Datasheet
VIPer53 - E Transconductance error amplifier
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11 Transconductance error amplifier
The VIPer53-E includes a transconductance error amplifier. Transconductance Gm is the
change in output current I
COMP
versus change in input voltage V
DD
. Thus:
Equation 8
The output impedance Z
COMP
at the output of this amplifier (COMP pin) can be defined as:
Equation 9
This last equation shows that the open loop gain A
VOL
can be related to Gm and Z
COMP
:
Equation 10
where Gm value for VIPer53 is typically 1.4mA/V.
Gm is well defined by specification, but Z
COMP,
and therefore A
VOL,
are subject to large
tolerances. An impedance Z must be connected between the COMP pin and ground in order
to accurately define the transfer function F of the error amplifier, the following equation, very
similar to the one above:
Equation 11
The error amplifier frequency response is shown in .0 for different values of a simple
resistance connected on the COMP pin. The unloaded transconductance error amplifier
shows an internal Z
COMP
of about 140KΩ. More complex impedances can be connected on
the COMP pin to achieve different compensation methods. A capacitor provides an
integrator function, thus eliminating the DC static error, and a resistance in series leads to a
flat gain at higher frequency, introducing a zero level and ensuring a correct phase margin.
This configuration illustrated in Figure 22, for the schematic and Figure 23 on page 28 for
the error amplifier transfer function for a typical set of values of C
COMP
and R
COMP
.
Note that a 10nF capacitor (8nF, minimum value) should always be connected to the COMP
pin to ensure a correct stability of the internal error amplifier.
The complete converter open loop transfer function can be built from both power cell and
error amplifier transfer functions. A theoretical example can be seen in Figure 24 for a
discontinuous mode flyback loaded by a simple resistor, regulated from primary side (no
Gm
∂I
COMP
∂V
DD
------------------- -=
Z
COMP
∂V
COMP
∂I
COMP
----------------------
1
Gm
--------- -
∂V
COMP
∂V
DD
----------------------
⋅==
A
VOL
Gm Z
COMP
⋅=
Fs() Gm Z s()⋅=