34704 datasheet(46/54 Pages) FREESCALE

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VRRM Vin Vo

–

≥

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:

Lmin

VoT

–

2Iomax

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

Vinmin

Vo Vinmin

–

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

⎝⎠

⎛⎞

≥

[H]

• C

OUT: 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:

COUT

IomaxDmax

FSW Vor

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

≥

[F]

•Where

ΔVO

r is the desired output voltage ripple.

• Now calculate the maximum allowed ESR to reach the

desired.

[

Ω]

ESR

Vor

Iomax

1Dmax

–

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

IOB

–

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

⎝⎠

⎛⎞

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

≤

• 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:

[

Ω]

0.9

1.5 Vo

–0.9

–

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

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):

[Hz]

FLC

′

min

LCOUT

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

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

′

min

Vinmin

Voutmax

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

• Cutoff frequency due to capacitor ESR:

FESR

π C

OUT

()ESR

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

[Hz]

• The right plane zero frequency:

[Hz]

RHPZ

′

min

()

LOAD

πL

•

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

• Crossover frequency (or bandwidth): select this

frequency as far away form the RHPZ as much as

possible:

[Hz]

FBW

RHPZ

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

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:

FPO

FBW

=FZ1

0.9FLC

=F22

1.1FLC

FP1

FESR

=F2P

FSW

-----------

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

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