MAX8655ETN(2007) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8655ETN
(Rev.:2007)

Highly Integrated, 25A, Wide-Input, Internal MOSFET, Step-Down Regulator

Maxim Integrated 

Highly Integrated, 25A, Wide-Input,

Internal MOSFET, Step-Down Regulator

obtained phase margin. A phase margin of at least 45°

is recommended:

[ ] ( ) GLOOP(s) =

⎡⎢1+

⎣

gmc

RLOAD ×

L × fS

× RLOAD

KS × (1− D)

− 0.5

⎤

⎥

⎦

×

(1+ s / 2π × fzMOD) ×

(1+ s / 2π × fpMOD)

(1+ s / 2π × fzEA )

×

(1+ s / 2π × fpEA ) × (1+ s / 2π × fpdEA )

gmEA

× Ro ×

VO

VFB

GS (s)

GS (s)

=

⎛

⎜1+

⎝⎜

1

s+

π.Qc.fS

s2

(π.fS )2

⎞

⎟

⎠⎟

where the sampling effect quality factor:

[ ] QC =

1

π.(KS.(1− D) − 0.5)

,

Below is a numerical example to calculate RC and CC

values of the typical operating circuit of Figure 3:

AVCS = 12

L = 0.56µH

RL = 1.8mΩ

fS = 600kHz

gmc = 1 / (AVCS x RL) = 1 / (12 x 0.0018) = 46.29S

VOUT = 1.2V

IOUT(MAX) = 20A

RLOAD = VOUT / IOUT(MAX) = 1.2 / 20 = 0.06Ω

COUT = 4 x 100µF = 400µF

ESR = 2mΩ/4 = 0.5mΩ

D = VOUT / VIN = 1.2/12 = 0.1:

KS

=1+

VSCOMP × L × fS

120 × (VIN − VO) × RL

= 1+ 1.25(0.56 x10−6 )(600000)

120(12 − 1.2)(0.0018)

= 1.18

[( ) ] ( ) GMOD(dc)

= gmc

×

⎡⎣⎢1+

RLOAD

L × fs

RLOAD

× KS ×

1− D

=

−

0.5

⎤

⎦⎥

[ ] 46.29 ×

1+

0.06

0.06

(0.56 x10−6 )(600000)

×

1.18(1− 0.1) − 0.5

= 2.53

fpMOD

=

2π

×

RLOAD

1

×

COUT

×

0.9

+

[ ] ⎡

⎢

⎣

2π

×

L

×

fS

1

× COUT

× 0.8

×

KS

× (1− D) − 0.5

⎤

⎥=

⎦

1

2π(400x10−6 )(0.06)

×

0.9

+

⎡

⎢

⎣⎢

2π(0.56

x10−6

1

)(600000)(400x10−6

)

×

0.8

(1.18(1

−

0.1)

−

⎤

0.5)⎥

⎦⎥

=

8.18kHz

fpMOD

<<

fC

≤

fS

5

8.18kHz << fC ≤ 120kHz, select fC = 60kHz.

fzMOD

=

2π

×

0.9

×

1

COUT

×

ESR

=

2π

×

0.9

×

(400

1

× 10−6

)

×

0.0005

=

884.2kHz

Since fzMOD > fC:

GMOD(fc)

= GMOD(dc)

×

fpMOD

fc

= 2.53 ×

8118

60000

= 0.345

RC

=

VOUT

VFB

×

gmEA

1

× GMOD(fc)

=

1.2

0.7

×

(110

1

x10−6

)(0.307)

RC = 44.7kΩ

Select the nearest standard value: RC = 40.2kΩ:

CC

=

1

2π × fpMOD

× RC

=

2π

1

× 8181× (40.2 x103 )

= 483.9pF

Select the nearest standard value: CC = 470pF:

CF

=

1

2π × RC × fzMOD

=

1

2π × (40.2 × 103 ) × (884.2 x103 )

= 5pF

R7 = RC = 40.2kΩ

C12 = CC = 470pF

C11 = CF = 5pF (not used)

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