Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 5 answers
  • Subscribers 64 subscribers
  • Views 1254 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

ULN2003A: Test condition for the figure 16 MAXIMUM AND TYPICAL INPUT CURRENT

ULN2003A: Input current limit

James Zhan
Expert 4365 points
Part Number: ULN2003A

Hi team,

My customer has MCU IO output with maximum current of 5mA, and they're wondering if they need to add an external resistor to limit the input current of ULN2003A?

Since the Rb in ULN2003A is only 2.7kohm with 15% variation, and customer concerns about the input current may damage ULN2003A.

Let me know if you have any ideas, thanks!

James

  • 0
    TI__Mastermind 32650 points

    Hi James,

    By adding a resistance to the base of the BJT, the timing may be affected.

    The input current will be a function of the collector current from through the channel so I do not think there is a need to add any external resistance to the device if the collector current is kept within spec.

  • 0 in reply to Shreyas Dmello
    TI__Expert 4365 points

    Hi Shreyas,

    But the MCU IO could output a 5mA current, would this damage the ULN2003? Since according to the datasheet, ULN2003A maximum input currentis 1.35mA(VI=3.85V). 

    Thanks.

  • 0 in reply to James Zhan
    TI__Mastermind 32650 points

    Hi James,

    To clarify that specification, a voltage of 3.85V is imparted on the input pin and the current is measured. Figure 12 on the datasheet depicts this test.

    At 3.85V input, the BJT will pull 1.35mA through the base. So if you had a weak driver (lets say a source that can only output 0.5mA) then there is a risk that the BJT would not turn on.

  • 0 in reply to Shreyas Dmello
    TI__Expert 4365 points

    Hi Shreyas,

    So the input current here means the minimum driven current needed for the ULN2003A? 

  • 0 in reply to James Zhan
    TI__Mastermind 32650 points

    Hi James,

    That spec is the typical and maximum current pulled into the base when the collector and flyback diode pins are open.

  • 0 in reply to Shreyas Dmello
    TI__Expert 4365 points

    Hi Shreyas,

    I am still confused with this:  MCU IO output with maximum current of 5mA;

    If the ULN2003A input current limit is less than 2mA according to Figure 6 in datasheet, would this IO current damage the ULN2003A?

  • 0 in reply to James Zhan
    TI__Mastermind 32650 points

    Hi James,

    That spec is not a limit. If the collector current is set to within spec, then the base current cannot be more than what is specified on the datasheet. This is because for a BJT, I_c = B*I_b when B is the current gain factor. 

    The BJT will not pull more current through the base than what is required for the load on the collector.

  • 0 in reply to Shreyas Dmello
    TI__Expert 4365 points

    Hi Shreyas,

    Do you mean the load for collector current determine the base current?

    Could you please help to share the data of B of "I_c = B*I_b"?

    By the way, what's the Vbe of two transistor in the input structure as below picture shows?

    Thanks.

    James

  • 0 in reply to James Zhan
    TI__Mastermind 32650 points

    Hi James,

    That equation is the BJT relation between collector current and base current. This is not specific to the ULN2003A but is rather the electrical characteristics of a BJT. B is the DC current gain, also referred to as h_FE. Please refer to figure 3 and figure 8 on the datasheet to see a pictorial representation of DC current gain.

    A BJT is a current device. The collector current is the base current magnified by B. Thus, if you have a collector current (set by your load), then your base current will be equal to the collector current divided by B. 

    Please refer to this App Note for a more detailed explanation: http://www.ti.com/lit/an/sloa026a/sloa026a.pdf

    TI does not spec the individual V_BE of the internal transistors. TI specs the ON state input voltage at a specific V_CE. Please refer to the datasheet for these values and to Figure 14 for a pictorial representation of the same.