L6910G Просмотр технического описания (PDF) - STMicroelectronics

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производитель

L6910G

ADJUSTABLE STEP DOWN CONTROLLER WITH SYNCHRONOUS RECTIFICATION

STMicroelectronics 

L6910G

Figure 10. Compensation Network

VIN

∆VOSC

L

VOUT

PWM

COMPARATOR

C18

ESR

COUT

C19

R5

R3

C20

R4

VCOMP

-EAREF

D03IN1512

+

The compensation network consists in the internal error amplifier and the impedance networks ZIN (R3, R4 and

C20) and ZFB (R5, C18 and C19). The compensation network has to provide a closed loop transfer function with

the highest 0dB crossing frequency to have fast response (but always lower than fsw/10) and the highest gain

in DC conditions to minimize the load regulation.

A stable control loop has a gain crossing with -20dB/decade slope and a phase margin greater than 45°. Include

worst-case component variations when determining phase margin.

To locate poles and zeroes of the compensation networks, the following suggestions may be used:

Modulator singularity frequencies:

ωLC

=

-------------1--------------

L ⋅ COUT

ωESR

=

----------------1----------------

ESR ⋅ COUT

Compensation network singularity frequency:

ωP1

=

-----------------------1------------------------

R5

⋅





C-C----1-1---8-8----+⋅---C-C----1-1---99--

ωZ1

=

-----------1------------

R5 ⋅ C19

ωP2 = R-----4-----⋅-1--C-----2---0--

ωZ2 = (---R-----3-----+----R----1-4----)----⋅---C----2----0-

– Put the gain R5/R3 in order to obtain the desired converter bandwidth;

– Place ωZ1 before the output filter resonance ωLC;

– Place ωZ2 at the output filter resonance ωLC;

– Place ωP1 at the output capacitor ESR zero ωESR;

– Place ωP2 at one half of the switching frequency;

– Check the loop gain considering the error amplifier open loop gain.

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