IRU3048CS datasheet(6/18 Pages) IRF

6 / 18 page

Rev. 1.7

09/12/02

IRU3048

www.irf.com

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 6TPC150M 150

mF, 6.3V has an ESR 40mV.

Selecting two of these capacitors in parallel for each

output, results to an ESR of

≅ 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.

The resulting output ripple current is smaller then each

channel ripple current due to the 180

8 phase shift. These

currents cancel each other. The cancellation is not the

maximum because of the different duty cycle for each

channel.

Inductor Selection

The inductor is selected based on output power, operat-

ing frequency and efficiency requirements. Low induc-

tor value causes large ripple current, resulting in the

smaller size, but poor efficiency and high output noise.

Generally, 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:

If the high value feedback resistors are used, the input

bias current of the Fb pin could cause a slight increase

in output voltage. The output voltage set point can be

more accurate by using precision resistor.

Soft-Start Programming

The soft-start timing can be programmed by selecting

the soft start capacitance value. The start up time of the

converter can be calculated by using:

Where:

CSS is the soft-start capacitor (

mF)

For a start-up time of 7.5ms, the soft-start capacitor will

be 0.1

mF. Choose a ceramic capacitor at 0.1mF.

Boost Supply Vc

To drive the high-side switch it is necessary to supply a

gate voltage at least 4V greater than the bus voltage.

This is achieved by using a charge pump configuration

as shown in Figure 9. The capacitor is charged up to

approximately twice the bus voltage. A capacitor in the

range of 0.1

mF to 1mF is generally adequate for most

applications.

Input Capacitor selection

The input filter capacitor should be based on how much

ripple the supply can tolerate on the DC input line. The

ripple current generated during the on time of control

MOSFET should be provided by input capacitor. The RMS

value of this ripple is expressed by:

Where:

D is the Duty Cycle, simply D=VOUT/VIN.

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

For VIN1=12V, IOUT1=4A and D1=0.275

Results to: IRMS1=1.78A

And for VIN2=5V, IOUT2=4A and D2=0.36

Results to: IRMS2 =1.92A

For higher efficiency, a low ESR capacitor is recom-

mended.

For VIN1=12V, choose one Poscap from Sanyo

16TPB47M (16V, 47

mF, 70mV, 1.4A)

For VIN2=5V, choose one 6TPC150M (6.3V, 150

mF,

40m

V, 1.9A).

tSTART = 75 3 Css (ms)

---(2)

IRMS = IOUT

3(1-D)

---(3)

ESR

[

---(4)

DVO

DIO

Where:

DVO = Output Voltage Ripple

DIO = Output Current

DVO=75mV and DIO=3A, results to: ESR=25mV

VIN - VOUT = L

;

Dt = D3

; D =

VOUT

VIN

L = (VIN - VOUT)

---(5)

VOUT

VIN

3Di3fS

Where:

VIN = Max Input Voltage

VOUT = Output Voltage

∆i = Inductor Ripple Current

fS = Switching Frequency

∆t = Turn On Time

D = Duty Cycle

IRU3048CS Datasheet(PDF) 6 Page - International Rectifier

IRU3048CS Datasheet(HTML) 6 Page - International Rectifier

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL