ꢀ ꢁꢁ ꢂꢃ ꢄꢅ
ꢆ
SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004
FUNCTIONAL DESCRIPTION
ǒ
Ǔ
When a wider than 1 * D
T
pulse is connected to the SYNC input, the oscillator is not able to
SYNC
MAX
maintain the maximum duty cycle, originally set by the timing resistor ratio (R , R
). Furthermore, the timing
ON OFF
capacitor waveform has a flat portion as highlighted by the vertical marker in the timing diagram. During this
flat portion of the waveform both outputs is off which state is not compatible with the operation of active clamp
power converters. Therefore, this operating mode is not recommended .
Note that both outputs of the UCC2897 controller are off if the synchronization signal stays continuously high.
When two UCC2897’s are synchronized by tying their SYNC pins together, they will operate in−phase. It is
possible to set different maximum duty cycle limits for the two UCC2897’s and still synchronize them by a simple
connection between their respective SYNC terminals.
APPLICATION INFORMATION: SETUP GUIDE
R
R
IN2
IN1
+V
IN
R
R
IN4
IN3
UCC2897
VIN
1
2
3
4
5
6
N/C 20
C
C
N/C
LINEOV 19
BIAS
HF
R
R
R
DEL
ON
18
RDEL
RTON
RTOFF
VREF
LINEUV
VDD 17
PVDD 16
OUT 15
OFF
C
VREF
7
8
9
SYNC
GND
CS
AUX 14
PGND 13
SS/SD 12
FB 11
−V
IN
C
F
R
SLOPE
10 RSLOPE
C
SS
RF
R
VREF
ISOLATED FEEDBACK
Figure 7. UCC2897 Typical Setup
17
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