34704 datasheet(46/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

• L: The minimum value of inductor to maintain CCM can be

determined by using the following procedure:

• COUT: The three elements of output capacitor that

contribute to its impedance and output voltage ripple are

the ESR, the ESL and the capacitance C. The minimum

capacitor value is approximately:

•Where

VO

r is the desired output voltage ripple.

• Now calculate the maximum allowed ESR to reach the

desired.

• R1 and RB: These two resistors help to set the output

voltage to the desire value using a VFB7=0.6V, select R1

between 10 k and 150 K and then calculate RB as follows:

NOTE: RB is not grounded, instead is connected to

VREF7 pin (VREF7=1.5 V) which provide a positive voltage

to assure a positive voltage at the FB7 pin.

• Compensation network. (C1,C2,C3, R2, R3)

For compensating a buck converter, 4 important

frequencies referring to the plant are:

• Output LC filter cutoff frequency (FLC):

Where D’min is the minimum off time percentage given by:

• Cutoff frequency due to capacitor ESR:

• The right plane zero frequency:

• Crossover frequency (or bandwidth): select this

frequency as far away form the RHPZ as much as

possible:

The Type 3 external compensation network will be in

charge of canceling some of these poles and zeros to

achieve stability in the system. The following poles and

zeroes frequencies are provided by the type 3 compensation:

VRRM Vin Vo

–



Lmin

VoT

–

2Iomax

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

Vinmin

Vo Vinmin

–

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







[H]

COUT

IomaxDmax

FSW Vor



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



[F]

[

]

ESR

Vor



Iomax

1Dmax

–

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

IOB

–

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





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



[

]

0.9

1.5 Vo

–0.9

–

----------------------------------R1

[Hz]

FLC



min

LCOUT



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



min

Vinmin

Voutmax

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

FESR

 C

OUT

ESR

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

[Hz]

RHPZ



min



LOAD

L



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

[Hz]

FBW

RHPZ

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

FPO

FBW

=FZ1

0.9FLC

=F22

1.1FLC

FP1

FESR

=F2P

FSW

----------

34704 Datasheet(PDF) 46 Page - Freescale Semiconductor, Inc

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