IRF7457 datasheet(5/21 Pages) IRF

5 / 21 page

IRU3037 / IRU3037A

Rev. 2.8

03/10/03

www.irf.com

Figure 3 - Typical application of the IRU3037 for

programming the output voltage.

APPLICATION INFORMATION

Design Example:

The following example is a typical application for IRU3037,

the schematic is Figure 18 on page 14.

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 internally referenced to

1.25V (0.8V for IRU3037A). The divider is ratioed to pro-

vide 1.25V at the Fb pin when the output is at its desired

value. The output voltage is defined by using the follow-

ing equation:

When an external resistor divider is connected to the

output as shown in Figure 3.

Equation (1) can be rewritten as:

Choose R5 = 1K

This will result to R6 = 1.65K

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 18. 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

larger capacitor, the less ripple expected but consider

should be taken for the higher surge current during the

power-up. The IRU3037 provides the soft-start function

which controls and limits the current surge. The value of

the input capacitor can be calculated by the following

formula:

Where:

CIN is the input capacitance (

mF)

IIN is the input current (A)

Dt is the turn on time of the high-side switch (ms)

DV is the allowable peak to peak voltage ripple (V)

IRU3037

VOUT

tSTART = 753Css (ms)

---(2)

VIN = 5V

VOUT = 3.3V

IOUT = 4A

DVOUT = 100mV

fS = 200KHz

R6 = R5

- 1

VOUT

VREF

(

)

VOUT = VREF

1 +

---(1)

(

)

CIN =

---(3)

IIN

3 Dt

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