LNBH25 datasheet(8/34 Pages) STMICROELECTRONICS

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Application information

LNBH25

8/34

Doc ID 022433 Rev 4

2.9

Surge protections and TVS diodes

The LNBH25 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

placed, as shown in the following schematic, to protect the STB output circuits where the

LNBH25 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

peak power dissipation that the diode is capable of supporting (see Ppp (W) parameter in

the LNBTVS datasheet for further details).

2.10

FLT: fault flag

In order to get an immediate feedback on diagnostic status, LNBH25 is equipped with a

dedicated fault flag pin (FLT). In the case of overload (OLF bit=1) or overheating (OTF bit=1)

or if Power No Good (PNG bit=1) condition is detected, the FLT pin (open drain output) is set

to low and is kept low until the relevant activating diagnostic bit is cleared. Be aware that

diagnostic bits OLF, OTF and PNG, once activated, are kept latched to “1” until the cause

origin is removed and a new register reading operation is performed by the microprocessor.

The FLT pin must be connected to a positive voltage (5 V max.) by means of a pull-up

resistor.

2.11

VMON: output voltage diagnostic

When device output voltage is activated (VOUT pin), its value is internally monitored and, as

long as the output voltage level is below the guaranteed limits, the VMON I²C bit is set to “1”.

See

Table 17 for more details.

2.12

TMON: 22 kHz tone diagnostic

The 22 kHz tone can be internally detected and monitored if the DETIN pin is connected to

the LNB output bus (see typical application circuit in

Figure 7) through a decoupling

capacitor.

The tone diagnostic function is provided with the TMON I²C bit. If the 22 kHz tone amplitude

and/or the tone frequency is out of the guaranteed limits (see

Table 19), the TMON I²C bit is

set to “1”.

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