment. A single resistor sets the maximum segment current for all the
segments, with a maximum of 40 mA per segment. The STLED325 drives any combination
of discrete LEDs and common anode (CA) digits for numeric displays.
The recommended value of RSET is the minimum allowed value, since it sets the display
driver to the maximum allowed segment current. RSET can be a higher value to set the
segment current to a lower maximum value where desired. The user must also ensure that
the maximum current specifications of the LEDs connected to the drivers are not exceeded.
3.9
Over temperature cut-off
The STLED325 contains an internal temperature sensor that turns off all outputs when the
die temperature exceeds 140°C. The outputs are enabled again when the die temperature
drops below 125°C. Register contents are not affected, so when a driver is over-dissipating,
the external symptom will be the load LEDs cycling between on and off as the driver
repeatedly overheats and cools, alternately turning the LEDs off and then back on again.
This feature will protect the device from damage due to excessive power dissipation. It is
important to have good thermal conduction with a proper lay-out to reduce thermal
resistance.
3.10
Standby mode
By utilizing the standby function, the host processor and other ICs can be turned off to
reduce power consumption. The STLED325 is able to wake-up the system when
programmed hotkeys are detected to signal that the full operation of the system is required.
The hotkeys can be entered to the system through the front panel keys or through the
infrared (IR) remote control or the Real Time Clock (RTC) alarm or through the wake-up pin.
STLED325 supports multiple remote control protocols decoding by setting the appropriate
register.
The STLED325 is able to cut-off the power to the main board for standby operation for good
power management. STBY will be set to high when READY signal goes from high to low, I²C
command for standby is seen or when the guard timer has finished counting down to 0,
whichever occurs first.
In the normal mode of operation, the STBY is asserted only when the guard timer has
finished counting down to 0. This is meant to put the system into stand-by even though
standby command was not issued by the host or READY signal did not go low. This occurs
as the guard timer register was not cleared before it finished counting down to 0.
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Doc ID 17576 Rev 1