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