MP8124 – 500mA, LNB POWER SUPPLY AND CONTROL REGULATOR

MP8124 Rev. 1.0

www.MonolithicPower.com

18

11/11/2015

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Soft-Start (SS) Capacitor Setting

With the required output voltage rising time

Equation (5):

CHA

RISE

TCAP

OUT

(13.5 I

T

)

C

V

××

=

(5)

6.8μA.

The TCAP soft-start capacitor also controls the

voltage transient slew rate between 13V and 18V.

TCAP is the feedback reference voltage of the

boost and LDO output. One small capacitor is

necessary to decouple TCAP. Typically, a 22nF

capacitor is recommended to control the soft start

and decouple the reference voltage.

Compensation Circuits Setting for the Boost

Converter

The output of the transconductance error

amplifier (COMP) is used to compensate the

regulation control system. R1 and C7 in series

compensate the feedback loop gain and phase.

Typically, R1 = 60.4kΩ and C7 = 3.3nF is

recommended for the ceramic output condition.

Please refer to Figure 4 on page 20 for details.

If the electrolytic capacitor is used to replace the

ceramic capacitor at the boost’s output during

application, one small capacitor from COMP to

GND is recommended. This forms a pole with R1

to compensate for the effect of the zero formed

by the electrolytic capacitor ESR.

BYPASS Capacitor

circuit bias at 5V, typically. One 0.22µF ceramic

bypass capacitor is necessary for the internal

control circuit and does not connect the external

BST Capacitor

The MP8124 uses one charge pump to power

the N-channel MOSFET for the LDO regulator.

One external bootstrap capacitor is necessary to

bypass the charge pump power. A 0.1µF ceramic

capacitor between

BST

and

VBOOST

is

recommended.

Logic Input Connection

For EN, LINEDROP, 13V/18V, and EXTM, one

high-level voltage (>2V) or low-level voltage

(<0.8V) sets the different function states. The

high-level input voltage connected to these pins

should not be higher than 6.5V. For high-voltage

inputs, <5.5V signals are recommended.

The EN signal can be connected to VIN through

a resistor divider to set the automatic start-up

once the input power is available. Do not pull up

to VIN through one resistor. The resistor divider

must keep the EN voltage within the 2V to 5.5V

range for an input power range of 8V to 14V.

Typically, a divider with a 249kΩ high-side

resistor and a 100kΩ low-side resistor are

recommended for normal application.

It is suggested that LINEDROP and 13V/18V are

pulled to the BYPASS voltage for a default high

input or pulled to GND for a default low input.

The EXTM input signal should also be lower than

5.5V.