Have you been had? Check the stated vs. actual current limit

TI’s SIMPLE SWITCHER® LM257x and LM259x regulators have been a popular choice for direct current (DC)/DC buck regulation for over 20 years. Due to their popularity, a number of manufacturers have created similar products that seem to mirror SIMPLE SWITCHER products, yet they may not. Here’s what to look for when you are choosing between seemingly identical products.

Other manufacturers have created pin-to-pin (P2P) drop-in alternative devices for TI’s SIMPLE SWITCHER LM257x and LM259x families. These similar products – let’s call them “regulator X” – claim to have identical performance and specifications. But what you can’t see under the shell is that regulator X contains silicon designed for a completely different regulator, and the manufacturer has modified the pinout to match the SIMPLE SWITCHER configuration. The reality is that it is very difficult to match the exact parameters of a part with different silicon. This means that regulator X will have to de-rate its data-sheet specifications compared to the SIMPLE SWITCHER device, or else its performance will not match those specifications.

The data sheet for regulator X closely resembles the data sheet for TI’s SIMPLE SWITCHER regulator; in fact, it may look identical. But how well does the performance of regulator X match the critical parameters stated in its data sheet, like switching frequency and current limit? Good question. As a test, the applications engineering team at TI created a custom evaluation board to take performance and reliability bench data on TI’s SIMPLE SWITCHER 40V, 3A LM2596 buck converter, as well as three other regulator X devices – which I’ll call Xa, Xb and Xc – from various manufacturers.

We evaluated all four devices for efficiency across input voltage and load current for a common 5VOUT application. We conducted tests at temperatures of -40°C, 25°C and 85°C. We also tested output current all the way to 7A on the nominal 3A devices in an effort to reveal performance limits, safety risks and reliability risks. We took oscilloscope screen captures at each operating point and recorded thermal-camera images during short-circuit conditions.

The most common performance inaccuracy we found was in the stated vs. actual current limit. High current limits cause excessive inductor, diode and integrated circuit (IC) heating during short-circuit conditions, which can damage the IC and prevent it from functioning correctly. This in turn can prevent the end product from functioning. On the datasheets for regulators Xa, Xb and Xc, the stated current limit is 4.5A (typical). However during evaluation, we found that regulators Xa, Xb and Xc’s current limits actually ranged from 5A to 10A; see Table 1.

Table 1: Current-limit comparison between TI’s SIMPLE SWITCHER LM2596 and alternative devices

All three of the P2P-alternative ICs we evaluated had very high current limits that were well above their data sheet’s stated parameter. Two devices did not limit current until it was between 5.5A to 6A, while  another device reached currents all the way up to 10A during a short-circuit condition. As shown in Figure 1, this particular IC heated all the way up to 110°C and the diode (not pictured) heated up to 133°C. At those temperatures, the potential for IC damage is high.

Figure 1: Thermal-camera image of regulator Xc during a short-circuit condition

An accurate and safe current limit is essential for the long-term reliability of the end equipment in which these devices are used. A short-circuit condition can cause the system to become inoperable.

So the next time you are shopping for LM257x and LM259x regulators, remember that it takes more than copying a data sheet to create a quality IC. Select TI’s SIMPLE SWITCHER regulators and rest easy knowing they will perform to spec: We guarantee it.

Additional resources:

  • Thermal management is a major concern for any power designer. Select a SIMPLE SWITCHER DC/DC switching regulator with thermal resistance as low as 10°C/W.