AST1S31HF datasheet(13/33 Pages) STMICROELECTRONICS

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DocID026968 Rev 2

13/33

AST1S31HF

Functional description

Equation 4

where:

Equation 5

Sn represents the ON time slope of the sensed inductor current, Se the slope of the external

ramp (VPP peak-to-peak amplitude - 0.55 V) that implements the slope compensation to

avoid sub-harmonic oscillations at the duty cycle over 50%.

The sampling effect contribution FH(s) is:

Equation 6

where:

Equation 7

and:

Equation 8

The resistor to adjust the output voltage gives the term from output voltage to the FB pin.

GDIV(s) is:

Equation 9

The transfer function from FB to Vc (output of E/A) introduces the singularities (poles and

zeroes) to stabilize the loop. In Figure 6 the small signal model of the error amplifier with the

internal compensation network is shown.



RLOAD COUT



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

mC 1D

–

 0.5

–



LCOUT fSW



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

------

Vpp fSW



VIN VOUT

–

------------------------------ Ri









FH s





n QP



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



------

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

 m

–

 0.5

–







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



 f



GDIV s



R1 R2

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

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