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HANDS-ON DESIGN TIPS

Cascode stageor “collector follower” Jean-Paul Brodier

All microprocessors from the 8051 family have inputs and outputs that are ‘quasi-bidirectional’.

This means that when power is first applied, the ports behave as inputs with a logic high level

and a weak pull-up.

from the moment power isapplied until the microprocessor

has had the chance to turn theoutput low. In addition, logichigh outputs are seldom able todeliver enough current to drivethe transistor into saturationbecause they have beendesigned to be active low.

To solve both of these problemsin one hit, we have to make the

active level logic low. This canb d i h diff

examples 2b and 2c is thatthey require more parts.

Collector followerThat leaves the open collectorbuffer in the form of an IC type7404. This solution, however,also has a few disadvantages.You do not always need all of the 6 buffers in one IC. Also, theSMD version can only handle12 V. This is too low and dan-

gerous if we happen to supplyh l d f l d

Px.x

µP 

R1T1

NPN

RE1D1

1N4148

+VCC   +VNR

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Px.x

µP 

R1

T1

NPN 

RE1D1

1N4148

+VCC

  +VNR

T1

NPN RE1D1

1N4148

+VNR

R2

R1

T2

PNP 

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a    b 

Figure 3. Cascode driver stage with discrete transistor.

power is switched on, both thebase and emitter see the samepotential, VCC, so the transistorremains blocked. One thing wehave to keep in mind: we maynot exceed the current rating of the microprocessor outputbecause it has to cope with allthe current flowing in the emitterof the transistor.In the case of the quite common80C51, this maximum current is

typically 3.2 mA (two LS TTLloads). This is sufficient to drivean LED without overloading the5-V regulator, or for driving aPNP power stage at the highside (Figure 3b).The parallel Philips PCF88574I2C interfaces can handle25 mA. For the AtmelAT89Cx051 as well as for the

Philips P89LPC9xx the limit is20 mA. For the latter type thecascode circuit or ‘collector fol-lower’ is even more interestingwhen the outputs are configuredas open-drain because the nom-inal voltage is only 3.6 V. In allcases we have to make sure thatthe maximum dissipation of the

package is not exceeded.Should this be the case, then thenumber of open collectorsrequired will probably justifyresorting to a 7404.A current of around 20 mA at

24 V is sufficient to energise ahalf Watt relay coil, which inturn can drive a load of 16 A at230 V.For loads driven from the positiveside, the voltage and current lim-

its are determined by the powerPNP (or VMOS) transistor.The cascode transistor can be a‘digital’ type with integratedbase and emitter resistors.

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