Obsolete Product(s) - Obsolete Product(s)
VIPer100/SP - VIPer100A/ASP
16/24
Transconductance Error Amplifier
The VIPer100/100A includes a transconductance error amplifier. Transconductance Gm is the change in
output current (I
COMP
) versus change in input voltage (V
DD
). Thus:
The output impedance Z
COMP
at the output of this amplifier (COMP pin) can be defined as:
This last equation shows that the open loop gain A
VOL
can be related to G
m
and Z
COMP
:
A
VOL
= G
m
x Z
COMP
where G
m
value for VIPer100/100A is 1.5 mA/V typically.
G
m
is defined by specification, but Z
COMP
and therefore A
VOL
are subject to large tolerances. An
impedance Z can be connected between the COMP pin and ground in order to define the transfer
function F of the error amplifier more accurately, according to the following equation (very similar to the
one above):
F
(S)
= Gm x Z(S)
The error amplifier frequency response is reported in figure 10 page 8 for different values of a simple
resistance connected on the COMP pin. The unloaded transconductance error amplifier shows an
internal Z
COMP
of about 330KΩ. More complex impedance can be connected on the COMP pin to achieve
different compensation level. A capacitor will provide an integrator function, thus eliminating the DC static
error, and a resistance in series leads to a flat gain at higher frequency, insuring a correct phase margin.
This configuration is illustrated in (see Figure 21) page 17.
As shown in (see Figure 21) an additional noise filtering capacitor of 2.2nF is generally needed to avoid
any high frequency interference.
Is also possible to implement a slope compensation when working in continuous mode with duty cycle
higher than 50%. (see Figure 22) shows such a configuration. Note: R1 and C2 build the classical
compensation network, and Q1 is injecting the slope compensation with the correct polarity from the
oscillator sawtooth.
External Clock Synchronization:
The OSC pin provides a synchronisation capability when connected to an external frequency source.
(see Figure 23) page17 shows one possible schematic to be adapted, depending the specific needs. If
the proposed schematic is used, the pulse duration must be kept at a low value (500ns is sufficient) for
minimizing consumption. The optocoupler must be able to provide 20mA through the optotransistor.
Primary Peak Current Limitation
The primary I
DPEAK
current and, consequently, the output power can be limited using the simple circuit
shown in (see Figure 24) page 18. The circuit based on Q1, R
1
and R
2
clamps the voltage on the COMP
pin in order to limit the primary peak current of the device to a value:
where:
The suggested value for R
1
+R
2
is in the range of 220KΩ.
m
∂
COM
∂
V
DD
---- --- --------- ----
=
CO MP
∂
COMP
∂
I
CO MP
--------------------------
1
m
G
---------
∂
COM
∂
V
DD
----------------- -----
×
==
DPEAK
COMP
H
ID
----- --- ------------------------
=
COMP
0.6
1
R
2
------------------
×
=
