rishispr said:
Hey andrewr,
Thankx for your reply.
Actually we are using 2 DM134B as sink only. It is not being used for controlling the mosfet. The gate of Mosfet is connected to 220E pullup resistor with the 'st 74hc595' shift register being used as latch.
And I think yes u've understood the design correctly. The source is connected to 5V after diode.
We are using FDN340P as the Mosfet,
2. The LED is amber (2.1V and 20mA current) required.
3. Earlier we were using 1K but i am using 220E now.
4. As I measured on CRO, the on time is 18.64 us.(I measured it on both legs one by one by taking common ground.).
we are latching DM134B with the nand gate latch HTC573D.
Kindly tell me what should we do to increase the intensity?
You appear to be hitting problems which are subtle in nature and not really that much a fault of your design... but of salesmanship of an idea with some problems by the SiTI company...
Long drawn out thought...
Ok. Let's do some calculations and datasheet probing here...
I Like ST brand HC chips, really robust!
A 74HCchip will go rail to rail pretty much unless overloaded on current. 10mA is a rule of thumb value/output pin (10 loads, 5V @ 4.7K for ttl designs.). If that is a 22 ohm pullup resistor 22 followed by 0 zeroes, you would be at 5v/22=227ma which isn't good, chances are the CMOS chip is incapable of pulling that down to less than 2V and you won't get a FET turned all the way on. If it is 220Ohm, which I assume it is -- then you are at 23ma which is probably OK but you're running your logic chip quite hard, as they really aren't meant for more than 20mA/terminal. A 470ohm resistor would be more conventional. The capacitance of a FET is multiplied by the slew rate of the output. The typical datasheet charge says 7.2nC -- so double that for safety: 14nc and compute current requred for turn on at 10% of "on" time: = 14Nc / 1.864us = 7.5mA. So the 74HC load is around 30mA conservative worst case; that's too high, so I would lighten the pull up resistor (higher value such as 470Ohm) or best solution -- remove it altogether. The 74HC chip you are using will pull up the gate to +5V as a turn off value on its own so long as the 74HC595 pin 1 is permanently grounded which makes a pull up resistor un-necessary. Quick thought: the latch is not inverting, so a logical zero on the input equals turn "ON" -- that is an easy software mistake; on time of led = off time of latch; so check that your software doesn't output "1" as meaning 'on' by mistake...
You are expecting 16 LED loads in parallel/chip. The DM134B with 220 Ohms resistance is 75mAx16=1.2A and that makes for around 0.9V output pin to ground minimum for it to function at all. 28mA*5V=140mW supply power besides output LED driver power. So, your are going to be walking the touchy power dissipation line with the DM134B...
OK! your LED is a 2.1V @ 20mA version; likely an InGaAlP type...
that puts your dynamic resistance at around 100Ohms above 20mA from other diodes which show 2.5V at 25ma, 2.1/0.02=~105ohm. So just estimate as if it was 100ohm always...
safe dissipation avg wattage = 20mA * 2.1V =~ 50mW.
75ma/20mA= 3.6; 3.6**2=12.96x the power at 70mA; so to keep the same average wattage you may only turn them on 1/12 of the time...
Your off time needs to be around ~250uS to prevent the LED from being over heated which will cause it to loose efficiency and become substantially dimmer... Since you are running 16 strings of LEDS you should be forced have an off time of 280us which is just about optimal...but you can't afford to waste time between runs -- you must always be refreshing as quickly as possible or the LED's will dim quite a bit more...
The LED's output is proportional to current (photon/carrier) with some extra loss as the device gets hot/current crowded.
Since you are 75mA/20mA = ~3.75 times as bright as normal, and your pulse operation time is 1/16th the normal time = composite effect is that roughly 1/4 the photons are output / LED as compared to normal. Part of your problem could be alleviated if you did not discharge the capacitance of the LED when turning off the FET -- but the gate-drain cap is quite large easily on the order of 10LED's worth. Secondly, for the charge storage on the LED's to be effective you need the RC constant / total cycle time to be on the same order of magnitude; and yours is waaay to slow for that. Faster refresh rate is one way to achieve this... but:
One other thing you are fighting is that power dissipation goes as I**2, so that doubling current quadruples power dissipation (and lowers LED efficiency) but only doubles the number of photons emitted. The eye detects brightness logarithmically so for low intensities, doubling the light will appear far more than 2x as bright to the eye -- but at higher intensities a doubling of brightness will have much less effect. Where you are heuristically, I can't know. You have to empirically find that out.
At least one, and likely both of these things are working against you in your current design; Time multiplexing saves wires but it majorly costs you in terms of power efficiency and sometimes apparent total light.
The most obvious fix is to get a much more efficient/brigher LED (4x as bright as your present one.) but there are other work-arounds you might consider -- placing capacitors across each of your LED's will cause the instant current going to the LED to drop, but extended in time by the RC constant.
Reverse current flow is a problem, and your FET has an intrinsic body diode which will potentially ruin this effect unless a blocking diode is added... but assuming that were fixed:
For example a 1uF capacitor on your present system assuming it is in parallel with a100ohms dynamic resistance LED is going to have a 63% voltage change every 100us when discharging; so a 10uF capacitor would have about a 6% change every 100us; and a 20uF capacitor would be in the ballpark to make the LED look as if it were not time multiplexed at all. There are low voltage electrolytic capacitors which are probably quite inexpensive as size drops rapidly with voltage. The capacitance swamps the bad capacitance of the FET gate to drain -- and the ability to have charge flow backward from one LED to another in the same set can have multiple solutions... it may even average out in a few cycles -- and as a bonus, holding the drain voltage much nearer to constant reduces the miller effect and load on your 75HC chip at the same time. So that seems like one reasonable possibility for increasing the brightness -- but at a small cost -- just add an electrolytic cap in parallel with each LED. (Also watch out for dynamic power dissipation of the cap...some of them are horrible). If you have a through hole board -- the capacitors can be mounted on the back side. You will then be free to raise the current level anywhere up to 300mA/LED (your chip can't...). Even if you leave the current level alone at 75ma you will still see a gain in brightness due to the increase in efficiency of the LED and the lengthening of the emission period; though the brightness v. capacitance is probably best found empirically in that case.
Since you are in the milli-ohm region with the mosfet; we can assume around a 2.25V drop across the LED and FET. And that means approximately 2.75V will have to appear at each pin of the DM134B. At 75mA only 0.9V is required, so you will be dissipating around 3.3Watts/chip once the capacitors are installed. That is likely going to destroy/overheat them. The supply voltage needs to be reduced to 2.25V+1.2V=3.45Volts for each LED string -- and that reduction can be by a voltage for the reverse blocking diode to protect the charged capacitors from discharge, although you are free to leave the 75HC and DM134B on the 5V rail. If the added power diode has a voltage drop of around 1.5V at 1.2A to 1.5A (75-90mA/LED) you will be quite happy with the result. It only needs around 200milliwatt average power dissipation capability.
1.2A*1.0V + 140mW =~ 1.34Watt of power in each DM134B avg w/ (all LED's ON).
The only chip I would be concerned about is the SSOP version. Also as ambient temperature goes up -- power dissipation is going to get worse.
You're in a bit of a complicated problem with several drawbacks, and are near the edge of the performance of the chips you are using. From the SMD part numbers, I would expect your prototyping to be difficult to change without re-making boards; are you open to discussing the nature of the problems and possible design alternatives which might prove effective?
eg: what are your constraints & is a redesign worth it if it provides benefits in cost/quality/uniformity? or is this a one shot where buying more expensive LEDs is probably the best route?
Cordially.
--Andrew.
