34844 datasheet(30/37 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34844

TYPICAL APPLICATIONS

COMPONENTS CALCULATION

The following formulas are intended for the calculation of

all external components related with the Boost converter and

Network compensation.

In order to calculate a Duty Cycle, the internal losses of the

MOSFET and Diode should be taken into consideration.

The average input current depends directly to the output

current when the internal switch is off.

Inductor

For calculating the Inductor we should consider the losses

of the internal switch and winding resistance of the inductor.

It is important to look for an inductor rated at least for the

maximum input current.

Input Capacitor

The input capacitor should handle at least the following

RMS current.

Output Capacitor

For the output capacitor selection the internal current

sense gain (CSG) and the Transconductance should be

taken in consideration.

The CSG is the internal RSense times the current sense

amplifier gain (ACSA).

The output voltage ripple (

ΔVout) depends on the ESR of

the Output capacitor, for a low output voltage ripple it is

recommended to use Ceramic capacitors that usually have

very low ESR. Since ceramic capacitor are expensive,

Electrolytic or Tantalum capacitors can be mixed with

ceramic capacitors to have a cheaper solution.

The output capacitor should handle at least the following

RMS current.

Network Compensation

Since this Boost converter is current controlled, Type II

compensation is needed.

I order to calculate the Network Compensation, first we

need to calculate all Boost Converter components.

For this type of compensations we need to push out the

Right Half Plane Zero to higher frequencies where it can’t

affect the overall loop significantly.

The Crossover frequency must be set much lower than the

location of the Right half plane zero

Since our system has a fixed Slope compensation set by

RSLOPE, RComp should be fixed for all configurations.

CComp1 and CComp2 should be calculated as follows:

Vout VD Vin

–

Vout VD VSW

–

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

Iinavg

Iout

–

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

Vin VSW

–Iinavg rw

()

–

() D

Iinavg

Iout FSW

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

Iinmax

Iinavg

Vin

Vout Vin

–

()

2LFSW Vout

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

IrmsCin

Vin

Vout Vin

–

()

2LFSW Vout

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

⎝⎠

⎛⎞ 0.3

CSG

ACSA RSense

Cout

RComp 5GM

Iout L

–

() Vout CSG

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

ESRCout

Vout

Vout FSW L

Vout

1 D

–

()

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

IrmsCout

Iout

–

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

fRHPZ

Vout

1 D

–

()

Iout 2

π L

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

fCross

fRHPZ

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

RComp

5.6Kohm

CComp1

fCross RComp π 2

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

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