MP8126 – LNB POWER SUPPLY AND CONTROL VOLTAGE REGULATOR

MP8126 Rev. 1.03

www.MonolithicPower.com

9

5/27/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The

MP8126

is

a

single-output

voltage

regulator that provides both the supply voltage

and the control signal from satellite set-top box

modules to the LNB (low noise block) of the

antenna port.

The MP8126 has an integrated boost converter

that—from a single supply source between 8V

and 14V—generates a voltage that enables the

linear

post-regulator

to

function

while

minimizing power dissipation.

Boost Converter/Linear Regulator

The boost converter is a fixed-frequency, non-

synchronous, voltage regulator with peak

current mode control. The operating frequency

is 342 kHz, typically.

To reduce power dissipation, the boost

converter operates in a pulse-skipping mode at

light load.

The output voltage of the boost converter tracks

the requested output voltage to allow the linear

regulator to work at the minimum drop-out

voltage.

13V/18V Switching

The 13V/18V logic input pin can set the output

voltage to select different LNB polarization

directions. A logic HIGH sets the output to 18V

LOW sets it to 13V.

LINEDROP Control

The MP8126 has a voltage compensation

function

that

compensates

for

excessive

voltage drop along the coaxial cable. Setting

the LINEDROP pin to HIGH allows the output

voltage increases by 1V while LOW disables

the function.

Soft-Start

the required slew rate (k) of the output voltage

using the following formula:

SS

CHA

C(15 I

) / k

=×

Current Limit

limits the output current dynamically. Select

LIMIT

LIMIT

I7700 / R

=

When an overload occurs, the device regulates

the output current at the current limit level for

50ms. If the overload is still detected after this

50ms, the output shuts down for 2 seconds

before the resuming the output.

The boost converter uses an integrated cycle-

by-cycle

over-current

limit

function

that

guarantees the part works with a full load.

To ensure normal current-limiting functionality,

Power Good (POK)

POK connects to an internal, open-drain device.

When the output voltage falls out of the ±12%

normal value range, POK is pulled down.

Otherwise, POK is pulled high.

Tone Generation

In accordance with DiSEqC standards, the part

can detect a 22kHz signal through EXTM pin,

shape it with the built-in amplitudes and rise/fall

times, and modulate the output (Figure 2). Tie

the EXTM pin to GND if the 22kHz signal isn’t

needed.

EXTM

Output

13V/18V

Figure 2: Tone Signal at Output

Thermal Protection

When

the

junction

temperature

exceeds

+150°C, the part shuts down. Once the junction

temperature cools—typically to 130°C—the part

restarts automatically.