AST1S31HF Datasheet, PDF(13/33 Page) STMicroelectronics

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AST1S31HF Datasheet, PDF (13/33 Pages) STMicroelectronics – Internal soft-start and enable

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AST1S31HF

Functional description

Equation 4

where:

ï·p = R-----L---O----A---D---1--ï---C-----O----U----T- + m-----L-C----ï-ï--C-ï¨--1-O----âU----TD----ï-ï©--f--âS----W0---.--5--

Equation 5

ï¦

ï§

1 + SS-----en-

ï§

Vpp ï fSW

ï§

ï¨

-V----I-N-----â---L--V----O----U----T- ï Ri

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:

FHï¨sï© = 1-----+------ï·---------n--------sï--1----Q----------P-----+-----ï·--s------2--n2--

Equation 7

QP = ï°-----ï---ï---m-----C-----ï---ï¨--1--1---â-----D----ï©----â----0----.-5----ï-

and:

Equation 8

ï·n = ï° ï fSW

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.

DocID026968 Rev 2

13/33

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