Transconductance error amplifier

VIPer53 - E

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optocoupler, the internal error amplifier is fully used for regulation). A typical schematic

corresponding to this situation can be seen on Figure 18.

The transfer function of the power cell is represented as G(s) in Figure 24 Iexhibits a pole

which depends on the output load and on the output capacitor value. As the load of a

converter may change, two curves are shown for two different values of output resistance

capacitor ESR. Note: The overall transfer function does not depend on the input voltage

because of the current mode control.

The error amplifier has a fixed behavior, similar to the one shown in Figure 23. Its bandwidth

is to avoid injection of high frequency noise in the current mode section. A zero due to the

The total transfer function is shown as F(s). G(s) at the bottom of Figure 24. For maximum

load (plain line), the load pole is exactly compensated by the zero of the error amplifier, and

the result is a perfect first order decreasing until it reaches the zero of the output capacitor

ESR. The error amplifier cut-off then definitely any further spurious noise or resonance from

disturbing the regulation loop.

The point where the complete transfer function has a unity gain is known as the regulation

bandwidth and has:

–

The higher it is, the faster the reaction will be to an eventual load change, and the

smaller the output voltage change will be.

–

The phase shift in the complete system at this point has to be less than 135° to

ensure good stability. Generally, a first-order slope gives 90° of phase shift, and a

second-order gives 180°.

In Figure 24, the unity gain is reached in a first order slope, so the stability is ensured.

The dynamic load regulation is improved by increasing the regulation bandwidth, but some

limitations have to be respected: As the transfer function above the zero due the capacitor

ESR is not reliable (The ESR itself is not well specified, and other parasitic effects may take

in Figure 24), the above criteria will be checked for this condition and allows to define the

range. The following formula can be derived:

Equation 12

R

COMP

P

OUT2

P

MAX

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

F

BW2

R

L2

C

OUT

⋅⋅

Gm

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

⋅

=

P

OUT2

V

OUT

2

R

L2

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

=

P

MAX

1

2

---

L

P

I

LIM

2

F

SW

⋅⋅

⋅

=

With:

and: