IRU3138

7

Rev. 1.0

01/29/04

www.irf.com

APPLICATION INFORMATION

Design Example:

The following example is a typical application for IRU3138,

the schematic is Figure 13 on page 15.

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. The output voltage

is defined by using the following equation:

When an external resistor divider is connected to the

output as shown in Figure 5.

Figure 5 - Typical application of the IRU3138 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 0.1

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 grater than the bus voltage. For

single supply applications, this is achieved by using a

charge pump configuration as shown in Figure 6. This

method is simple and inexpensive. The operation of the

circuit is as follows: when the lower MOSFET is turned

on, the capacitor (C1) is pulled down to ground and

bus voltage will be added to this voltage when upper

MOSFET turns on in next cycle, and providing supply

voltage (Vc) through diode (D2). Vc is approximately:

Capacitors in the range of 0.1

mF and 1mF are generally

adequate for most applications. The diode must be a

fast recovery device to minimize the amount of charge

fed back from the charge pump capacitor into Vc. The

diodes need to be able to block the full power rail volt-

age, which is seen when the high side MOSFET is

switched on. For low voltage application, schottky di-

odes can be used to minimize forward drop across the

diodes at start up. For this application, Vc is biased by

an external 12V supply.

Figure 6 - Charge pump circuit.

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:

Css = 20

3

---(8)

(output voltage ripple

Supply Voltage

L2

IRU3138

C1

Vc

HDrv

Q1

Q2

C2

D2

D1

3

- 1

(

)

3 1 +

---(7)

(

)

Fb

IRU3138