Analog Integrated Circuit Device Data



Freescale Semiconductor

27

34844

TYPICAL APPLICATIONS

COMPONENTS CALCULATION

COMPONENTS CALCULATION

The following formulas are intended for the calculation of all external components related with the Boost converter and Network

compensation. 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 (

VOUT) depends on the ESR of the Output capacitor. For a low output voltage ripple, it is recommended to

use Ceramic capacitors usually having very low ESR. Since ceramic capacitor are expensive, Electrolytic or Tantalum capacitors can be

mixed with ceramic capacitors for an inexpensive solution.

D

Vout VD Vin

–

+

Vout VD VSW

–

+

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

=

Iinavg

Iout

1D

–

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

=

L

Vin VSW

–Iinavg rw





–

 D



Iinavg r



FSW



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

=

Iinmax Iinavg

Vin

Vout Vin

–





2LFSW Vout







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

+

=

IrmsCin

Vin

Vout Vin

–





2L FSW Vout







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



 0.3



=

CSG

ACSA RSense



=

Cout

RComp 5GM



Iout L







1D

–

 Vout CSG





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

=