TISP5xxxH3BJ Overvoltage Protection Series
APPLICATIONS INFORMATION (CONTINUED)
The star-connection of three TISP5xxxH3BJ protectors gives a protection circuit which has a low differential capacitance to ground (Figure 13).
This example, a -100 V ISDN line is protected. In Figure 13, the circuit illustration A shows that protector Th1 will be forward biased as it is
connected to the most negative potential. The other two protectors, Th2 and Th3 will be reverse biased as protector Th1 will pull their common
connection to within 0.5 V of the negative voltage supply.
Th1
Th3
SIGNAL
Th2
C
0.5 V
600 pF
26 pF
29 pF
26 pF
C
-99.5 V
1 pF
C
-99.5 V
29 pF
A) STAR-CONNECTED
U-INTERFACE
PROTECTOR
- 100 V
B) EQUIVALENT
TISP5150H3BJ
CAPACITANCES
- 100 V
C) DELTA EQUIVALENT
SHOWS 25 pF
LINE UNBALANCE
- 100 V
AI4XAB
Figure 13. ISDN Low Capacitance U-Interface Protection
Illustration B shows the equivalent capacitances of the two reverse biased protectors (Th2 and Th3) as 29 pF each and the capacitance of the
forward biased protector (Th1) as 600 pF. Illustration C shows the delta equivalent of the star capacitances of illustration B. The protector
circuit differential capacitance will be 26 - 1 = 25 pF. In this circuit, the differential capacitance value cannot exceed the capacitance value of
the ground protector (Th3).
A bridge circuit can be used for low capacitance differential. Whatever the potential of the ring and tip conductors are in Figure 14, the array of
steering diodes, D1 through to D6, ensure that terminal 1 of protector Th1 is always positive with respect to terminal 2. The protection voltage
will be the sum of the protector Th1, V(BO), and the forward voltage of the appropriate series diodes. It is important to select the correct
diodes. Diodes D3 through to D6 divert the currents from the ring and tip lines. Diodes D1 and D2 will carry the sum of the ring and tip currents
and so conduct twice the current of the other four diodes. The diodes need to be specified for forward recovery voltage, VFRM, under the
expected impulse conditions. (Some conventional a.c. rectifiers can produce as much as 70 V of forward recovery voltage, which would be an
extra 140 V added to the V(BO) of Th1). In principle the bridge circuit can be extended to protect more than two conductors by adding extra
legs to the bridge.
RING TIP
1
D1
D3
D5
2 Th1
D2
D4
D6
AI5XAC
Figure 14. Low Capacitance Bridge Protection Circuit
JANUARY 1998 - REVISED FEBRUARY 2005
Specifications are subject to change without notice.
Customers should verify actual device performance in their specific applications.
BOURNS [ BOURNS ELECTRONIC SOLUTIONS ]
