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SN74LVC2G34: SN74AUP and SN74LVC family LED driving and current consumption

Part Number: SN74LVC2G34
Other Parts Discussed in Thread: SN74AUP2G34, SN74AUP1G125

I have a couple of inquiries and questions about the SN74AUP and SN74LVC family of ICs.

I’m trying to drive and quickly toggle or turn on/off an LED with a 24 MHz, 0 – 3.3V ~40% duty cycle signal, at 6-7 mA and 3.3V VCC with the SN74LVC2G34 and SN74AUPG34 drivers.

The LED used: Vishay VSMA1085600, which has a forward voltage of ~2.55V at 10 mA.

Unclear from the datasheet for the drivers, but by interpolating the values provided, VOH minimum should be over 2.55 if VCC is 3.3V and target drive current is 6 mA.
When driving the LED directly with an MCU output, and using a 15 Ohm resistor in series, the current draw is about 6 mA with the required brightness. However, when the MCU output is connected to the SN74LVC2G34 or SN74AUP2G34 input, and the driver output to the LED with the same resistor in series - the current draw not only becomes huge (20 mA and 5 – 10 mA respectively), but the brightness is also low for SN74AUP2G34. Only one input pin is used for now, the other is tied to ground. Not sure what the load capacitance of the LED is. The static current consumption is negligible and the dynamic current consumption for the drivers should be dictated by the below equations. For the SN74LVC2G34 and assuming 30 pF LED load (which may be an over estimation) this should be:

P_T = Cpd*Vcc*fi*N_sw = 15pF*(3.3V)*24MHz 1 = 1.188 mA
P_CL = C_L*fo*Vcc = 30pF*24 MHz*(3.3V) = 2.376 mA
Total = 3.564 mA

For the SN74AUP2G34 this same calculation yields 2.717 mA

Any ideas what could cause the increased current consumption?

Unrelated but according for the datasheet the SN74AUPG106 and SN74AUPG206 the Cpd is 1 pF and 4.3 pF respectively, while for the SN74AUPG107 and SN74AUPG207 it is the other way around, 4.3 pF and 1 pF respectively. Is this correct?

Also the for the SN74AUP1G125 it says on page 5 that the maximum current should be 4 mA, however on page 17 it says that it can drive an LED up to 24 mA at 3.3v. Which one is correct? Does this hold for all AUP family ICs?

  •  Schematics for reference 

  • Power dissipation is measured in watts.

    Please note that the MCU and logic gate outputs have a certain resistance, so the total resistance is larger than 15 Ω; see [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?

    Are you really using 24 MHz? Why not 24 kHz?

    The 24 mA value in the AUP datasheet is an error (probably copied from an LVC datasheet); AUP should not be used with more than 4 mA.

  • Yes, it's 24 MHz, used for free space optical data transmission. The point still stands if calculating power consumption by squaring the voltage in the above formulas. This yields 11.761 mW and 7.841 mW respectively which is much lower than the measured (> 50 mW). 

    What is the output resistance of the SN74AUP and SN74LVC ICs?

    Noted regarding the error in the datasheet. The datasheet also mentions and absolute rating for continuous output current at 20 mA, which is much higher than 4 mA? What about the Cpd values for the SN74AUPG106 and SN74AUPG107 (along with the dual input versions)?

  • I do not know how accurate your 30 pF estimate is. From your measured power dissipation, I would suspect that it is higher.

    The worst-case output resistance can be computed from the worst-case VOL and IOL specifications; the typical value is roughly half that.

    4 mA is the highest current for which a voltage drop is specified; larger currents up to 20 mA will not damage the device, but the output transistor might saturate, i.e., the output resistance might increase.

    The Cpd values in the datasheets appear to be consistent.

  • I'll check with the manufacturer what the load capacitance of the LED is. However, why would the current consumption when directly driven by the MCU be lower with the optical output actually being higher? If it's the same LED/load capacitance used, and if the output resistance of the MCU is lower, it should actually be a higher current consumption?

  • I cannot explain that observation.

    I guess you need an oscilloscope to show how the LED voltage/current actually behaves.