UCC2960_14 Datasheet, PDF (10/14 Pages) Texas Instruments – PRIMARY-SIDE STARTUP CONTROLLER

UCC2960, UCC3960
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PRIMARYĆSIDE STARTUP CONTROLLER
SLUS430A – APRIL 1999 – REVISED DECEMBER 2000
APPLICATION INFORMATION
pulse-edge transmission circuit
The UCC3960 uses a pulse-edge-transmission (PET) circuit to transmit isolated gate-pulse information from
the secondary-side controller. It is important for the PET circuitry to have proper frequency response and
adequately high damping (low Q-factor) in order to precent excessive overshoot. The circuit is shown in
Figure 3.
FB 1
UCC3960
SS 4
R2
C_SS
C1
L_M
R1
+
V2P
0
–
SECONDARY
GATE PULSE
VFB
VSS+1.0
VSS
VSS–1.0
25 ns < T < 200 ns
t
25 ns < T < 200 ns
Figure 3. Pulse Edge Transmission (PET) Circuit
UDG–99097
The pulse width measured at the FB pin must be less than 25 ns when measured at 1 V above the soft-start
voltage and 200 ns when measured at 1 V below the soft-start voltage. The feedback (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
feedback 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 those specifications
for a 12-V segondary gate pulse signal, over the full range of UCC3960 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).
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