ST1S40 datasheet(10/29 Pages) STMICROELECTRONICS

10 / 29 page

Functional description

ST1S40

10/29

DocID17928 Rev 5

Equation 3

where:

Equation 4

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

ramp (VPP peak-to-peak amplitude 1.25 V) that implements the slope compensation to

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

The sampling effect contribution FH(s) is:

Equation 5

where:

Equation 6

and

Equation 7

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

GDIV(s) is:

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

zeros) to stabilize the loop. Figure 5 shows the small signal model of the error amplifier with

the internal compensation network.



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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