Datasheet
SLUS431A – MAY 2000 – REVISED DECEMBER 2000
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
pulse edge transmission circuit
The UCC3961 uses a pulse-edge-transmission (PET) circuit to transmit isolated gate-pulse information from
the secondary-side controller. It is important for the PET circuit to have proper frequency response and
adequately high damping (low Q-factor) in order to prevent excessive overshoot. The circuit is shown in
Figure 3.
4
3
FB
SS
R2
C
SS
UCC3961
C1
L
M
R1
+
V2P
0
SECONDARY
GATE PULSE
25 ns < T < 200 ns
t
VSS+1.0
VSS–1.0
VFB
VSS
25 ns < T < 200 ns
Figure 3. Pulse Edge Transmission Circuit
The pulse width measured at the FB pin must be between 25 ns when measured at 1 V above the soft-start
voltage and and 200 ns when measured at 1 V below the soft-start voltage. The FB voltage must not be
overdriven by more than 5 V above or 5 V below the soft-start voltage. In order to prevent false triggering, the
FB voltage must not ring below the soft-start voltage by more than ±0.9 V. This can be met if the PET circuit has
a resonant frequency of 880 kHz and a Q of 0.25. The following values meet the specifications for a 12-V
secondary-gate pulse signal, over the full range of UCC3961 operating frequencies.
T1 1:1 turns ratio, LM = 5.4-µH, Ferronics 11–622J, N1 = N2 = 4 turns
R1 300 Ω
C1 2700 pF
R2 200 Ω
Pulse-edge-transmission (PET) circuits in standard surface-mount packages are available from Pulse
Engineering (Part #PA0128, Part #PA0115) and from Cooper Electronic Technologies (Coiltronix),
(Part #CTX01–15157).