IRU3073CQTR【PDF 9/21 页】IC REG DL BCK/LINEAR 16-QSOP

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Sheet目录40

5页 6页 7页 8页 [9页]10页 11页 12页 13页

IRU3073

Rev. 1.0

09/17/03

www.irf.com

If Di = 25%(IO), then the output inductor will be:

The Coilcraft DO5022HC series provides a range of in-

ductors in different values, low profile suitable for large

currents. 3.3mH is a good choice for this application.

This will result to a ripple approximately 23% of output

current.

Output Capacitor Selection

The criteria to select the output capacitor is normally

based on the value of the Effective Series Resistance

(ESR). In general, the output capacitor must have low

enough ESR to meet output ripple and load transient

requirements, yet have high enough ESR to satisfy sta-

bility requirements. The ESR of the output capacitor is

calculated by the following relationship:

The Sanyo TPC series, Poscap capacitor is a good choice.

The 6TPC330M, 330mF, 6.3V has an ESR 40mV. Se-

lecting two of these capacitors in parallel, results to an

ESR of E 20mV which achieves our low ESR goal.

The capacitor value must be high enough to absorb the

inductor's ripple current. The larger the value of capaci-

tor, the lower will be the output ripple voltage.

Power MOSFET Selection

The IRU3073 uses two N-Channel MOSFETs. The se-

lections criteria to meet power transfer requirements is

based on maximum drain-source voltage (VDSS), gate-

source drive voltage (VGS), maximum output current, On-

resistance RDS(ON) and thermal management.

The MOSFET must have a maximum operating voltage

(VDSS) exceeding the maximum input voltage (VIN).

The gate drive requirement is almost the same for both

MOSFETs. Logic-level transistor can be used and cau-

tion should be taken with devices at very low VGS to pre-

vent undesired turn-on of the complementary MOSFET,

which results a shoot-through current.

The total power dissipation for MOSFETs includes con-

duction and switching losses. For the Buck converter,

the average inductor current is equal to the DC load cur-

rent. The conduction loss is defined as:

The RDS(ON) temperature dependency should be consid-

ered for the worst case operation. This is typically given

in the MOSFET data sheet. Ensure that the conduction

losses and switching losses do not exceed the package

ratings or violate the overall thermal budget.

Choose IRF7832 for both control MOSFET and synchro-

nous MOSFET. This device provides low on-resistance

in a compact SOIC 8-Pin package.

The MOSFETs have the following data:

The total conduction losses will be:

The switching loss is more difficult to calculate, even

though the switching transition is well understood. The

reason is the effect of the parasitic components and

switching times during the switching procedures such

as turn-on / turnoff delays and rise and fall times. The

control MOSFET contributes to the majority of the switch-

ing losses in synchronous Buck converter. The synchro-

nous MOSFET turns on under zero voltage conditions,

therefore, the turn on losses for synchronous MOSFET

can be neglected. With a linear approximation, the total

switching loss can be expressed as:

The switching time waveform is shown in Figure 9.

PCOND(Upper Switch) = ILOAD3RDS(ON)3D3q

PCOND(Lower Switch) = ILOAD3RDS(ON)3(1 - D)3q

q = RDS(ON) Temperature Dependency

L = 3.125mH

Where:

DVO = Output Voltage Ripple

Di = Inductor Ripple Current

DVO = 50mV and DI E 23% of 8A = 1.89A

This results to: ESR=26.5mV

ESR [ ---(10)

DVO

DIO

PCON(TOTAL) = PCON(UPPER) + PCON(LOWER)

PCON(TOTAL) = 0.38W

Where:

VDS(OFF) = Drain to Source Voltage at off time

tr = Rise Time

tf = Fall Time

T = Switching Period

ILOAD = Load Current

PSW = ILOAD ---(12)

VDS(OFF)

tr + tf

IRF7832

VDSS = 30V

ID = 16A @ 708C

RDS(ON) = 4mV

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