7/18

AN1606 APPLICATION NOTE

Figure 7.

It is based on the following consideration: the frequency of the signal of interest (tens of Hz), is much lower than

the switching frequency (tens of kHz). The boost inductor, for the low frequency, behaves like a short circuit.

Since the Powermos's drains are, in turns, close to ground (via the body diode), the resulting equivalent circuit

is shown in fig7b.

The relation between the voltage (from the inductor) and the current that flows in to Iac pin is:

a)

Where:

and

The net introduces one pole at:

The pole must be located at a frequency high enough not to distort the input waveform and at the same time,

low enough to filter the switching frequency.

In this application the equivalent resistance has been choosen

Req.=324-k

Ω that fits well with the current amplifier design.

The resulting R1 is 300k

Ω and R2 is 12kΩ

The pole has been placed a decade before the switching frequency:

Fp = 5kHz

that gives:

b)

In practice, in our test, a standard value of 2.7 nF as ben used.

Input voltage sensing (A), and equivalent circuit (B).

A

CURRENT

MIRROR

R

1

R

1

R

2

C

1

i

v

i

R

1

R

1

R

2

C

1

+

B

v

Coupled

Inductor

Ys

()

1

Req

-----------

1

1st

+

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

⋅

=

Req

R1

2 R2

⋅

+

=

t

R1

2

--------

//R2

C1

⋅

=

fp

1

2

π t

⋅⋅

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

=

C1

1

2

π fp

R1

2

--------

//R2





⋅⋅

⋅

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

2.87nF

==