Datasheet

B
MAX
3 V
R
I / n
=
CS
OUTA
OUTB
COMP
UCC28250
SLUSA29C –APRIL 2010– REVISED JULY 2011 www.ti.com
CS: Current Mode Control
For current mode control, the RAMP/CS pin is driven by a signal representative of the transformer primary-side
current. The current signal has to have compatible input range of the COMP pin. As shown in Figure 12, the
COMP pin voltage is used as the reference for peak current. The primary-side signals OUTA and OUTB are
turned on by the internal clock signal and turned off when sensed peak current reaches the COMP pin voltage.
Choose the current sense transformer turns ratio (1:n) and the burden resistor value (R
B
) based on the peak
current at maximum load I
MAX
. Refer to the Functional Description section for more details on the current mode
control.
(7)
Figure 12. Peak Current Mode Control and PWM Generation
REF/EA+ (1/8)
REF/EA+ is the non-inverting input of the UCC28250’s internal error amplifier.
When the UCC28250 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 UCC28250 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.
18 Submit Documentation Feedback Copyright © 2010–2011, Texas Instruments Incorporated
Product Folder Link(s): UCC28250