Datasheet

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SLUS542F − OCTOBER 2003 − REVISED JULY 2009
10
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DETAILED PIN DESCRIPTIONS (continued)
FB (pin 9)
This pin is an input for the control voltage of the pulse width modulator of the UCC2891 family. The control
voltage is generated by an external error amplifier by comparing the converters output voltage to a voltage
reference and employing the compensation for the voltage regulation loop. Usually, the error amplifier is located
on the secondary side of the isolated power converter and its output voltage is sent across the isolation
boundary by an opto coupler. Thus, the FB pin is usually driven by the opto coupler. An external pull-up resistor
to the VREF pin (pin 4) is also needed for proper operation as part of the feedback circuitry.
The control voltage is internally buffered and connected to the PWM comparator through a voltage divider to
make it compatible to the signal level of the current sense circuit. The useful voltage range of the FB pin is
between approximately 1.25 V and 4.5 V. Control voltages below the 1.25-V threshold result in zero duty cycle
(pulse skipping) while voltages above 4.5 V result in full duty cycle (D
MAX
) operation.
SS/SD (pin 10)
A capacitor (C
SS
) connected between this pin and GND (pin 6) programs the soft start time of the power
converter. The soft-start capacitor is charged by a precise, internal DC current source which is programmed by
the R
ON
resistor connected to pin 2. The soft-start current is defined as:
I
SS
+ 0.43 I
RTON
+ 0.43
V
REF
2
1
R
ON
This DC current charges C
SS
from 0 V to approximately 5 V. Internal to the UCC2891 family of controllers, the
soft start capacitor voltage is buffered and ORed with the control voltage present at the FB pin (pin 9). The lower
of the two voltages manipulates the controller’s PWM engine through the voltage divider described with regards
to the FB pin. Accordingly, the useful control range on the SS pin is similar to the control range of the FB pin
and it is between 1.25 V and 4.5 V approximately.
PGND (pin 11)
This pin serves as a dedicated connection to all high-current circuits inside the UCC2891 family of parts. The
high-current portion of the controller consists of the two high-current gate drivers, and the various bias
connections except VREF (pin 4). The PGND (pin 11) and GND (pin 6) pins are not connected internally, a
low-impedance, external connection between the two ground pins is also required. It is recommended to form
a separate ground plane for the low current setup components (R
DEL
, R
ON
, R
OFF
, C
VREF
, C
F
, R
SLOPE
, C
SS
and
the emitter of the opto-coupler in the feedback circuit). This separate ground plane (GND) should have a single
connection to the rest of the ground of the power converter (PGND) and this connection should be between pin
6 and pin 11 of the controller.
AUX (pin 12)
This is a high-current gate drive output for the auxiliary switch to implement the active clamp operation for the
power stage. The auxiliary output (AUX) of the UCC2891 and UCC2892 drives a P-channel device as the clamp
switch therefore it requires an active low operation (the switch is ON when the output is low). The UCC2893
and UCC2894 controllers are optimized for N-channel auxiliary switch therefore it employs the traditional active
high drive signal.
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