Datasheet

LNBH25L Application information
Doc ID 022634 Rev 2 7/28
2.6 Diagnostic and protection functions
The LNBH25L has 3 diagnostic internal functions provided via the I²C bus, by reading 3 bits
on the STATUS1 register (in read mode). All the diagnostic bits are, in normal operation (that
is no failure detected), set to LOW. Two diagnostic bits are dedicated to the overtemperature
and overload protection status (OTF and OLF). One bit is dedicated to the input voltage
power not good function (PNG). Once the OLF (or OTF or PNG) bit has been activated (set
to “1”), it is latched to “1” until the relevant cause is removed and a new register reading
operation is done.
2.7 Surge protection and TVS diodes
The LNBH25L device is directly connected to the antenna cable in a set-top box.
Atmospheric phenomenon can cause high voltage discharges on the antenna cable causing
damage to the attached devices. Surge pulses occur due to direct or indirect lightning
strikes to an external (outdoor) circuit. This leads to currents or electromagnetic fields
causing high voltage or current transients. Transient voltage suppressor (TVS) devices are
usually used, as shown in the following schematic, to protect the STB output circuits where
the LNBH25L and other devices are electrically connected to the antenna cable.
Figure 4. Surge protection circuit
For this purpose we recommend the use of LNBTVSxx surge protection diodes specifically
designed by ST. The selection of the LNBTVS diode should be made based on the
maximum peak power dissipation that the diode is capable of supporting (see the LNBTVS
datasheet for further details).
2.8 Power-on I²C interface reset and undervoltage lockout
The I²C interface built into the LNBH25L is automatically reset at power-on. As long as the
V
CC
stays below the undervoltage lockout (UVLO) threshold (4.7 V typ.), the interface does
not respond to any I²C command and all data register bits are initialized to zeroes, therefore
keeping the power blocks disabled. Once the V
CC
rises above 4.8 V typ. the I²C interface
becomes operative and the data registers can be configured by the main microprocessor.
2.9 PNG: input voltage minimum detection
When input voltage (V
CC
pin) is lower than LPD (low power diagnostic) minimum thresholds,
the PNG I²C bit is set to “1” and the FLT pin is set low. Refer to
Tabl e 12 for threshold details.