IR3621 datasheet(13/29 Pages) IRF

13 / 29 page

IR3621&(PbF)

www.irf.com

Choose IRF7821 for control MOSFETs and IRF8113 for

synchronous MOSFETs. These devices provide low on-

resistance in a compact SOIC 8-Pin package.

The MOSFETs have the following data:

The total conduction losses for each output 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 a synchronous Buck converter. The syn-

chronous MOSFET turns on under zero voltage condi-

tions, therefore, the switching losses for synchronous

MOSFET can be neglected. With a linear approxima-

tion, the total switching loss can be expressed as:

These values are taken under a certain condition test.

For more details please refer to the IRF7821 data sheet.

By using equation (9), we can calculate the total switch-

ing losses.

Programming the Over-Current Limit

The over-current threshold can be set by connecting a

resistor (RSET) from drain of low side MOSFET to the

OCSet pin. The resistor can be calculated by using equa-

tion (3).

The RDS(on) has a positive temperature coefficient and it

should be considered for the worse case operation.

PCON(TOTAL, 2.5V) = PCON(UPPER) + PCON(LOWER)

PCON(TOTAL, 2.5V) = 1.0W

PSW(TOTAL,2.5V) = 0.18W

PSW(TOTAL,1.8V) = 0.18W

IRF7821

VDSS = 30V

RDS(on) = 9m

Ω

Where:

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

tr = Rise Time

tf = Fall Time

T = Switching Period

ILOAD = Load Current

PSW =

ILOAD

---(9)

VDS(OFF)

tr + tf

IRF7821

tr = 2.7ns

tf = 7.3ns

RDS(on) = 6m

Ω×1.5 = 9mΩ

ISET

≅ IO(LIM) = 10A×1.5 = 15A

(50% over nominal output current)

This results to:

RSET = R1=R6=6.75K

Ω

VDS

VGS

10%

90%

td(ON)

td(OFF)

Figure 13 - Switching time waveforms.

From IRF7821 data sheet we obtain:

PCOND(Upper Switch) = ILOAD

×RDS(on)×D×ϑ

PCOND(Lower Switch) = ILOAD

×RDS(on)×(1 - D)×ϑ

ϑ = RDS(on) Temperature Dependency

The gate drive requirement is almost the same for both

MOSFETs. Logic-level transistors 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 in a shoot-through.

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 current. 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.

PCON(TOTAL, 1.8V) = PCON(UPPER) + PCON(LOWER)

PCON(TOTAL, 1.8V) = 1.0W

IRF8113

VDSS = 30V

RDS(on) = 6m

Ω

This resistor must be placed close to the IC, place a

small ceramic capacitor from this pin to ground for noise

rejection purposes.

IR3621 датащи(PDF) 13 Page - International Rectifier

IR3621 датащи(HTML) 13 Page - International Rectifier

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

датащи скачать

ссылки URL