ST1S40 datasheet(9/29 Pages) STMICROELECTRONICS

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DocID17928 Rev 5

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ST1S40

Functional description

Figure 4 shows the simple small signal model for the peak current mode control loop.

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 the FB pin, GDIV(s)

from the 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 (0.3 ) the equivalent sensing resistor of

the current sense circuitry,

p the single pole introduced by the LC filter and 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

GCO(s)

GDIV(s)

GEA(s)

VIN

VOUT

VFB

GCO s



RLOAD

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

Rout TSW



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

mC 1D

–

 0.5

–







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



------







------





---------------------- FH s









ESR COUT



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

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