Datasheet

UCC2897A
SLUS829D -- AUGUST 2008 -- REVISED JULY 2009
11
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DETAILED PIN DESCRIPTIONS (continued)
SS/SD
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
RON
= 0.43 ×
V
REF
2
×
1
R
ON
This DC current charges C
SS
from 0 V to approximately 5 V. Internal to the UCC2897A, 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
2.5 V and 4.5 V approximately. During line undervoltage protection, the PWM follows this pin capacitor
discharge to achieve soft stop funtion.
PGND
This pin serves as a dedicated connection to all high-current circuits inside the UCC2897A. 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
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 UCC2897A 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).
OUT
This high-current output drives an external N-channel MOSFET. The UCC2897A controller uses an active high
drive signal for the main switch of the converter.
Due to the high speed and high-drive current capability of these outputs (AUX, OUT) the parasitic inductance
of the external circuit components connected to these pins should be carefully minimized. A potential way of
avoiding unnecessary parasitic inductances in the gate drive circuit is to place the controller in close proximity
to the MOSFETs and by ensuring that the outputs (AUX, OUT) and the gates of the MOSFET devices are
connected by wide, overlapping traces. It is recommend connecting a 10--kΩ resistor from this pin to PGND pin
to reduce possible parasitic effect from layout.
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