rickman wrote:
> On Dec 2, 2:12 pm, Jim Granville <no.s...@designtools.maps.co.nz>
> wrote:
>
>>rickman wrote:
>>
>>
>>>Good point about the driver current, but I think you have the topology
>>>(at least as I am describing it) wrong. In my approach, the row
>>>drivers are the ones that enable a single row at a time. Of course,
>>>it is arbitrary which are called the "row" and which are the
>>>"column". But in my approach it is important to distinguish between
>>>the drivers that are controlled by the shift register and which are
>>>controlled directly by the MCU. I use the MCU outputs to directly
>>>control the high current drivers for the rows with only one of the
>>>drivers enabled at a time. The columns use low current drivers (the
>>>HBDM60V600W mentioned earlier) and they are all active all the time.
>>
>>>Zetex makes a part, ZDT6753T, which is in a very small package with a
>>>max current rating of 2 Amps for both the NPN and PNP transitors.
>>>Trimming back the current to match the driver will reduce the average
>>>current to just under 18 mA vs. the OPs goal of just under 19 mA.
>>>These parts are only 7 mm square and are available from Mouser in
>>>single quantities.
>>
>>Not sure if the OP is into small SMD, or if this is a vero-board
>>student project ?
>>
>>
>>
>>
>>>As to the output impedance of the driver, if you consider the emitter
>>>follower driver arrangement, which is what I am suggesting for the
>>>high current drivers, you will see that this is a *very* low impedance
>>>configuration since the load provides feedback to the input.
>>
>> I did not notice any feedback, but a Logic+emitter follower has a
>>number of impedance sources.
>
>
> The feedback is due to the fact that the load is in the input voltage
> loop as well as the output. If the load voltage increases, the
> voltage across the B-E decreases lowering the current to the load. A
> lower voltage on the load likewise increases the B-E current and
> increases the load current. A common emitter arrangement has no such
> feedback and changes in the load will affect the output voltage much
> more significantly.
You would need to draw this, as I cannot picture a circuit from that
description.
>
>
>> The Vbe slope, gives 600-700mV @ 300mA and 1.0-1.2V at 2.1A, so that's
>>a change in drive-drop of ~400mV from that alone.
>
>
> I don't see this data in the sheet for the ZDT6753T. Regardless, the
> current variation from the changes you describe are not large. There
> is significant variation between LEDs and this should be within that
> typical variation in brightness.
LEDs are quite good within a batch. I've measured under 10mV of
variation, on a sample of 100. There IS, of course, variation between
Red / Green, but the issue is trying to minimise crosstalk or brightness
modulation, caused by common mode resistance.
>
>
>> Then the base current also matters - 2.1A at Hfe of 100 is 21mA, which
>>is quite high for HCMOS, Even at 210Hfe, (rarer, but doable) it is 10mA
>
>
> Yes, you are right, the ZDT6753T may not be the best choice for this
> circuit dut to its low hfe. The ZDT6790T or a similar part should
> work much better with an hfe of 150 min at 2 amps requiring only 13.3
> mA base current.
>
>
>
>>Even a high drive SN74LV595 specs 16mA @ .55V max Nch, and 0.8V Pch
>>- that drop is current linear, so adds to the modulation effects.
>
>
> I'm not sure why you are discussing this part since it is not in the
> circuit.
See my other notes, the HC165 is a poor choice, as it lacks a Latch, and
OE control. The LV595 is the highest drive, 5V latched shift register
I could find quickly.
The high current drivers are controlled directly from the
> MCU outputs. Many MCUs can directly drive 20 mA LEDs. I don't know
> the specs on the OP's MCU, but 13 mA should be doable without
> excessive voltage drop.
with a high drive LV595, and your numbers, that's 675mV, which is
significant in the voltage budget. It also varies from 96mV to 675mV
as the load current (number of LEDs ON) changes.
To me, that is intolerable to have in the voltage budget - so
a circuit change is needed.
>
>
>>Then the LED drop at 300mA needs to be included (esp non-red), so the
>>whole thing looks tight at 5V, if using ths simple emitter-followers in
>>both paths.
>
>
> I don't think you understand the circuit I am describing (I may have
> mistyped getting common collector/emitter follower confused with
> common emitter). The emitter follower push-pull circuit is only used
> in the row drivers. The column drivers are standard common emitter
> push-pull drivers. The load resistor should be duplicated from each
> common emitter collector output to the load to help minimize shoot
> through, rather than tying the collectors together with a single
> resistor to the load.
A drawing would help - a picture is worth 100 words :)
The voltage budget is getting tight.
Few Display LEDs specs to 300mA, but slope of 500mv/50mA is not
uncommon, for Green.
Most matrix displays expect to be driven at lower than 16:1 duty cycle.
The dominant resistive voltage drop, should be the current limiting
resistor, not any drivers, and especially not any column drivers.
>
>
>>That starts to dictate N fet and Pfet column drivers.
>>vanilla 100mOhm fets are 210mV at 2.1A, with no base-drive factors.
>>50mOhm fets are ~100mV
>
>
> This is not so much an issue of FET vs bipolar as common collector vs.
> common emitter. Common emitter allows the device to be in saturation
> with a lower voltage drop across the transistor output. But that
> circuit requires more control circuitry. The point of the emitter
> follower stage is that it duplicates a high impedance on the MCU
> control pin to the driver output. This is not doable with a common
> emitter stage without two control lines per driver. This is also not
> doable with FETs in a source follower configuration since the
> threshold voltage is so much higher for a FET than for a bipolar
> transistor.
>
> If you use the correct voltage levels for the drivers, you will see
> that there is adequate drive from a 5 volt power rail.
Depends on the LED. The specs I have for 5x7 matrix displays show none
rated to 300mA, and suggest ~3V drop at 100mA drive (green).
That can barely tolerate one follower, so it does need a low saturation
column driver. If we lower to 100mA, for the sheet I have, and allow
0.8V for emitter follower + base effects, that leaves 1.2V for column
driver, and limiting resistor. Not enough room for a second follower,
so it needs a open-collector saturating driver. At 5V targets, FETs
are better than bipolars.
[also not nearly enough voltage budget for the OPs original Emitter
follower/darlington follower/LED/resistor chain ]
Std 5mm leds got nowhere near 300mA ratings (even peak) but I found
a 'traffic light' green led, specs 3.8V max Vf, at 350mA. (typ 3.3V)
-jg
-jg