ST1S41 datasheet(9/27 Pages) STMICROELECTRONICS

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DocID023654 Rev 2

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ST1S41

Functional description

Figure 4. Block diagram of the loop for the small signal analysis

Three main terms can be identified to obtain the loop transfer function:

from control (output of E/A) to output, GCO(s);

from output (Vout) to FB pin, GDIV(s);

from FB pin to control (output of E/A), GEA(s).

The transfer function from control to output GCO(s) results:

Equation 1

where RLOAD represents the load resistance, Ri the equivalent sensing resistor of the

current sense circuitry,

ω p the single pole introduced by the LC filter and ω z the zero given

by the ESR of the output capacitor.

FH(s) accounts for 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 2

Cout

Current sense

Logic

and

Driver

Slope

Compensation

PWM comparator

Error Amp

0.8V

High side

Switch

Low side

Switch

G CO(s )

G DIV(s )

G EA(s )

VIN

VOUT

VFB

AM15061v1

CO s

()

LOAD

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

out

⋅

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

–

() 0.5

–

⋅

[]

⋅

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

-------

⎝⎠

⎛⎞

-------

⎝⎠

⎛⎞

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

H s

()

⋅⋅

⋅

ESR

OUT

⋅

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

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