LNBH25S

Application information

DocID026736 Rev 2

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distortion during tone transmission, the LNBH25S is provided with the BPSW pin to be

connected to an external transistor, which allows the output RL filter to be bypassed in

DiSEqC 2.x applications while in transmission mode. Before starting tone transmission,

TEN bit has to be set to “1” and after ending tone transmission, TEN bit has to be set to “0”.

2.6

Output current limit selection

The linear regulator current limit threshold can be set by an external resistor connected to

the ISEL pin. The resistor value defines the output current limit by the equation:

with ISET=0

with ISET=1

see ISET bit description in Table 9: "Data 3 (read/write register. Register address = 0X4)",

where RSEL is the resistor connected between ISEL and GND expressed in kΩ and

2.7

Output voltage selection

The linear regulator output voltage level can be easily programmed in order to accomplish

application specific requirements, using 4 bits of an internal data 1 register, see Section

7.3: "Data registers" and Table 14: "Output voltage selection table (data1 register, write

mode)" for exact programmable values. Register writing is accessible via I²C bus.

2.8

Diagnostic and protection functions

The LNBH25S has 8 diagnostic internal functions provided by I²C bus, by reading 8 bits on

two status registers (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) while the remaining 6 bits are dedicated to

the output voltage level (VMON), to 22 kHz tone characteristics (TMON), to the minimum

load current (IMON), to external voltage source presence on the VOUT pin (PDO), to the

input voltage power not good function (PNG) and to the 22 kHz tone presence on the

DETIN pin (TDET). Once the OLF (or OTF or PNG) bit is active

(set to “1”), it is latched to

“1” until relevant cause is removed and a new register reading operation is performed.

2.9

Surge protections and TVS diodes

The LNBH25S 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 LNBH25S and other devices are electrically connected to the antenna

cable.

I

LIM

(typ.) = 13915

RSEL

1.111

I

LIM

(typ.) = 6808

RSEL

1.068