Datasheet

UCC28700, UCC28701
UCC28702, UCC28703
www.ti.com SLUSB41 – JULY 2012
FUNCTIONAL DESCRIPTION
The UCC28700 is a flyback power supply controller which provides accurate voltage and constant current
regulation with primary-side feedback, eliminating the need for opto-coupler feedback circuits. The controller
operates in discontinuous conduction mode with valley-switching to minimize switching losses. The modulation
scheme is a combination of frequency and primary peak current modulation to provide high conversion efficiency
across the load range. The control law provides a wide-dynamic operating range of output power to achieve the
<30-mW stand-by power requirement.
Another feature beneficial to achieve low stand-by power without excessive start-up time is a wide operating
VDD range to allow a high-value VDD start-up resistance and low-value VDD capacitance. During low-power
operating ranges the device has power management features to reduce the device operating current at operating
frequencies below 44 kHz. The UCC28700 controller includes features in the modulator to reduce the EMI peak
energy of the fundamental switching frequency and harmonics. Accurate voltage and constant current regulation,
fast dynamic response, and fault protection are achieved with primary-side control. A complete charger solution
can be realized with a straightforward design process, low cost and low component count.
Primary-Side Voltage Regulation
Figure 12 illustrates a simplified flyback convertor with the main voltage regulation blocks of the device shown.
The power train operation is the same as any DCM flyback circuit but accurate output voltage and current
sensing is the key to primary-side control.
Figure 12. Simplified Flyback Convertor
(with the main voltage regulation blocks)
In primary-side control, the output voltage is sensed on the auxiliary winding during the transfer of transformer
energy to the secondary. As shown in Figure 13 it is clear there is a down slope representing a decreasing total
rectifier V
F
and resistance voltage drop (I
S
R
S
) as the secondary current decreases to zero. To achieve an
accurate representation of the secondary output voltage on the auxiliary winding, the discriminator reliably blocks
the leakage inductance reset and ringing, continuously samples the auxiliary voltage during the down slope after
the ringing is diminished, and captures the error signal at the time the secondary winding reaches zero current.
The internal reference on VS is 4.05 V; the resistor divider is selected as outlined in the VS pin description.
Figure 13. Auxiliary Winding Voltage
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Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703