New DC-DC Converter from TI Emphasizes Flexibility and Low Component CountJanuary 20, 2020 by Robert Keim
The LM63635 is a wide-input-voltage-range buck regulator that is specifically intended for automotive applications.
The essence of a switch-mode regulator is producing a stable output voltage by turning a transistor on and off, such that the switching element spends most of its time in a fully conductive state or a fully non-conductive state. It’s not surprising, then, that the particular methodology that we use when turning the transistor on and off is a major aspect of the circuit’s overall performance. One important parameter is the regulator’s switching frequency, which influences efficiency, output ripple, and required PCB area.
The term “foldback” refers to a circuit-protection technique whereby a regulator or amplifier gradually reduces the output current that occurs during a fault condition.
In a circuit with a standard “current limit” form of protection, the regulator or amplifier prevents the output current from rising above a specified value. This behavior is shown in the plot below:
The problem with this approach is that a large amount of output current is still present after the protection circuitry has become active. The output current itself will not damage the device, but the high power dissipation associated with this current may lead to excessively high temperatures.
Foldback current limiting addresses both the current and the power dissipation. As you can see in the next plot, this approach to short-circuit protection involves “folding back” the curve toward lower current values.
By reducing current in response to an output short circuit instead of merely limiting it, a foldback circuit helps to maintain acceptable power dissipation during a fault condition.
Flexible Regulation for Automotive Systems
The LM63635 is a step-down DC/DC converter with a wide input-voltage range (3.5 V to 32 V), high output current (3.25 A), and AEC-Q100 qualification. It offers 23 µA quiescent current, efficiencies in the 65–90% range even with very small load currents, and pin-selectable output voltage.
As you can see in the simplified schematic and PCB implementation, this looks like a good regulator for people (such as myself) who really do not enjoy designing complicated switcher circuits.
Simplified schematic of LM63635. Image used courtesy of Texas Instruments
PCB implementation of LM63635. Note that IOUT = 3.25 A, ƒSW = 2200 kHz. Image used courtesy of Texas Instruments
The LM63635 includes an interesting feature that TI calls “frequency foldback.” In this mode of operation, the regulator’s switching frequency automatically folds back in response to light-load conditions, dropout, minimum on-time operation, or current limiting. Using frequency foldback with small load currents leads to higher efficiency.
The designer can choose a switching frequency using the device’s RT pin, but the LM63635 will deviate from this setting when the foldback functionality is active. If the designer wants to prioritize constant switching frequency over efficiency, this can be achieved by placing the device in “forced PWM” mode. In some cases, it is desirable to avoid switching-frequency fluctuations to ensure that the regulator does not shift into a frequency band where conducted or radiated emissions will interfere with nearby circuitry.
The following plot gives you an idea of how the switching frequency is modified in response to light-load conditions.
Switching frequency vs. output current of LM63635. Image (modified) used courtesy of Texas Instruments
I can imagine that some designers would be uncomfortable with such large variations in switching frequency. The next plot shows typical efficiency when the output voltage is 5 V. It seems to me that 90% efficiency at 1 mA output current (with VIN = 8 V) is quite good, and I would probably be willing to tolerate major changes in switching frequency if I were designing a battery-powered device that frequently goes into some sort of low-current standby mode.
Graph showing typical efficiency of LM63635 at 5 V. Image (modified) used courtesy of Texas Instruments
What are your thoughts on the value of this frequency-foldback feature? Do you prefer fixed-frequency switchers, or is efficiency usually a higher priority?