VIPER53-E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'VIPER53-E' PDF : 36 Pages View PDF
VIPer53 - E
Transconductance error amplifier
11 Transconductance error amplifier
The VIPer53-E includes a transconductance error amplifier. Transconductance Gm is the
change in output current ICOMP versus change in input voltage VDD. Thus:
Equation 8
Gm = -∂--∂-I-C-V---O--D--M-D---P--
The output impedance ZCOMP at the output of this amplifier (COMP pin) can be defined as:
Equation 9
ZCOMP
=
∂----V----C----O----M----P-
∂ICOMP
=
----1-----
Gm
⋅
-∂---V----C----O----M----P-
∂VDD
This last equation shows that the open loop gain AVOL can be related to Gm and ZCOMP:
Equation 10
AVOL = Gm ⋅ ZCOMP
where Gm value for VIPer53 is typically 1.4mA/V.
Gm is well defined by specification, but ZCOMP, and therefore AVOL, 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
F(s) = Gm ⋅ Z(s)
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 ZCOMP 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 CCOMP and RCOMP.
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
25/36
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