I have gone through the below TI document page number 3


I want to simulate the fig2.2 using LTspice using transistors and main aim is to generate 1.5V with 54 Ohm load. The LTspice model is as below

According to my understanding i need to find the RC1 and RC2. Is it correct? Do i need to consider the base resistors as well? Please help to solve the circuit.

 

TI document does not show such resistors. why do you have them?

 

TI document does not show such resistors. why do you have them?

I wanted to create one driver for simulation purpose, i am not sure if i have to use resistor or not.

I have to generate current how do i do?

 

hi V123,
If ALL the switches have the same parameters, Label them all as SW1

Also add the working voltage V1

Also add the D,Dinv pulse train signal parameters,
E

 

RS485 is pretty fast so transistors should not go into saturation. they are low power high speed types and as such have significant Rds_on. to produce 1.5V across 54 Ohm you need current to me at least 1.5V/54Ohm=27.77mA.
since the supply is 5V, voltage drop across two transistors must be less than 5V - 1.5V = 3.5V or 1.75V per transistor (Assuming they are matching). so maximum resistance of one transistor is 1.75V/27.77mA = 63Ohm. designing for 20 Ohm or less give you decent safety margin.

20+54+20 = 94 Ohm.

5V/94 Ohm = 53.19mA which is practically double of the 27mA mentioned before.

so to make transistor act as 20-30 Ohm resistor, you need to bias it
btw if you want to simulate MOSFET stage, why are you using BJTs? do you know the difference? if you did, you would not bring input voltages to transistor bases like that. BJTs are current controlled devices, MOSFETs are voltage controlled devices.
this means BJTs require base resistors (series), while MOSFETs do not.

 

I know BJT little better than FET so i have started with BJT, i will update the model using MOSFET.

 

Hi V123,
If you add the requested voltages and signals to the circuit it should highlight to You the number circuit errors in your proposed design.

E

 

all discrete NMOS types in LTSpice have Rds_on 6Ohm or less. so if you want to get closer to 20-30Ohm you need to make custom model

 

i have updated and the LTspice does not show errors, what should i do next create bias circuit?

 

hi V123,
Reduce the Tran time so that you see the waveforms.
Not to spec,
E

 

Hi V123,
This is a clip from your d/s.
Check the P/N FET types in your circuit.
E

 

Can you please clarify the below point, i have downloaded the data sheet of BSP89 the VGS is 1.4V and the RDS(on) is 6Ohm, so when M1 and M3 are On the total resistance is 6 Ohm + 6Ohm + 54Ohm = 66 Ohm. The current is 75mA and the voltage drop across 54Ohm = 75mA * 54 Ohm = 4.05V, how do i bring it to 1.5V, am i missing some additional circuit?

 

main aim is to generate 1.5V with 54 Ohm load

Ok, but if you just want an RS-485 compliant driver, note that the differential voltage must be greater than 1.5V.

 

Ok, but if you just want an RS-485 compliant driver, note that the differential voltage must be greater than 1.5V.

My main aim is to understand how a voltage is generated. So, if i can know for example 1.5V then i can modify the circuit to 1.8V etc.

 

hi V,
This is a simplified circuit.
Note the voltage on the M1 Source when the M1 Gate is positive.
E

 

Ok i understand now that the source is floating, and the gate voltage to be more than the source by 1.4V, so need to add the bootstrap capacitor, am i correct?

 

Ok i understand now that the source is floating, and the gate voltage to be more than the source by 1.4V, so need to add the bootstrap capacitor, am i correct?

Hi V.
The source is not floating when it is positive, there is voltage drop across the 54R resistor, which pulls the Source voltage high.
How will a bootstrap cap help???

You need to study transistor theory before designing circuits
E

 

the VGS is 1.4V and the RDS(on) is 6Ohm

N.b. those values only apply under the conditions specified in the datasheet.
1.4V is the threshold voltage at which the drain current is only 108μA.
Rds(on) is 6Ω when Vgs is 10V and the drain current is 0.35A.

 

Hi V,
This basic circuit should help you get started.
E

 

actual terminating resistors (found at both ends of a bus) are usually 120 Ohm and - they are in parallel. so actual load is normally 60 Ohm...
but... to ensure circuit can drive that, specs are demanding more... and that is where 54 Ohm and minimum 1.5V come from.
in reality voltage differential is higher than 1.5V (2.5-3.5V... or even up to 5V). it needs to be at least 1.5V to reliably receive data... because there can and will be not just your signal but some noise as well.