• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 249 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.

LM338: Datasheet clarifications

Roland Melad
Intellectual 1185 points
Part Number: LM338
Other Parts Discussed in Thread: LM138,

Hello,

We have a customer with below questions. Appreciate your help to advice

1) The graph of Figure 5 on data sheet page 6 indicates drop-out voltages for load levels at 5A, 3A and 1A. Is there any data for this component that specifies the drop-out voltage at lower load levels? For example, what is the drop-out voltage at 500mA, 250mA and 100mA (for both 25C and 50C)?

2) In the same Figure 5, is the C temperature shown referring to ambient, case or junction? Also, what is being indicated by "delta Vout = 100mV" in the upper-left corner of the graph? I don't understand the implication.

3) For LM338 regulator, the minimum load required to maintain regulation is listed at 3.5mA (typical) and 10mA (max) on data sheet page-5. For LM138, the minimum required load listed is 3.5mA (typical) and 5mA (max). Are the required worst-case minimum levels actually higher for the LM338 than for the LM138, or is this an error in the data sheet? In section 7.4.3 on page 10 (Operation at Light Loads), it indicates that worst-case minimum load is 5mA, but it doesn't specify that this applies only to LM138 and not to LM338. Please clarify.

4) On data sheet page 10 and 11, it indicates that either the load or feedback must consume the approx 50uA bias current passed to the output terminal for the output to remain in regulation at the desired voltage level. To do this, the design recommendation is to install a 250-ohm feedback resistor between the output and adjust terminal to consume the worst-case minimum load current of 5mA: (1.25V/0.005A = 250-ohm). Otherwise, the output voltage will increase above the target level. In the application under consideration, the load will not always be connected to the LM338 output, but installing a 250-ohm resistor to consume the required 5mA minimum current for maintaining regulation may be too excessive (as installed, this 5mA would be parasitic load on the system battery). If the worst-case minimum load for the LM338 is actually 10mA instead of 5mA (question 3), this would definitely be too excessive. If the Vref resistor (R1) value was chosen so that only 1mA was consumed by the feedback circuit when the main load to the regulator was disconnected, this would be the only parasitic load on the battery and could be tolerated. This 1mA load is obviously below the required 5mA level to keep the output in regulation, however with the main load disconnected, the increase in regulator output voltage will not actually have any adverse effect on the circuit external to the regulator. With only a 1mA Vref load, approximately how much will the regulator output voltage increase, will it be stable or oscillatory and will this have any adverse effect on the regulator itself?

Regards,

Roland

  • 0
    TI__Mastermind 29080 points

    Hi Roland,

    I am reviewing your question and will respond within 2 business days.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Mastermind 29080 points

    Hi Roland,

    Here are my comments:

    1. This is an older component so data not already published on the datasheet will be hard to come by.  I reviewed the archives (several of them, actually) and could not locate any additional information which would answer the question regarding dropout at different load current values.

    2. Dropout is defined as an output voltage deviation of 100mV.  This is a standard definition at TI and helps to standardize the testing between different parts.  Without it we would be left questioning if what we were seeing was dropout, vs when a component is dropping due to board IR drop, load regulation, line regulation, etc.  The graph was most likely completed at ambient temperature, however the tests are captured in fractions of a second (as fast as we can take them).  So it is safe to assume that the junction is nearly at the same temperature as ambient.

    3. The datasheet is correct on the values of minimum load current.  Each device has a slightly different minimum load current.  Section 7.4.3 on page 10 is referring to the LM138 device.  I have made a note in our datasheet correction log to clarify this in the next revision of the datasheet.

    4. We have not characterized this device under various modes of unstable operation, and by definition a device that is unstable can have unpredictable behavior.  With a load under the minimum load for stability, it may oscillate.  This device uses a floating reference topology and as the customer noted, the output will float unless loaded by the minimum load requirements.  At a minimum, and given enough time based on the load capacitance, the output may float up to Vin of the component, where it would be clamped by the parasitics of the internal BJT pass element.  The LDO will be fine as long as absolute maximum ratings are not exceeded.

    If the customer would like to provide their LDO requirements (Vin, Vout, Iout, temperature, and any special features or constraints) then I can have one of my team members review our catalog for another component that would better meet their needs.

    Thanks,

    - Stephen