630
6 Reverse Engineering of Microcircuits
Fig. 6.99 Non-standard circuit of protection from the positive ESC discharge in the output-power
source circuit (a) and the output stage of I 2 L-TTL with increased immunity to voltage bursts (b)
lower than the direct gain, the collector current VT3 is correspondingly lower than
the emitter current, which limits the discharge current (by cutting off the noise level)
and eliminates the possibility of destruction of the protected circuit.
Figure 6.99b shows a fragment of the electrical circuit of the output stage of
the I
2 L-TTL LSIC with increased noise immunity. The additional n-p-n transistor
VT2 with a floating base protects the pin from destruction in case of positive pulses
of noise voltage with regard to the common bus. The VT6 transistor in the output
stages protects the LSIC from destruction when exposed to short-term surge voltage
in the external power supply circuit by switching the output transistors VT5 to the
short-term active mode, which limits the total discharge current of the surge voltage.
Any program for recovery of the electrical circuit from topology will register
an error during description of these elements (transistor base is stuck in the air or
connected to the common base).
In order to understand the purpose and operation principle of these elements, an
intruder would need to perform instrumental and technological modeling of these
unknown elements, the result of which cannot give a certain result.
6 Reverse Engineering of Microcircuits
Fig. 6.99 Non-standard circuit of protection from the positive ESC discharge in the output-power
source circuit (a) and the output stage of I 2 L-TTL with increased immunity to voltage bursts (b)
lower than the direct gain, the collector current VT3 is correspondingly lower than
the emitter current, which limits the discharge current (by cutting off the noise level)
and eliminates the possibility of destruction of the protected circuit.
Figure 6.99b shows a fragment of the electrical circuit of the output stage of
the I
2 L-TTL LSIC with increased noise immunity. The additional n-p-n transistor
VT2 with a floating base protects the pin from destruction in case of positive pulses
of noise voltage with regard to the common bus. The VT6 transistor in the output
stages protects the LSIC from destruction when exposed to short-term surge voltage
in the external power supply circuit by switching the output transistors VT5 to the
short-term active mode, which limits the total discharge current of the surge voltage.
Any program for recovery of the electrical circuit from topology will register
an error during description of these elements (transistor base is stuck in the air or
connected to the common base).
In order to understand the purpose and operation principle of these elements, an
intruder would need to perform instrumental and technological modeling of these
unknown elements, the result of which cannot give a certain result.