LM1205 datasheet(16/24 Pages) NSC

16 / 24 page

Circuit Description (Continued)

TLH11881 – 19

FIGURE 11 Simplified Schematic of LM1205LM1207 Clamp Comparator Circuit

CLAMP GATE CIRCUIT

Figure 12 is a simplified schematic of the Clamp Gate cir-

cuit A voltage reference is setup by Z3 and by Q104 and

Q105 connected as diodes generating a 7V base drive to

Q94 Q99 and Q101 Q94 is used to bias the input stage

This stage is designed to accept TTL levels at pin 14 Q95

and Q97 form a differential pair The base of Q97 is set to

21V by Q99 driving the voltage divider formed by R77 and

R78 In a balanced condition the base of Q95 is also at

21V Q96 is connected as a diode and the current flow thru

it is mirrored into Q98 Also the input to pin 14 would be one

diode drop below 21V or around 14V R74 is added to the

input for current limiting during any possible voltage surge at

pin 14 With no resistors at the emitters of Q96 and Q98 this

circuit will quickly switch Below 14V (12V typical) Q95 is

turned on and Q97 is turned off Above 14V (16V typical)

Q97 is turned on and Q95 is turned off With Q97 turned on

Q100 is also turned on This pulls the current thru R79 to

ground turning off Q102 and Q103 Remember Q102 is a

current mirror to Q86 in the clamp comparator With Q102

turned off the clamp comparator is also turned off When

the input signal goes below 12V Q97 and Q100 will be

turned off This allows Q102 to turn on turning on the clamp

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LM1205 Datasheet(PDF) 16 Page - National Semiconductor (TI)