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UC1848 Datasheet(PDF) 6 Page - Texas Instruments

Part No. UC1848
Description  Average Current Mode PWM Controller
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UC1848 Datasheet(HTML) 6 Page - Texas Instruments

   
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UC1848
UC2848
If the system is not sensitive to short circuit requirements,
Figure 5 shows the simplest method of downslope gener-
ation: a single resistor (ROFF = 40k) from IOFF to VREF.
The discharge current is then 100
µA. The disadvantage
to this approach is that the synthesizer continues to gen-
erate a down slope when the switch is off even during
short circuit conditions. Actual inductor down slope is
closer to zero during a short circuit. The penalty is that
the average current is understated by an amount approxi-
mately equal to the nominal inductor ripple current. Out-
put short circuit is therefore higher than the designed
maximum output current.
A third method of generating IOFF is to add a second
winding to the output inductor core (Fig. 6). When the
power switch is off and inductor current flows in the free
wheeling diode, the voltage across the inductor is equal
to the output voltage plus the diode drop. This voltage is
then transformed by the second winding to the primary
side of the converter. The advantages to this approach
are its inherent accuracy and bandwidth. Winding the
second coil on the output inductor core while maintaining
the required isolation makes this a more costly solution.
In the example, ROFF = VO / 100µA. The 4 • ROFF re-
sistor is added to compensate the one volt input level of
the IOFF pin. Without this compensation, a minor current
foldback behavior will be observed.
APPLICATION INFORMATION (cont.)
Figure 5: Fixed IOFF.
Figure 6: Second inductor winding generation of
IOFF.
UDG-93010
UDG-93011
Figure 4: Inductor current waveform synthesizer.
UDG-93009


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