IRU3073CQTR【PDF 8/21 页】IC REG DL BCK/LINEAR 16-QSOP

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Sheet目录40

4页 5页 6页 7页 [8页]9页 10页 11页 12页

Rev. 1.0

09/17/03

IRU3073

www.irf.com

VIN - VOUT = L3 ; Dt = D3 ; D =

VOUT

VIN

L = (VIN - VOUT)3 ---(11)

VOUT

VIN3Di3fS

Where:

VIN = Maximum Input Voltage

VOUT = Output Voltage

i = Inductor Ripple Current

fS = Switching Frequency

t = Turn On Time

D = Duty Cycle

APPLICATION INFORMATION

Design Example:

The following example is a typical application for IRU3073,

the schematic is Figure 17 on page 16.

Output Voltage Programming

Output voltage is programmed by reference voltage and

external voltage divider. The Fb pin is the inverting input

of the error amplifier, which is referenced to the voltage

on non-inverting pin of error amplifier. For this applica-

tion, this pin (VP) is connected to reference voltage (VREF).

The output voltage is defined by using the following equa-

tion:

When an external resistor divider is connected to the

output as shown in Figure 8.

Figure 8 - Typical application of the IRU3039 for

programming the output voltage.

Equation (7) can be rewritten as:

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:

For a start-up time of 5ms, the soft-start capacitor will

be 0.1mF. Choose a ceramic capacitor at 0.1mF.

Supply VcL and VcH

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

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

For this application, VcL and VcH are biased with a sepa-

rate 12V supply.

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 upper

MOSFET should be provided by input capacitor. The RMS

value of this ripple is expressed by:

For higher efficiency, a low ESR capacitor is recom-

mended. Choose two Poscap from Sanyo 6TPB47M

(16V, 47mF) with a max allowable ripple current of 5.2A.

Inductor Selection

The inductor is selected based on operating frequency,

transient performance and allowable output voltage ripple.

Low inductor value results to faster response to step

load (high di/dt) and smaller size but will cause larger

output ripple due to increase of inductor ripple current.

As a rule of thumb, select an inductor that produces a

ripple current of 10-40% of full load DC.

For the buck converter, the inductor value for desired

operating ripple current can be determined using the fol-

lowing relation:

VOUT = VP 3 1 + ---(7)

VP = VREF = 0.8V

( )

IRU3073

VOUT

VREF

Css E 203tSTART (mF) ---(8)

Where tSTART is the desirable start-up time (s)

For VIN=5V, IOUT=8A and D=0.5, the IRMS=4A

IRMS = IOUT D3(1-D) ---(9)

Where:

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

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

R6 = R5 3 - 1

VOUT

( )

Choose R5 = 1K. This will result to R6 = 2.15K

Switcher

VIN = 5V

VOUT = 2.5V

IOUT = 8A

DVOUT = 50mV

fS = 200KHz

Linear Regulator

VIN = 2.5V

VOUT = 1.6V

IOUT = 2A

Supply Voltage

VCC=VCL=VCH=12V

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