AST1S31HF Datasheet, PDF (17/33 Pages) STMicroelectronics – Internal soft-start and enable

AST1S31HF
Functional description
It’s also possible define the output voltage in function of input voltage, on phase time and
switching frequency:
Equation 19
V_OUTSet = V_IN  D_MIN = V_IN  ^T----_T^O----_Sn---^MW-----ⁱ-ⁿ-
So the on phase the equation results:
Equation 20
I_TONV_FB = -^V----^I-^N-----–-----^V----^F---^B-----^------^V--------^I0---^N----.---8---^{-------}--^T--------^TO--------^SN------^W--^M--------ⁱ---^{n-----–----}--_L-^D----^C-----^R-----^L----+-----^R----^D----^S---^--^O----^N----^--^H----ⁱ-^g---^h---^S---ⁱ--^d---^e---^---^-----^I  T_ONMin
Off phase:
Equation 21
I_TOFF = ^{–----}--^R-----^D---^S----^--^O----^N----^-^L---^o---^w----^S----ⁱ-^d---^e---_L-+-----^D-----^C----^R-----^L---^---^-----^I---–-----^V----^O----^U----^T-  T_SW
It is possible to repeat the considerations realized to the on phase equation. So it's possible
to write the off phase equation in the following manner:
Equation 22
I_TOFFV_FB = ^{–----}--^R-----^D---^S----^--^O----^N----^-^L---^o---^w----^S----ⁱ-^d---^e----+-----^D-----^C----^R----_L-^L---^---^-----^I---–-----^V----^F---^B-----^------^V--------^I0---^N----.---8---^{-------}--^T--------^TO--------^SN------^W--^M--------ⁱ---^n--  T_SW
The peak current escalates over the peak current threshold (called “OCP1”) if :
Equation 23
I_TONV_FB  I_TOFFV_FB
In case the current escalates up to a further current threshold (called “OCP2”), slightly
higher than OCP1, the converter stops the switching activity, the reference of the error
amplifier is pulled down and then it restarts with a new soft-start procedure. If the
overcurrent condition is still active, the current foldback with frequency reduction properly
limit the output current.
DocID026968 Rev 2
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