MP8124 – 500mA, LNB POWER SUPPLY AND CONTROL REGULATOR

MP8124 Rev. 1.0

www.MonolithicPower.com

16

11/11/2015

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Tone Generation

In accordance with DiSEqC 1.x standards, the

part can detect a 22kHz signal through EXTM,

shape it with the built-in amplitudes and rise

and fall times, and modulate the output (see

Figure 2).

The external tone source on EXTM should be

within the 20kHz to 24kHz range to meet

DiSEqC 1.x standards. The MP8124 responds

to the tone rising and returns to a normal

voltage if there is no signal input from EXTM

after about 40μs. Tie EXTM to GND if the

22kHz signal is not needed.

The 22kHz tone signal is shaped on VOUT by

the linear regulator. The MP8124 is able to

provide an optimal tone signal, even when there

is no load on VOUT.

Figure 2: Tone Signal on Output

LDO Pull-Down Current and Protection

VOUT sinks the current when the output is

higher than the set voltage, so the voltage

transient from 18V to 13V or the 22kHz tone

signal works well, even though there is no load

current on the output. To prevent power loss

and thermal issues caused by the sink current

when the output is biased to a higher voltage, a

timer with a typical 9ms period is enabled when

the sink current triggers the sink current limit. If

the LDO sink current limit signal still remains

after 9ms, the sink current circuit is turned off.

The sink current circuit is enabled again after a

delay of about 450ms and continues monitoring

the sink over-current condition.

The LDO has different sink current capabilities

based on whether the tone is available or not.

Thermal Protection

When

the

junction

temperature

exceeds

+160°C, the part shuts down. Once the junction

temperature drops below 140°C, the part

restarts automatically.