IRU3038CS datasheet(5/18 Pages) IRF

5 / 18 page

IRU3038

Rev. 2.0

09/12/02

www.irf.com

APPLICATION INFORMATION

Design Example:

The following example is a typical application for IRU3038,

the schematic is Figure 11 on page 12.

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 3.

Figure 3 - Typical application of the IRU3038 for

programming the output voltage.

Equation (1) can be rewritten as:

Choose R5 = 1K

This will result to R6 = 1K

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.

Shutdown

The converter can be shutdown by pulling the soft-start

pin below 0.5V. The control MOSFET turns off and the

synchronous MOSFET turns on during shutdown.

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 11. 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. In application, when a separate voltage

source is available the boost circuit can be avoided as

shown in Figure 1.

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, D=VOUT/VIN.

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

For higher efficiency, a low ESR capacitor is recom-

mended. Choose two Poscap from Sanyo 10TPB100ML

(10V, 100

mF, 55mV) with a maximum allowable ripple

current of 1.9A.

tSTART = 753Css (ms)

---(2)

VIN = 5V

VOUT = 2.5V

IOUT = 8A

DVOUT = 100mV

fS = 200KHz

IRU3038

VOUT

VREF

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

VOUT = VP

3 1 +

---(1)

VP = VREF = 1.25V

(

)

R6 = R5

- 1

VOUT

(

)

IRMS = IOUT

3(1-D)

---(3)

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