MP5600 datasheet(21/26 Pages) MPS

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MP5600-COMPACT MULTIPLE-OUTPUT POWER SUPPLY FOR TV-LCD PANEL

MP5600 Rev. 0.9

www.MonolithicPower.com

10/19/2009

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

Selecting the Output Capacitor

The output capacitor (C8) is required to maintain

the DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to reduce the

output voltage ripple. The output voltage ripple

can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

ΔV

ESR

OBUCK

Where L is the inductance of the inductor and

RESR is the equivalent series resistance (ESR)

value of the output capacitor.

In the case of ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

OBUCK

ΔV

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the output

ripple can be approximated by:

ESR

OBUCK

ΔV

⎟

⎠

⎞

⎜

⎝

⎛ −

Selecting the Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, Schottky diode is preferred.

Choose a diode whose maximum reverse voltage

rating is greater than the maximum input voltage,

and whose current rating is greater than the

maximum load current.

Boost Regulator

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The R2 is low-side resistor

of the voltage divider. Determine the high-side

resistor R1 by the equation:

OBOOST

)

R2(V

−

Where VOBOOST is the output voltage of the boost

regulator.

Selecting the Inductor

The inductor is required to force the higher output

voltage while being driven by the input voltage. A

larger value inductor results in less ripple current

that results in lower peak inductor current,

reducing stress on the internal N-Channel.switch.

However, the larger value inductor has a larger

physical size, higher series resistance, and/or

lower saturation current.

The inductance value can be exactly calculated.

A good rule of thumb is to allow the peak-to-peak

ripple current to be approximately 30-50% of the

maximum input current. Make sure that the peak

inductor current is below 75% of the current limit

value at the operating duty cycle to prevent loss

of regulation due to the current limit. Also make

sure that the inductor does not saturate under the

worst-case load transient and startup conditions.

Calculate the required inductance value by the

equation:

ΔI

)

OBOOST

(MAX)

LOAD

OBOOST

IN(MAX)

(

)

IN(MAX)

50%

30%

ΔI

−

Where ILOAD(MAX) is the maximum load current, ΔI is

the peak-to-peak inductor ripple current, and η is

efficiency.

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current of the inductor, while limiting noise

at the input source. A low ESR capacitor is

required minimize the noise of the IC. Ceramic

capacitors are preferred, but tantalum or low-

ESR electrolytic capacitors may also suffice.

The capacitance of the input capacitor should be

greater than 4.7μF. The capacitor can be

electrolytic, tantalum or ceramic. However since

it absorbs the input switching current, it requires

an adequate ripple current rating. Use a

capacitor with RMS current rating which is

greater than the inductor ripple current.

Place the input capacitor as close to the IC as

possible to reduce the noise. Alternately a

smaller and high quality ceramic capacitor should

be placed closer to the IC if the larger capacitor

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