IRU3018 datasheet(12/16 Pages) IRF

12 / 16 page

Rev. 1.5

07/24/01

IRU3018

Output Inductor Selection

The output inductance must be selected such that un-

der low line and the maximum output voltage condition,

the inductor current slope times the output capacitor

ESR is ramping up faster than the capacitor voltage is

drooping during a load current step.

However, if the inductor is too small, the output ripple

current and ripple voltage become too large. One solu-

tion to bring the ripple current down is to increase the

switching frequency, however that will be at the cost of

reduced efficiency and higher system cost. The follow-

ing set of formulas are derived to achieve the optimum

performance without many design iterations.

The maximum output inductance is calculated using the

following equation:

Where :

Vin(min) = Minimum input voltage

For Vo=2.8V, ∆I=14.2A

Assuming that the programmed switching frequency is

set at 200KHz, an inductor is designed using the

Micrometals’ Powder Iron core material. The summary

of the design is outlined below:

The selected core material is Powder Iron, the selected

core is T50-52D from Micro Metal wound with 8 turns of

#16 AWG wire, resulting in 3µH inductance with

≈ 3mΩ

of DC resistance.

Assuming L=3µH and Fsw=200KHz (switching fre-

quency), the inductor ripple current and the output ripple

voltage is calculated using the following set of equations:

In our example for Vo=2.8V and 14.2 A load, assuming

IRL3103 MOSFET for both switches with maximum on

resistance of 19mΩ, we have:

Power Component Selection

Assuming IRL3103 MOSFETs as power components,

we will calculate the maximum power dissipation as fol-

lows:

For high-side switch the maximum power dissipation

happens at maximum Vo and maximum duty cycle.

For synch MOSFET, maximum power dissipation hap-

pens at minimum Vo and minimum duty cycle.

Heat Sink Selection

Selection of the heat sink is based on the maximum

allowable junction temperature of the MOSFETS. Since

we previously selected the maximum RDS(on) at 125!C,

then we must keep the junction below this temperature.

Selecting TO-220 package gives θJC=1.8!C/W (from the

venders’ data sheet) and assuming that the selected

heat sink is black anodized, the heat-sink-to-case ther-

mal resistance is: θcs=0.05!C/W, the maximum heat

sink temperature is then calculated as:

With the maximum heat sink temperature calculated in

the previous step, the heat-sink-to-air thermal resistance

(θSA) is calculated as follows:

Assuming TA = 35!C:

∆T = Ts - TA = 118 - 35 = 83!C

Temperature Rise Above Ambient

L = ESR×C×[Vin(min) - Vo(max)] / ( 2×∆I )

L = 0.006×9000×(4.75 - 2.8) / (2×14.2) = 3.7µH

T = 1 / 200000 = 5µs

Vsw = Vsync = 14.2 × 0.019 = 0.27V

≈ (2.8 + 0.27) / (5 - 0.27 + 0.27) = 0.61

Ton = 0.61 × 5 = 3.1µs

Toff = 5 - 3.1 = 1.9µs

∆Ir = (2.8 + 0.27) × 1.9 / 3 = 1.94A

∆Vo = 1.94 × 0.006 = 0.011V = 11mV

≡ Switching Period

≡ Duty Cycle

Vsw

≡ High-side MOSFET ON Voltage

RDS

≡ MOSFET On-resistance

Vsync

≡ Synchronous MOSFET ON Voltage

∆Ir

≡ Inductor Ripple Current

∆Vo

≡Output Ripple Voltage

T = 1 / Fsw

Vsw = Vsync = Io × RDS

≈ (Vo + Vsync) / (Vin - Vsw + Vsync)

Ton = D × T

Toff = T - Ton

∆Ir = (Vo + Vsync) × Toff / L

∆Vo = ∆Ir × ESR

Dmin

≈ (2 + 0.27) / (5.25 - 0.27 + 0.27) = 0.43

PDS = (1-Dmin) × Io2 × RDS(max)

PDS = (1 - 0.43) × 14.22 × 0.029 = 3.33 W

Dmax

≈ (2.8 + 0.27) / (4.75 - 0.27 + 0.27) = 0.65

PDH = Dmax × Io2 × RDS(max)

PDH = 0.65 × 14.22 × 0.029 = 3.8W

RDS(max) = Maximum RDS(on) of the MOSFET at 125!C

Ts = TJ - PD × (θJC + θcs)

Ts = 125 - 3.82 × (1.8 + 0.05) = 118!C

θSA = ∆T / PD

= 83 / 3.82 = 22!C/W

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