MAP3613 datasheet(13/16 Pages) MGCHIP

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Datasheet Version 1.0

Mar 28

th 2016

External Components Selection

Inductor

In order to achieve accurate constant current output,

the MAP3613 is required to operate in Continuous

Conduction Mode (CCM) under all operating conditions.

In general, the magnitude of the inductor ripple current

should be kept as small as possible. If the PCB size is

not limited, higher inductance values result in better

accuracy of the output current. However, in order to

minimize the physical size of the circuit, an inductor

with minimum physical outline should be selected such

that the converter always operates in CCM and the

peak inductor current does not exceed the saturation

current limit of the inductor.

The Min. inductance(boundary inductance) which

guarantees CCM operation can be calculated as;

OUT





OUT

OFF

OUT

MIN















)

(

The ripple current through chosen inductor is as

following equation;

OUT









)

(

For

example,

case

VIN=175V,

VOUT=135V,

IOUT(ILED)=425mA,

fSW=50kHz

and

target

ripple

current=300mA;

175

135 



OUT

]

[

425

)

(

135

)

(

OUT

MIN



















The ripple current at LMIN is 2*IOUT=850[mA] and this is

too large to use.

For target ripple current(ΔIL=300mA);

]

[

)

135

175

(

)

(

OUT



















In this case, the chosen conventional inductor is

2mH/1A.

MOSFET

The power MOSFET is chosen based on maximum

stress voltage, maximum peak MOSFET current, total

power losses, maximum allowed operating temperature

and the driver capability of the MAP3613.

Maximum stress voltage on the power MOSFET

(drain-source voltage) for this buck converter is equal

to the input voltage. The power MOSFET must be

selected with some voltage margin. For example, if the

input voltage is maximally 400 V, then maximum drain-

source voltage should be 450 V or higher.

Maximum peak MOSFET current was selected in

order to calculate the inductor size. Also in this case,

the power MOSFET must be chosen with some current

margin.

The power losses in the MOSFET can be separated

into conduction losses and switching losses. The

conduction loss, PCOND, is the I2R loss across the

MOSFET. The conduction loss is given by;

RMS

COND





)

(

Where, k is the temperature coefficient of the

MOSFET.

The switching loss is related to QGD and QGS1 which

determine the commutation time. QGS1 is the charge

between the threshold voltage and the plateau voltage

when a driver charges the gate, which can be read in

the chart of VGS vs. QG of the MOSFET datasheet. QGD

is the charge during the plateau voltage. These two

parameters are needed to estimate the turn on and turn

off loss.

PLT

















where VTH is the threshold voltage, VPLT is the plateau

voltage, RG is the gate resistance, VDS is the drain-

source voltage, VDR is the drive voltage

The total gate charge, QG, is used to calculate the

gate drive loss. The expression is





Fast switching MOSFETs can cause noise spikes

which may affect performance. To reduce these spikes

a drive resistor can be placed between GATE pin and

the MOSFET gate.

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