UCC2897
SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004
DETAILED PIN DESCRIPTIONS
VIN (pin 1)
The UCC2897 controller are equipped with a high voltage, P-channel JFET start up device to initiate operation
from the input power source of the converter in applications where the input voltage does not exceed the 110-V
maximum rating of the start up transistor. In these applications, the VIN pin can be connected directly to the
positive terminal of the input power source. The internal JFET start up transistor provides approximately 15-mA
charge current for the energy storage capacitor (C
BIAS
) connected across the VDD (pin 14) and PGND (pin 11)
terminals. Note that the start up device is turned off immediately when the voltage on the VDD pin exceeds
approximately 13.5 V, the controller’s undervoltage lockout threshold for turn-on. The JFET is also disabled at
all times when the high-current gate drivers are switching to protect against excessive power dissipation and
current through the device.
For more information on biasing the UCC2897, refer to the
Setup Guide
and
Additional Application Sections
of this datasheet.
RDEL (pin 3)
This pin is internally connected to an approximately 2.5-V DC source. A resistor (R
DEL
) to GND (pin 6) sets the
turn-on delay for both gate drive signals of the UCC2987 controller. The delay time is identical for both switching
transitions, between OUT (pin 13) is turning off and AUX (pin 14) is turning on as well as when AUX (pin 14)
is turning off and OUT (pin 13) is turning on. The delay time is defined as:
t
DEL
+
R
DEL
1.1
10
*11
(1)
For proper selection of the delay time refer to the various references describing the design of active clamp power
converters.
RTON (pin 4)
This pin is internally connected to an approximately 2.5-V DC source. A resistor (R
ON
) to GND (pin 6) sets the
charge current of the internal timing capacitor. The RTON pin, in conjunction with the RTOFF pin (pin 3) are used
to set the operating frequency and maximum operating duty cycle of the UCC2897.
RTOFF (pin 5)
This pin is internally connected to an approximately 2.5-V DC source. A resistor (R
OFF
) to GND (pin 6) sets the
discharge current of the internal timing capacitor. The RTON and RTOFF pins are used to set the switching
period (T
SW
) and maximum operating duty cycle (D
MAX
) according to the following equations:
t
ON
+
37.33
t
OFF
+
16
10
*12
10
*12
R
ON
R
OFF
(2)
(3)
(4)
T
SW
+
t
ON
)
t
OFF
D
MAX
+
t
ON
T
SW
(5)
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