IRU3037 View Datasheet(PDF) - International Rectifier

Part Name

Description

Manufacturer

IRU3037

8-PIN SYNCHRONOUS PWM CONTROLLER

International Rectifier 

IRU3037/IRU3037A & (PbF)

Assuming the following:

∆V = 1%(VIN), Efficiency(η) = 90%

∆t = D × 1

fS

VO × IO

IIN = η × VIN

∆t = 3.3µs

IIN = 2.93A

By using equation (3), CIN = 193.3µF

For higher efficiency, low ESR capacitor is recommended.

Choose two 100µF capacitors.

The Sanyo TPB series PosCap capacitor 100µF, 10V

with 55mΩ ESR is a good choice.

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:

ESR ≤

∆VO

∆IO

---(4)

Where:

∆VO = Output Voltage Ripple

∆IO = Output Current

∆VO=100mV and ∆IO=4A

Results to ESR=25mΩ

The Sanyo TPC series, PosCap capacitor is a good

choice. The 6TPC150M 150µF, 6.3V has an ESR 40mΩ.

Selecting two of these capacitors in parallel, results to

an ESR of ≅ 20mΩ 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.

Inductor Selection

The inductor is selected based on output power, operat-

ing frequency and efficiency requirements. Low inductor

value causes large ripple current, resulting in the smaller

size, but poor efficiency and high output noise. Gener-

ally, the selection of inductor value can be reduced to

desired maximum ripple current in the inductor (∆i). The

optimum point is usually found between 20% and 50%

ripple of the output current.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

VIN

-

VOUT

=

L×

∆i

∆t

;

∆t

=

D×

1

fS

;

D

=

VOUT

VIN

L = (VIN - VOUT)×VIN×VO∆UiT×fS

---(5)

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

If ∆i = 20%(IO), then the output inductor will be:

L = 7µH

The Toko D124C series provides a range of inductors in

different values, low profile suitable for large currents,

10µH, 4.2A is a good choice for this application. This

will result to a ripple approximately 14% of output cur-

rent.

Power MOSFET Selection

The IRU3037 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:

PCOND (Upper Switch) = I2LOAD × RDS(ON) × D × ϑ

PCOND (Lower Switch) = I2LOAD × RDS(ON) × (1 - D) × ϑ

ϑ = RDS(ON) Temperature Dependency

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.

6

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