34704 datasheet(41/54 Pages) FREESCALE

41 / 54 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

34704

FUNCTIONAL DEVICE OPERATION

COMPONENT CALCULATION

1. First calculate the dt_I (inductor current rising time)

given by:

dtI

IomaxT

ΔIostep

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

[s]

Where the parameter

ΔIo_step is the maximum current

step during the current rising time and is define as:

ΔIostep

Dmax

Fsw

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

⎝⎠

⎛⎞ Vinmin Vo

–

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

⎝⎠

⎛⎞

[A]

2. Then the output capacitor can be chosen as follow:

COUT

IomaxdtI

ΔVo

max

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

≤

[A]

•Where

ΔVO

max is the maximum allowed transient

overshoot expressed as a percentage of the output

voltage, typically from 3 to 5% of Vo.

3. Finally find the maximum allowed ESR to allow the

desired transient response:

ESRmax

ΔVo

r Fsw

() L

()

Vo 1 Dmin

–

()

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

[

Ω]

NOTE: Do not use the parameters

ΔVO

r and ΔVOmax

indistinctly, the first one indicates the output voltage ripple,

while the second one is the maximum output voltage

overshoot (transient response).

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

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

between 10 k and 100 K and then calculate RB as follows:

Vref

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

–

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

[

Ω]

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

compensating a buck converter, 3 important frequencies

referring to the plant are:

1. Output LC filter cutoff frequency (FLC):

FLC

LCOUT

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

[Hz]

2. Cutoff frequency due to capacitor ESR:

FESR

π C

OUT

()ESR

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

[Hz]

3. Crossover frequency (or bandwidth):

FBW

FSW

-----------

[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.

34704 Datasheet(PDF) 41 Page - Freescale Semiconductor, Inc

34704 Datasheet(HTML) 41 Page - Freescale Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL