34704 datasheet(43/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

Compensating for boost operation:

• L: A boost power stage can be designed to operate in

CCM for load currents above a certain level usually 5 to

15% of full load. The minimum value of inductor to

maintain CCM can be determined by using the following

procedure:

1. Define IOB as the minimum current to maintain CCM as

15% of full load:

Lmin

Vo D

() 1D

–

()

2IOB

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

≥

[H]

However the worst case condition for the boost power

stage is when the input voltage is equal to one half of the

output voltage, which results in the Maximum

ΔI

L, then:

Lmin

Vo T

()

16IOB

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

≥

[H]

• 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:

COUT

IomaxDmax

Fsw

ΔVo

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

≥

[F]

•Where

ΔVO

r is the desired output voltage ripple.

• Now calculate the maximum allowed ESR to reach the

desired

ΔVO

ESR

ΔVo

Iomax

1Dmax

–

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

IOB

⎝⎠

⎛⎞

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

≤

[

Ω]

• R1 and RB:

These two resistors help to set the output voltage to the

desire value using a Vref=0.6V, select R1 between 10k and

100K and then calculate RB as follows:

VREF

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

–

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

[

Ω]

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

For compensating a boost converter, 4 important

frequencies referring to the plant are:

1. Output LC filter cutoff frequency (FLC):

FLC

′

min

LCOUT

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

[Hz]

•Where D’min is the minimum off time percentage given

by:

′

min

Vinmin

Voutmax

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

2. Cutoff frequency due to capacitor ESR:

FESR

π C

OUT

()ESR

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

[Hz]

3. The right plane zero frequency:

RHPZ

′

min

()

LOAD

πL

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

[Hz]

4. Crossover frequency (or bandwidth): select this

frequency as far away form the RHPZ as much as

possible:

FBW

RHPZ

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

[Hz]

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

-----------

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