www.irf.com

16

RD-0608

The resistance between VBUS and VCC (RS1) should be large enough to minimize the current sourced directly

from the input voltage line. Through this supply resistor a current will flow to charge the VCC capacitor. Once the

capacitor is charged up to the VCCUV+ threshold, the IRS2540/1/01/11 begins to operate activating the LO and HO

outputs. After the first few cycles of switching the resistor RS2 connected between the output and VCC will take

over and source current for VCC from the output. The RS2 resistor provided in the evaluation board has been

designed for an output of 16 to 24V. If a higher output voltage is desired, RS2 will need to be increased accordingly.

Conversely, if the output voltage is below 20V the value of RS2 may need to be reduced in order to supply sufficient

voltage to VCC.

A 10uF capacitor has been used at VCC of the IRS2540/1/01/11, which removes most low frequency ripple that

could originate from VBUS due to a rectified voltage waveform.

If the input and output voltages are defined for the evaluation board, enough information is provided to calculate

values for RS1, RS2, and RS3 (see Fig. 23 for component definition). All three supply resistors were chosen to be

1W devices since they source all current to the chip. Efficiency can be improved by optimizing these values for

specific applications.

By making each component 1W, the work in supplying VCC can be split up equally making it a more robust

solution instead of relying entirely on one component. This also allows the chip to turn on at a lower bus voltage.

Assuming that a 14V external zener diode will be used on Vcc, exact values of RS1, RS2, and RS3 are calculated as

follows (values calculated to operate the components just below half their rated power):

R

V

P

2

=

RS1

()

Ω

≈

Ω

=

−

=

−

=

−

=

k

k

RS

W

V

V

W

V

V

RS

RS

V

V

W

Bus

Bus

56

6

.

48

1

2

1

14

170

2

1

14

1

1

14

2

1

2

2

2

max

max

RS3

()

()

()

)

Ω

≈

Ω

=

−

⋅

−

=

−

⋅

−

=

−

⋅

−

=

≈

k

k

RS

W

V

V

W

V

V

RS

RS

V

V

W

Bus

Bus

47

8

.

43

1

2

1

14

170

1

.

0

1

2

1

14

1

.

0

1

3

3

14

1

.

0

1

2

1

%

10

ratio

duty

min

2

2

2

max

max