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UCC28251 Datasheet(PDF) 22 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 22 Page - Texas Instruments

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C
CS
R
CS
REF
COMP
GND
UCC28251
O
O2
O1
REF / EA
V
R
1
R
V
+
æ
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=
-
´
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UCC28251
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
www.ti.com
Feature Description (continued)
When the UCC28251 is configured for secondary-side control, the internal error amplifier is used as the control
loop error amplifier. Connect REF/EA+ directly to the VREF pin to provide the reference voltage for the feedback
loop.
When the UCC28251 is configured for primary-side control, the error amplifier is connected as a voltage follower.
Connect REF/EA+ to the opto-coupler output.
The voltage range on REF/EA+ pin is 0 V to 3.7 V.
8.3.9 FB/EA– (2/9)
FB/EA- is the inverting input of the UCC28251’s internal error amplifier.
When the UCC28251 is configured for secondary-side control, connect the output voltage sensing divider to this
pin. The voltage divider can be selected according to the voltage on REF/EA+ pin. Referring to Figure 32, pick
the lower resistor RO1 value arbitrarily, and choose the upper resistor RO2 value as:
(7)
Because the control loop gain is affected by voltage divider resistor values, choose an appropriate RO1 value so
that the voltage loop DC gain is larger than 40 dB to prevent interference between the primary-side control loop
and the SR control loop during start up.
When the UCC28251 is sitting on the primary side, the error amplifier is connected as a voltage follower.
Connect FB/EA- directly with COMP pin.
The maximum voltage allowed on FB/EA- pin is 3.7 V.
8.3.10 COMP (3/10)
The COMP pin is the internal error amplifier’s output and also the input signal for PWM comparator. The
maximum input common voltage of the PWM comparator is 2.8 V. It is suggested to program the peak value of
RAMP to be lower than 2.3 V. Otherwise, the voltage of COMP pin should be clamp to be lower than 2.8 V by
external circuit in order to make the internal PWM comparator work properly. An external circuit detailed as below
is recommended for voltage clamp function. Both the primary-side switches’ duty cycle and secondary-side SRs’
duty cycle is controlled by the COMP pin voltage. At steady state, a higher COMP pin voltage results in a larger
duty cycle for the primary-side switches and a smaller duty cycle on the SRs.
When the UCC28251 controller is set up for secondary-side control, connect the compensation network from the
FB/EA- pin to the COMP pin.
For primary-side control, the error amplifier is connected as a voltage follower. Directly connect the COMP pin to
the FB/EA- pin.
Figure 31. Comp Clamp Circuit
22
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