L6986I datasheet(23/60 Pages) STMICROELECTRONICS

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Closing the loop

The iso-buck, compared to a standard buck, includes the transformer in place of the inductor and all the

components connected to the secondary winding (at least the Schottky diode and the secondary output

capacitor). Nevertheless, the regulation loop does not include the secondary side components. As a first

approximation the control loop of an iso-buck can be assimilated to the one of a standard buck. Therefore,

the block diagram in figure below can be still used (where the inductor symbol identifies the inductance of the

primary winding) as well as all the equations and calculations in the next sections.

Figure 26. Block diagram of the loop

5.1

GCO(s)control to output transfer function

The accurate control to output transfer function for a buck peak current mode converter can be written as:

GCO(s)= RLOAD∙gCS∙

1+RLOAD∙TSW

L ∙mC∙1‐D‐0.5∙

1+sωZ1+sωp∙FH(s

(7)

where RLOAD represents the load resistance, gCS the equivalent sensing transconductance of the current sense

circuitry, wp the single pole introduced by the power stage and wz 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.

ωZ= 1ESR∙COUT

(8)

ωp= 1

RLOAD∙COUT∙mC∙1‐D‐0.5

L∙COUT∙fSW

(9)

where:

mC=1+SeSn

Se=VPP∙gCS∙fSW

Sn=VIN−VOUTL

(10)

L6986I

Closing the loop

DS13647 - Rev 1

page 23/60

L6986I Datasheet(PDF) 23 Page - STMicroelectronics