L6985F datasheet(42/65 Pages) STMICROELECTRONICS

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Closing the loop

L6985F

42/65

DocID027836 Rev 2

5.5

Compensation network design

The maximum bandwidth of the system can be designed up to fSW/6 or up to 150 kHz to

guarantee a valid small signal model.

Equation 33

Equation 34

where gCS represents the current sense transconductance (see Table 5: Electrical

characteristics on page 8) and gm TYP the error amplifier transconductance.

Equation 35

Example 2

Considering L6985F, VIN = 12 V, VOUT = 3.3 V, L = 4.7 H, COUT = 10 F, fSW = 2 MHz.

Assuming to design the compensation network to achieve system bandwidth of 110 kHz:

Equation 36

Equation 37

so accordingly with Equation 34 and Equation 35:

Equation 38

Equation 39

min

fSW

--------- 150kHz







 BW C

OUT VOUT





0.85V gCS gm TYP



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

 R

C BW





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

fPOLE

·· 4kHz

RLOAD

VOUT

IOUT

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

6.6



107k

 110k



66pF 68pF



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