MP8124 – 500mA, LNB POWER SUPPLY AND CONTROL REGULATOR

MP8124 Rev. 1.0

www.MonolithicPower.com

15

11/11/2015

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© 2015 MPS. All Rights Reserved.

OPERATION

The

MP8124

is

a

single-output

voltage

regulator that provides both a supply voltage

and a control signal from satellite set-top box

modules to the low-noise block (LNB) of the

antenna port.

The

MP8124

uses

an

integrated

boost

converter between 8V and 14V from a single

supply source. It generates the voltage to

enable the linear regulator to work at minimum

dissipated power.

Boost Converter/Linear Regulator

The boost converter is a fixed-frequency, non-

synchronous voltage regulator with peak-

current-mode control. The operating frequency

is 440kHz 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 to allow

the linear regulator to work with minimum drop-

out voltage.

13V/18V Switching

The output voltage can be set to 13V or 18V to

select different polarization directions of the

LNB. A logic high level on 13V/18V sets the

output to 18V; a low level sets it to 13V. Pull

13V/18V up to BYPASS for a default 18V

output during start-up.

LINEDROP Control

To compensate for the excess voltage drop

along the coaxial cable, the MP8124 employs a

voltage compensation function. If LINEDROP is

set at a high level, the output voltage can be

increased by 1V. A low level disables the

function. Pull LINEDROP to BYPASS for a

default 1V compensation.

Output Slew Rate Control

The MP8124 integrates a soft-start function to

reduce the inrush current during start-up. A soft

start is implemented via the external capacitor

Equation (1):

CHA

RISE

TCAP

OUT

(13.5 I

T

)

C

V

××

=

(1)

1V immediately after the IC is enabled, and it is

then controlled by the TCAP voltage.

The output voltage rising and falling times are

also controlled by the soft-start circuit during the

13V/18V transient.

Current Limit

The output current is limited to about 600mA.

When an overload or a short circuit is detected,

the output current is regulated at the current

limit level for 50ms. If the overload is still

detected after this time period, the output shuts

down for 1.9 seconds before it resumes. If an

overload occurs, the window is registered and

lasts for 30µs. If another overload is detected

within this 30µs window, another 30µs window

begins. This detection mode ensures that OCP

hiccup mode occurs in the event of a high

oscillatory current.

The boost converter integrates a cycle-by-cycle

over-current limit function that supports the part

in a full load.

Boost Over-Voltage Protection (OVP)

The

MP8124

features

an

over-voltage

protection (OVP) circuit on SW. If VBOOST is

disconnected on the board, the boost output

voltage cannot feed back to the boost control

circuit, and the boost output voltage runs away.

A typical 32V OVP circuit on SW shuts down

the MP8124 to prevent damage to the IC once

the over-voltage condition is detected. A

blanking time in the detection circuit is added to

avoid mistriggering OVP by the SW spike

voltage. Once the MP8124 experiences OVP,

the part cannot restart until power is applied at

VIN or the EN input.