SN74AHCT244-Q1: output current calculation

Haiwen Huang

Part Number: SN74AHCT244-Q1
Other Parts Discussed in Thread: UCC21750-Q1

Tool/software:

Dears:

SN74AHCT244-Q1 Continuous output current is +/- 25mA, Do we have formula to calculate this current ?

Below is the test waveform, blue waveform is the output current , you can see the current pulse peak is 40mA, time is 40nS, Please help check if this has risk. Thanks.

3 months ago

0 Ian Graham 3 months ago

TI__Genius 9591 points

Hi Haiwen,

Yes, violating the absolute maximum output current can damage the part.

Best,

Ian

0 Haiwen Huang 3 months ago in reply to Ian Graham

TI__Intellectual 2170 points

Ian:

Do you have document to help calculate the current ?

Haiwen Huang said:
SN74AHCT244-Q1 Continuous output current is +/- 25mA, Do we have formula to calculate this current ?

0 Clemens Ladisch 3 months ago in reply to Haiwen Huang

Guru 316890 points

The actual output current depends on the load. What is your load?

0 Haiwen Huang 3 months ago in reply to Clemens Ladisch

TI__Intellectual 2170 points

Clemens:

It's load is UCC21750-Q1 IN+ and IN-, you can refer to below sch.

0 Clemens Ladisch 3 months ago in reply to Haiwen Huang

Guru 316890 points

The UCC21750-Q1 inputs have 55 kΩ inputs. The input current is negligible.

But there also are several capacitors connected to the outputs. A capacitor initially acts like a short, so the current is limited only the output's internal impedance, which is about 23 Ω. And 5 V / 23 Ω = 217 mA.

Remove those 100 pF capacitors, or add current-limiting resistors between them and the output pins.

0 Haiwen Huang 3 months ago in reply to Clemens Ladisch

TI__Intellectual 2170 points

Clemens:

about 23 Ω impedance, I can't find it in datasheet, Could you please help share where can find this spec ? Thanks.

0 Clemens Ladisch 3 months ago in reply to Haiwen Huang

Guru 316890 points

The worst-case output impedance is V_OL / I_OL = 0.44 V / 8 mA = 45 Ω. The typical value is roughly half that.

0 Haiwen Huang 3 months ago in reply to Clemens Ladisch

TI__Intellectual 2170 points

Clemens:

Customer said SN74AHCT244-Q1 continuous output current is +/- 25mA, but not the surge current , Can BU help provide surge current ? They think 40mA and 40nS surge current is safe for this part.

0 Clemens Ladisch 3 months ago in reply to Haiwen Huang

Guru 316890 points

The absolute maximum rating can be exceeded only when the total capacitive load is less than about 70 pF. This is not the case here.

0 Haiwen Huang 3 months ago in reply to Clemens Ladisch

TI__Intellectual 2170 points

Could you please help provide more detailed document to help customer understand it ? Thanks.

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SN74AHCT244-Q1: output current calculation