7

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FN6439.4

April 2, 2010

Maximum Output Current

The MOSFET current limit is nominally 4.0A and guaranteed

based on Equation 7:

where:

D = MOSFET turn-on ratio:

switching frequency and 10µH inductor.

Cascaded MOSFET Application

A 24V N-Channel MOSFET is integrated in the boost

regulator. For the applications where the output voltage is

greater than 24V, an external cascaded MOSFET is needed

as shown in Figure 11. The voltage rating of the external

DC PATH BLOCK APPLICATION

Note that there is a DC path in the boost converter from the

input to the output through the inductor and diode, hence the

input voltage will be seen at output with a forward voltage

drop of diode before the part is enabled. If this voltage is not

desired, the following circuit (see Figure 12) can be inserted

between input and inductor to disconnect the DC path when

the part is disabled.

2.5

5

1790

2.5

9

990

2.5

12

750

3.3

5

2370

3.3

9

1300

3.3

12

970

5

9

1970

5

12

1470

()

12

⁄ΔI

L

×

()

+

=

(EQ. 7)

ΔI

L

+

() V

IN

–

[]

×

(

×

+

×

------------------------------------------------------------------------------

=

(EQ. 8)

1D

–

-------------

=

(EQ. 9)

D1

+

--------------------------------------------

–

=

(EQ. 10)

FIGURE 11. CASCADED MOSFET TOPOLOGY FOR HIGH

OUTPUT VOLTAGE APPLICATIONS

INTERSIL

ISL97656

LX

FB

FIGURE 12. CIRCUIT TO DISCONNECT THE DC PATH OF

BOOST CONVERTER

INPUT

EN

TO INDUCTOR

ISL97656