UCC1883 Unitrode Semiconductor, UCC1883 Datasheet - Page 8

• Manufacturer Part Number: UCC1883
• Description: Micropower Peak Current Mode Controller
• Manufacturer: Unitrode Semiconductor

APPLICATION INFORMATION (cont.)
A ramp voltage running from VSS to 2.5V is created on
the CT pin. This oscillator is automatically synchronized
to the secondary-side clock frequency when data commu-
nication occurs across the isolation interface. Proper syn-
chronization will only occur if the frequency of the
secondary-side clock exceeds the natural frequency of
the UCC1883 oscillator. A new oscillator cycle (ramp re-
turns to VSS) will be initiated on the rising edge of each
positive isolation interface input pulse, with a correspond-
ing decrease in ramp amplitude. The UCC1883 is de-
signed so that the dV/dt established on its CT pin will be
nominally the same as that of the UCC1885 CT pin, if
equivalent components are used on the RBIAS and CT
f
OSC
CT RBIAS
0.72
Figure 7. Input Current Limit Application
Figure 8. Fault Waveforms
8
pins of both devices. However, the natural frequency of
the UCC1883 oscillator will automatically be lower, to al-
low synchronization. Proper CT ramp slope is important
to the UCC1883, because it uses this information, along
with the pulses received over the isolation interface, to
reconstruct the analog output of the UCC1885 voltage er-
ror amplifier. This reconstructed voltage is in turn used to
control the peak current mode PWM. The ratio of the
slope of the UCC1883 timing ramp to the UCC1885 tim-
ing ramp sets the gain applied by the UCC1883 to the
UCC1885 error amplifier output.
Other variations of the UCC1883 implementing voltage
mode control, duty cycles greater than 50%, or faster
natural oscillator frequencies may be available. Contact
the factory for further information.
UCC1883
UCC2883
UCC3883
UDG-94087
UDG-94088