MAP3204 Просмотр технического описания (PDF) - MagnaChip Semiconductor

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MAP3204

4-channelLED Driverfor High Brightness LEDs

MagnaChip Semiconductor 

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Datasheet Version 1.5

EXTERNAL COMPONENTS SELECTION

Inductor

The inductor value should be decided before system

design. Because the selection of the inductor affects

the operating mode of CCM(Continuous Conduction

Mode) or DCM(Discontinuous Conduction Mode). In

CCM operation, inductor size should be bigger, even

though the ripple current and peak current of inductor

can be small. In DCM operation, even ripple current

and peak current of inductor should be large while the

inductor size can be smaller.

The inductor DC current or input current can be

calculated as following equations.

IIN = VOUT × IOUT

VIN ×η

η – Efficiency of the boost converter

Then the duty ratio is,

D = VOUT − VIN + VD

VOUT + VD

VD – Forward voltage drop of the output rectifying

diode

When the boost converter runs in DCM ( L < Lcritical),

it takes the advantages of small inductance and quick

transient response. The inductor peak current is,

IL _ peak _ DCM = VIN × D

fSW × L

The converter will work in CCM if L > Lcritical,

generally the converter has higher efficiency under

CCM and the inductor peak current is,

IL _ peak _ CCM = IIN + VIN × D

2 fSW × L

Input Capacitor

In boost converter, input current flows continuously

into the inductor; AC ripple component is only

proportional to the rate of the inductor charging, thus,

smaller value input capacitors may be used. Ensure

the voltage rating of the input capacitor is suitable to

handle the full supply range.

A capacitor with low ESR should be chosen to

minimize heating effects and improve system

efficiency.

Output Capacitor

The output capacitor acts to smooth the output voltage

and supplies load current directly during the conduction

phase of the power switch. Output ripple voltage

consists of the discharge of the output capacitor during

the FET ton period and the voltage drop due to load

current flowing through the ESR of the output capacitor.

The ripple voltage is shown in following equation.

∆VOUT = IOUT × D + IOUT × ESR

COUT × fSW

Assume a ceramic capacitor is used. The minimum

capacitance needed for a given ripple can be estimated

by following equation.

COUT = (VIN − VOUT ) × IOUT

VOUT × fSW × ∆VOUT

Output Rectifying Diode

Schottky diodes are the ideal choice for MAP3204

due to their low forward voltage drop and fast

switching speed. Make sure that the diode has a

voltage rating greater that the possible maximum

output voltage. The diode conducts current only when

the power switch is turned off.

Loop Compensation

The MAP3204 controls in current mode. Current

mode easily achieves compensation by consisting

simple single pole from double pole that LC filer makes

at voltage mode. In general, crossover frequency is

selected from 1/3 ~ 1/6 range of the switching

frequency. If fc is large, there is possibility of oscillation

to occur, although time response gets better.

On the other hand, if fc is small, time response will be

bad, while it has improved stability, which may cause

over shoot or under shoot in abnormal condition.

Layout Consideration

A gate drive signal output from GATE pin becomes

noise source, which may cause malfunction of IC due to

cross talk if placed by the side of an analog line. It is

recommended to avoid placing the output line

especially by the side of CS, ISET, FSW, OVP, COMP

as far as possible.

Feb 28th 2012.

9

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