AST1S31HF Datasheet, PDF (12/33 Pages) STMicroelectronics – Internal soft-start and enable

Functional description
AST1S31HF
Figure 5. Block diagram of the loop for the small signal analysis
Slope
Compensation
Current sense
PWM comparator
0.8V
Logic
And
Driver
VIN
GCO(s)
High side
Switch
L
VOUT
Low side
Switch
Cout
VC
Rc _{Error Amp}
Cc
V_FB
G _EA( s)
G_DIV (s)
R1
R2
Three main terms can be identified to obtain the loop transfer function:
1. From control (output of E/A) to output, G_CO(s)
2. From output (Vout) to the FB pin, G_DIV(s)
3. From the FB pin to control (output of E/A), G_EA(s).
The transfer function from control to output G_CO(s) results:
Equation 2
G_COs = -^R----^L--_R-^O---_i-^A---^D--  -₁----+-----_{R----------o------u------t----L--------T--------S------W---------}---_--_m1------_C-----_---_--₁-----–----_D-----_----–----₀---_.--₅----_  _-^--₁₁-----++------_^{------ss-------}_p-^{z----}-  F_Hs
where R_LOAD represents the load resistance, R_i the equivalent sensing resistor of the
current sense circuitry (0.38 ), _p the single pole introduced by the LC filter and _z the
zero given by the ESR of the output capacitor.
F_H(s) accounts the sampling effect performed by the PWM comparator on the output of the
error amplifier that introduces a double pole at one half of the switching frequency.
Equation 3
_Z = _E-----_S---_R------_-¹--_C-----_O----_U----_T-
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DocID026968 Rev 2