ST1S50 datasheet(10/29 Pages) STMICROELECTRONICS

10 / 29 page

Functional description

ST1S50

10/29

DocID025723 Rev 4

In Figure 4 is shown 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 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 load resistance, Ri 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 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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