MP8124 – 500mA, LNB POWER SUPPLY AND CONTROL REGULATOR

MP8124 Rev. 1.0

www.MonolithicPower.com

17

11/11/2015

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© 2015 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Input Capacitor

The input capacitor (C1) is used to maintain the

DC input voltage. Low ESR ceramic capacitors

with 10μF X7R dielectrics are recommended.

The input voltage ripple can be estimated with

Equation (2):

IN

IN

IN

2

sOUT

VV

V1

8f L C1

V

⎛⎞

Δ=

⋅ −

⎜⎟

⋅⋅

⎝⎠

(2)

is the boost inductor value.

Setting the Boost Output Capacitor

The output current to the boost converter is

discontinuous, and therefore requires an output

capacitor (C2) to supply AC current to the load.

For best performance, low ESR capacitors are

recommended. The output voltage ripple can be

estimated with Equation (3):

OUT

IN

OUT

sL

OUT

V

V

V1

fR C2

V

⎛⎞

Δ=

⋅ −

⎜⎟

⋅⋅

⎝⎠

(3)

Ceramic capacitors with X7R dielectrics are

highly recommended because of their low ESR

and small temperature coefficients. Typically,

two-piece 10μF X7R ceramic capacitors are

recommended. Place one close to the boost

switching loop and the other close to the LDO

input.

Tantalum or low ESR electrolytic capacitors are

also sufficient for the boost output. In this

condition, it is recommended to place two small

ceramic capacitors (0.1µF or higher) close to

both the boost-switching loop output and the

LDO input. When using electrolytic capacitors,

ESRs that are too high may introduce more

voltage ripple on the boost output. Additional

filters may be necessary to minimize the ripple.

Selecting the Inductor of the Boost Converter

An inductor is required to transfer the energy

between the input source and the boost output

capacitors. An inductor with a larger value results

in less ripple current and a lower peak inductor

current, and therefore it reduces the stress on the

power MOSFET. However, the larger value

inductor has a larger physical size, a higher

series resistance, and a lower saturation current.

For most designs, the inductance value can be

calculated with Equation (4):

IN

OUT

IN

sOUT

L

V(V

V )

L

fV

I

−

=

⋅⋅ Δ

(4)

Choose the inductor ripple current to be

approximately

30%~50%

of

the

maximum

inductor peak current. Typically, a 10μH inductor

is recommended. Ensure that the inductor does

not saturate under a worst-case load transient

condition. The inductor should have a low series

resistance (DCR) to reduce the resistive power

loss.

Selecting the Rectifier Diode of the Boost

Converter

The high switching frequency demands high-

speed

rectifiers.

Schottky

diodes

are

recommended for most applications because of

their fast recovery times and low forward voltage.

Typically, a 2A or 3A high efficiency Schottky

diode is recommended for the boost converter;

the diode voltage rating should be higher than

the output voltage. A higher rating may be

needed based on other protections, such as

surge protection.

LDO Output Capacitor and Diode

To permit transport of the 22kHz tone signal, a

0.1μF output capacitor is recommended for the

LDO regulator. A capacitance that is too high

may affect the 22kHz signal shape, and a

capacitance that is too low may lead to LDO

instability.

For the OCP or SCP conditions on the far end of

the bus line (cable), the MP8124 shuts down for

protection, and VOUT may drop to a negative

voltage due to the long cable parasitic inductance.

One low-voltage drop Schottky diode (i.e.,

1N5819 or better) placed between VOUT and

GND is recommended to clamp the negative

voltage.