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SN74HC164: Temp issue

GARETH OU
Guru 10125 points
Part Number: SN74HC164
Other Parts Discussed in Thread: SN74LV595A, TPIC6C595, SN74LV164A, , ULN2003A, TPL7407L

 Hi

Customers use SN74HC164PWT_ Shift Register to control the LED, It has been found that it will reach 107°C in a 75°C chamber test.

1. What is the cause of high temperature? FW control?

2. Please confirm whether the schematic 3.3V needs to be adjusted?

3. Parallel Output: 4mA?

  • 0
    Guru 281530 points

    HC devices have a drive strength of about 2 mA at 3.3 V. Larger currents indeed result in a higher power dissipation.

    Consider the SN74LV164A (6 mA), SN74LV595A (8 mA), or TPIC6C595 (100 mA).

    Using a 5 V supply or paralleling outputs also would help.

  • 0
    TI__Guru* 96862 points

    First, I should say that this is well within the operating parameters of the device -- it's rated for up to 125C ambient temperature and 150C junction temperature, so a case temperature of 107C at 75C ambient is no issue.

    You're seeing a rise of 32C, so I'd expect that the power consumption of the devices is about 0.25W.

    At 4mA drive, we'd expect to see the output voltage around 0.132V, which means each channel is dissipating 0.5mW from DC operation. With 8 channels, that's only about 4mW total. Even if the resistance was significantly more than I expect, say double, then it's still only 8mW.

    My guess is that there's some PWM involved -- is the device switching quickly? You mentioned "FW control" -- what does that mean?

    Another possibility is a floating input -- floating inputs cause a lot of power to be consumed by the device.

  • 0 in reply to Emrys Maier
    Guru 10125 points

    Hi Emrys

    1. The total output power consumption of SN74HC164PWT is 4mA? Or is it 4mA per port?

    2. Please suggest P2P SN74HC164PWT to drive >80mA products?

  • 0 in reply to GARETH OU
    TI__Guru* 96862 points

    Hello,

    4mA is the test current used for measuring VOH and VOL for each output of the device. It is not a maximum rating.

    The total output current of the SN74HC164 is in the datasheet:

    Exceeding 50mA total could damage the device.

    I don't have anything in my product line (logic and voltage translation) that can drive 80 mA per output channel -- and I don't think anyone else will have a pin to pin match for that much power.

    There are some driver products from TI, for example, the TPL7407L and ULN2003A, that could support that much current per channel, however they do not perform the logic function of the serial shift register.