Could anyone explain the output stage of the print head driver ?.  - Page 1
Forum section: electronics / beginners

author: sangar
author_rank: Regular Contributor
timestamp: January 07, 2019, 10:03:51 am
content: Hi,I have been trying to understand the attached circuit since last two days. Though I got something, I am still not sure. There are 64 output channels in which each can output up to 400mA. My assumption is that the feature "16-step grey scale" helps to configure the current of the output channel.Coming to the attached circuit, it is a channel output stage circuit of 64 channels. The datasheet says that it converts 4-bit parallel input to 16 steps (4-bit) grey scale output. So, I think, at a time, a step (ex.0100) in 16 steps feeds as input to the push-pull configuration. And, the output of the push-pull goes to Transmission gate to switch the COM voltage to the output. Please share your input. Attached the datasheet and image.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 07, 2019, 11:13:24 am
content: This is a rather complicated logic to connect the outputs to one of the COM lines. It's kind of staggered, to reduce the amount of current drawn through the COM lines at the same time.Anyway, the output circuit does nothing but connecting the output to the respective COM lines. The switches are implemented in a similar way as a usual analog switch, so the COM lines can have any voltage within the rated voltage. That might be used for some kind of temperature depending adjustment of the print head, and maybe reversing the driver voltage for whatever reason. I've never dealed with such stuff, so I'm just guessing.The gray scaling is done by a PWM scheme (varying the turn-on time of the output, in turn varying the amount of energy delivered to the print head. You'll find the PWM scheme at some other page within the data sheet.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: January 07, 2019, 04:05:38 pm
content: Your pictures shows 64 outputs. 4 to the power of 3 ==

author: sangar
author_rank: Regular Contributor
timestamp: January 08, 2019, 09:41:48 am
content: Hi,I don't understand that what you mean by "4 to the power of 3"...I think that each output channel circuit has individual 4 input lines, not a common "4 input lines" for all the output channel. The block diagram in the datasheet looks like not completed. Please correct me if I am wrong.Thanks,Muthu

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: January 08, 2019, 08:38:52 pm
content: You have 4 inputs, but each one can be -1,0, or 1. 4 to the power of three ... 64.

author: sangar
author_rank: Regular Contributor
timestamp: January 09, 2019, 11:59:08 am
content: Hi,Thanks all. So, there is two different configuration in the output stage of each channel, 4---push-pull configuration and 2 transmission gate. I need to implement the output stage in discrete since the IC is no longer in production.For the push-pull configuration, I have found a solution like L2293Q which feature 4--push-pull channel...I am yet to find a solution to the transmission gate. In our case, we need a total of 128 transmission gates for 64-output channels.Q1: Is there any solution which offers the 128 transmission gate in a single package and it has an option to control from push-pull stage's output as in the attached diagram?.Q2: The solution we had for the push-pull configuration is having only 4-channels .Hence, the application will need 64-chips of L2293Q for 64-output channels which in turn increase the board size. So, I appreciate if anyone suggests a greater number of channels with the same output current as in the L2293Q.Thanks,Muthu

author: capt bullshot
author_rank: Super Contributor
timestamp: January 09, 2019, 12:11:04 pm
content: regarding your Q1:usually the push-pull stage is part of the transmission gate (as its only is purpose to drive the transmission gate). So you may want to look for a bunch of powerful transmission gates in a chip - e.g. ready made analog switches, these all are transmission gates controlled by (typically) logic level inputs, simplifying your circuit.Anyway, I doubt one would find as many as 64 general purpose TGs in a single package, and most general analog switches have rather high Rds on / low current capabilities. You'll have to check your requirements of what level of current the TGs have to switch.Driving "normal" transistors / MOSFETS to be used as TGs is quite problematic, since the gate drive level (VH up to 40V according to the datasheet) exceeds the usual max. Gate-Source voltages, and the intrinsic reverse diode voids the TG at all. You may want to use back-to-back connected MOSFETs as the switches and control them by photovoltaic optocouplers - if this is feasible depends on your speed requirements.