LNBH25LS

Application information

DocID026753 Rev 3

9/30

2.7

Surge protections and TVS diodes

The LNBH25LS 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 LNBH25LS 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 LNBTVS diodes should be based on the maximum peak

power dissipation supported by the diode, see the LNBTVS datasheet for further details.

2.8

Power-on I²C interface reset and undervoltage lockout

The I²C interface built into the LNBH25LS is automatically reset at power-on. As long as

does not respond to any I²C command and all data register bits are initialized to zeros,

interface becomes operative and the data registers can be configured by the main

microprocessor.

2.9

PNG: input voltage minimum detection

When input voltage (VCC pin) is lower than LPD (low power diagnostic) minimum

thresholds, the PNG I²C bit is set to “1”.

2.10

COMP: boost capacitors and inductor

The DC-DC converter compensation loop can be optimized in order to properly work with

both ceramic and electrolytic capacitors (VUP pin). For this purpose, one I²C bit in the data

4 register, see COMP Table 9: "Data 4 (read/write register. Register address = 0X5)" can

be set to “1” or “0” as follows:



COMP = 0 for electrolytic capacitors



COMP = 1 for ceramic capacitors

For recommended DC-DC capacitor and inductor values refer to Section 5: "Typical

application circuits" and to the BOM in Table 5: "DiSEqC 1.x bill of material".

2.11

OLF: overcurrent, short-circuit protection and diagnostic

In order to reduce the total power dissipation during an overload or a short-circuit condition,

the device is provided with a dynamic short-circuit protection. It is possible to set short-