Feb21-07

SP7661 Evaluation Manual

©2007 Sipex Corporation

Page 8 of 11

Select the ratio of RZ2 / R1 gain for the desired gain bandwidth (from above, we will use 60kHz)













⋅













⋅

=

LC

fp

fco

Vin

pp

Vramp

R

RZ

_

max

_

_

1

2

Ω

≅

Ω

=













⋅









⋅

=

4020

4166

12

60

12

1

1

2

kHz

kHz

V

V

R

RZ

Calculate CZ2 by placing the zero at ½ of the output filter pole frequency

LC

fp

RZ

CZ

_

2

1

2

⋅

⋅

=

π

nF

nF

kHz

CZ

8

.

6

59

.

6

12

4020

1

2

≈

=

⋅

Ω

⋅

=

π

Calculate CP1 by placing the first pole at ESR zero frequency

)

_

2

(

2

1

1

ESR

fz

Rz

CP

⋅

⋅

=

π

pF

pF

KHz

CP

56

7

.

59

)

663

4020

(

2

1

1

≈

=

⋅

⋅

=

π

Calculate RZ3 by setting the second pole at ½ of the switching frequency and the second zero

at the output filter double pole frequency

LC

fp

fs

LC

fp

R

RZ

_

2

)

_

(

)

1

(

2

3

−

⋅

⋅

=

Ω

≅

Ω

=

−

⋅

Ω

⋅

=

400

408

12

600

)

12

(

)

10

(

2

3

kHz

KHz

kHz

k

RZ

Calculate CZ3 from RZ3 component value above

fs

RZ

CZ

⋅

⋅

=

3

1

3

π

pF

nF

KHz

CZ

1500

3

.

1

600

400

1

3

≈

=

⋅

Ω

⋅

=

π

Choose 100pF

≤ CF1 ≤ 220pF to stabilize the SP7661ER internal Error Amplifier. For this

example let’s select 100pF.