UC19431
UC29431
UC39431
UC39431B
APPLICATION INFORMATION
the error amplifier, either a pole or a pole-zero can be
added.
Magnetic Amplifier Controller Application
The 0.4% initial reference makes the UC39431 ideal as a
programmable shunt regulator. By adding two external
resistors, the on-chip 1.3V reference can be gained to
any voltage between 2.2V (2.4V for the UC39431) and
36.0V. The input bias current is typically maintained at
0.2µA for the output voltage range. Since the
non-inverting error amplifier input is not available, a 5.1k
non-inverting input impedance is added to the input of
the error amplifier. This allows the user to choose the
SENSE pin input impedance to cancel the minimal offset
voltage caused by the input bias current.
The second gain stage is the transconductance (gm) am-
plifier. The gm amplifier is designed with a known linear
140mS of transconductance. The voltage gain is conse-
quently gm • Ro, where Ro is the output impedance at
the collector pin. The frequency response of the
transconductance amplifier is controlled with the COLL
pin. The gain bandwidth product of the gm amplifier is
typically 3MHz. A pole or pole-zero can be added to this
stage by connecting a capacitor or a series capacitor and
resistor between COLL and GND.
The compensation of a control loop containing the
UC39431 is made easier due to the independant com-
pensation capability of the error amplifier and gm ampli-
fier. As shown in the applications information, a pole-zero
is created with a series resistor and capacitor between
SENSE and COMP. The pole created is dominant, while
the zero is used to increase the bandwidth and cancel
the effects of the pole created by the capacitor between
the COLL and GND pins.
Frequency Compensation
The UC39431 shunt regulator is designed with two
independant gain stages. The error amplifier provides
90dB of gain with a typical gain bandwidth product of
5MHz. The error amplifier provides sufficient gain in or-
der for the sense voltage to be accurately compared to
the 1.3V on-chip reference. Complete control of the fre-
quency response of the error amplifier is accomplished
with the COMP pin. By putting negative feedback across
V
SENSE
21K
6.8nF
40.2K
COMP
VCC
2
3
R1
4
I
COLL
20k
R2
6
COLL
1
4k
R3
5
0.5V
REF
1.3V
22.4k
SENSE
7
16k
2.85k
5.1Ω
2K
GND
8
UDG-99051
Figure 2. 15.0V optocoupler application.
4
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