AN4086 datasheet(4/11 Pages) STMICROELECTRONICS

4 / 11 page

Circuit description

AN4086

4/11

Doc ID 023053 Rev 1

Circuit description

●

Output voltage setting

In Equation 1 below, the output voltage is programmed by ROS and RFB using the

formula:

Equation 1

where VREF is 0.6 V and RFB is selected to obtain the desired regulator bandwidth (see

section 6.1 of datasheet for details).

●

Inductor selection

Choosing an inductor involves a compromise between dynamic response, efficiency,

cost and size. A higher inductor value will decrease the output voltage ripple, but will

increase the regulator response time to load changes.

The inductance has to be calculated to keep the ripple current (

ΔIL) between 20% and

30% of the maximum output current, using the following equation:

Equation 2

where FSW is the switching frequency, VIN is the input voltage, and VOUT is the output

voltage.

●

Output capacitor selection

The output capacitor bank will define the ripple voltage and affect the transient

response of the regulator.

During steady state operation, the output voltage ripple is affected by the ESR and the

capacitance value according to the following equations:

Equation 3

Equation 4

where

ΔIL is the inductor current ripple.

ROS = RFB * VREF / (VOUT – VREF)

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