Datasheet

8
UCC2750
UCC3750
Sine Reference Generator
The IC has a versatile low frequency sinewave reference
generator with low harmonic distortion and good fre
-
quency accuracy. In its intended mode as shown in Fig.
5, the reference generator will take an input from a
32kHz crystal (connected between XTAL1 and XTAL2)
and generate a sine-wave at 20Hz, 25Hz or 50Hz based
on the programming of pins FS0 and FS1 (See Table 2).
If the crystal frequency is changed, the output frequen
-
cies will be appropriately shifted. C-2 type Quartz crystals
(Epson makes available through DigiKey) are recom
-
mended for this application. If the frequency accuracy is
not a major concern, the more common and less expen
-
sive clock crystal (C-type) at 32.768kHz can be used with
a minor output frequency offset (20.5Hz instead of 20Hz).
Additionally, the XTAL1 input can be clocked at a desired
frequency to get a different set of output frequencies at
the sine-wave output (with divide ratios of 1600, 1280
and 640). The sine-wave output is centered around an in
-
ternal reference of 3V. A capacitor from SINREF to GND
helps provide smoothing of the sine wave reference.
Recommended value is at least 0.01µF and maximum of
0.1µF. When FS0 and FS1 are both 1 (high), the sine ref-
erence is disabled and external sine-wave can be fed
into the SINREF pin. This signal should have the same
DC offset as the internal sine-wave (3V).
Reference and Error Amplifier
The recommended circuit connections for these circuits
are shown in Fig. 6. The sine-wave is added to a DC off
-
set to create the composite reference signal for the error
amplifier. The DC reference can vary over a wide range.
For pure AC outputs it is zero, while in many common
applications, it is the talk battery voltage (–48V). The
UCC3750 accomplishes this task by summing the two
signals weighted by resisting R14 and R15. The output of
AMP1 also helps determine the mode of the circuit.
Referring to Fig. 6, the output of AMP1 is given by :
VO UT1 = 1+
R13
R14
+
R13
R26
•VCM–
R13
R26
•REF






(1)
–
R13
R14
•VB–
R13
R15
• VAC
In order to nullify the effect of V
CM
on this value, the ratio
of R26 to R14 should be made 1.5. With this ratio, the
equation becomes:
VO UT1 = VCM –
R13
R14
•VB–
R13
R15
• VAC
(2)
VOUT1 is the reference voltage that the second amplifier
(AMP2) uses to program the output voltage. Assuming
that Z4 is high DC impedance, the output voltage is de
-
rived by summing the currents into pin 18. The output is
given as:
VO = 1+
R10
R27
+
R10
R12
•VCM






(3)
–
R10
R27
•REF–
R10
R12
•VOUT1
Again, if the ratio of R27 to R10 is made 1.5, the effect of
V
CM is nullified and the output voltage becomes (after
substituting for VOUT1):
VO =
R10•R13
R12•R14
•VB+
R10•R13
R12•R15
• VAC
(4)
From equation 4, it can be seen that if the output voltage
DC value has to track VB directly, the following condition
should be forced:
R10 • R13 = R12 • R14 (5)
However, in some cases, this becomes impractical due
to large AC gain required form V
AC
to V
O.
Only a small
part of the gain can be accommodated in the first ampli
-
fier stage due to its output voltage limitations. As a result,
the required resistance values become very high. This
APPLICATION INFORMATION (cont.)
Figure 5. Sine-wave generator. Figure 6. Error amplifier setup.
UDG-96166-1
UDG-96167-1