TISP61521 Bourns, Inc., TISP61521 Datasheet - Page 6

• Manufacturer Part Number: TISP61521
• Description: Dual Forward-conducting P-gate Thyristors Programmable Overvoltage Protectors
• Manufacturer: Bourns, Inc.

This section covers three topics. First, it is explained why gated protectors are needed. Second, the voltage limiting action of the protector is
described. Third, an example application circuit is described.
Purpose of Gated Protectors
Fixed voltage thyristor overvoltage protectors have been used since the early 1980s to protect monolithic SLICs (Subscriber Line Interface
Circuits) against overvoltages on the telephone line caused by lightning, a.c. power contact and induction. As the SLIC was usually powered
from a fixed voltage negative supply rail, the limiting voltage of the protector could also be a fixed value. The TISP1072F3 is a typical example
of a fixed voltage SLIC protector.
SLICs have become more sophisticated. To minimize power consumption, some designs automatically adjust the driver supply voltage to a
value that is just sufficient to drive the required line current. For short lines, the supply voltage would be set low, but for long lines, a higher
supply voltage would be generated to drive sufficient line current. The optimum protection for this type of SLIC would be given by a protection
voltage which tracks the SLIC supply voltage. This can be achieved by connecting the protection thyristor gate to the SLIC V BATH supply,
Figure 3. This gated (programmable) protection arrangement minimizes the voltage stress on the SLIC, no matter what value of supply voltage.
Operation of Gated Protectors
Figure 3 and Figure 4 show how the TISP61521 limits negative and positive overvoltages. Positive overvoltages (Figure 4) are clipped by the
antiparallel diode of Th5 and the resulting current is diverted to ground. Negative overvoltages (Figure 3) are initially clipped close to the SLIC
negative supply rail value (V BATH ). If sufficient current is available from the overvoltage, then Th5 will switch into a low voltage on-state
condition. As the overvoltage subsides, the high holding current of Th5 prevents d.c. latchup. The protection voltage will be the sum of the
gate supply (V BATH ) and the peak gate-cathode voltage (V GK(BO) ). The protection voltage will be increased if there is a long connection
between the gate decoupling capacitor, C1, and the gate terminal. During the initial rise of a fast impulse, the gate current (I G ) is the same as
the cathode current (I K ). Rates of 70 A/µs can cause inductive voltages of 0.7 V in 2.5 cm of printed wiring track. To minimize this inductive
voltage increase of protection voltage, the length of the capacitor to gate terminal tracking should be minimized. Inductive voltages in the
protector cathode wiring will also increase the protection voltage. These voltages can be minimized by routing the SLIC connection through
the protector as shown in Figure 6.
Figure 5, which has a 10 A/µs rate of impulse current rise, shows a positive gate charge (Q GS ) of about 0.1 µC. With the 0.1 µF gate
decoupling capacitor used, the increase in gate supply is about 1 V (= Q GS /C1). This change is just visible on the -72 V gate voltage, V BATH .
But the voltage increase does not directly add to the protection voltage, as the supply voltage change reaches a maximum at 0.4 µs, when the
gate current reverses polarity, and the protection voltage peaks earlier at 0.3 µs. In Figure 5, the peak clamping voltage (V (BO) ) is -77.5 V, an
increase of 5.5 V on the nominal gate supply voltage. This 5.5 V increase is the sum of the supply rail increase at that time, (0.5 V), and the
protection circuit’s cathode diode to supply rail breakover voltage (5 V). In practice, use of the recommended 220 nF gate decoupling capacitor
would give a supply rail increase of about 0.3 V and a V (BO) value of about -77.3 V.
APRIL 2001 REVISED JULY 2008
Specifications are subject to change without notice.
Customers should verify actual device performance in their specific applications.
Gated Protectors
TISP61521 SLIC Protector
Figure 3. Negative Overvoltage Condition
I
K
PROTECTOR
61521
SLIC
TISP
220 nF
C1
Th5
I
G
V
SLIC
AI6XABA
BAT
APPLICATIONS INFORMATION
Figure 4. Positive Overvoltage Condition
I
F
PROTECTOR
61521
SLIC
TISP
220 nF
C1
Th5
V
SLIC
AI6XACA
BAT