Datasheet

O2 O1

REF / EA

R 1 R

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UCC28250

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SLUSA29C –APRIL 2010– REVISED JULY 2011

FB/EA- (2/9)

FB/EA- is the inverting input of the UCC28250’s internal error amplifier.

When the UCC28250 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 13, pick

the lower resistor R

value arbitrarily, and choose the upper resistor R

value as:

(8)

Because the control loop gain is affected by voltage divider resistor values, choose an appropriate R

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 UCC28250 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.

COMP (3/10)

The COMP pin is the internal error amplifier’s output. The voltage range of COMP pin is 0 V to 3 V. 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 UCC28250 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.

VSENSE (14/1)

The VSENSE pin is used to directly sense the output voltage and to feed it into a transconductance error

amplifier. The measured voltage allows the UCC28250 to achieve optimal pre-biased start up performance.

When configured as a secondary-side controller, the output voltage is sensed and fed into the FB/EA- pin. The

UCC28250 uses a conventional error amplifier approach to allow type III compensation. Therefore, the FB/EA-

pin voltage always follows the REF/EA+ voltage. The FB/EA- pin does not reflect the true output voltage and

therefore this dedicated VSENSE pin is required. The voltage divider connected to VSENSE is discussed in the

Pre-Biased Start-Up Section.

When UCC28250 is set up as primary-side control, connect VSENSE pin to VREF.

Product Folder Link(s): UCC28250